m.sc. chemistry of natural products

M.Sc.

CHEMISTRY OF NATURAL PRODUCTS

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M.Sc. ChemistryPaper-XIVCHE-114

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CONTENTS

Chapters Page No.

1. Terpenoids and Carotenoids 1

2. Alkaloids 101

3. Vitamins 118

4. Steroids and Hormones 136

5. Rotenoids and Porphyrins 170

NOTES

Terpenoids and Carotenoids

Self-Instructional Material 1

CHAPTER – 1

TERPENOIDS AND CAROTENOIDS

STRUCTURE 1.1 Learning Objectives 1.2 Introduction 1.3 Isolation of Monoterpenes and Sesquiterpenes 1.4 General Methods of Determining Structure 1.5 Monoterpenes 1.6 Monocyclic Monoterpenes 1.7 Bicyclic Monoterpenes 1.8 Sesquiterpenes 1.9 Acyclic Sesquiterpenes 1.10 Monocyclic Sesquiterpenes 1.11 Bicyclic Sesquiterpenes 1.12 Diterpenes 1.13 Carotenes 1.14 Summary 1.15 Review Questions 1.16 Further Readings

1.1 LEARNING OBJECTIVESAfter studying the chapter, students will be able to:

zz To definition the Diterpenes & Caroteneszz To discuss the Monocyclic Monoterpeneszz To describe the General Methods of Determining Structurezz The understand the Monoterpenes & Monocyclic Monoterpenes

1.2 INTRODUCTIONThe terpenes form a group of compounds the majority of which occur in the plant kingdom; a few terpenes have been obtained from other sources. The simpler mono- and sesquicentennial are the chief constituents of the essential oils; these are the volatile oils obtained from the

NOTES

Chemistry of Natural Products

Self-Instructional Material2

sap and tissues of certain plants and trees. The essential oils have been used in perfumery from the earliest times. The di- and triterpenes, which are not steam volatile, are obtained from plant and tree gums and resins. The tetraterpenes form a group of compounds known as the caro tenoids, and it is usual to treat these as a separate group. Rubber is the most important polyterpene.

Most natural terpene hydrocarbons have the molecular formula {C5H8)n, and the value of n is used as a basis of classification. Thus we have the following classes {these have already been mentioned above):

{i) Monoterpenes, C10H16 {ii) Sesquiterpenes, C15H24

{iii) Diterpenes, C20H32 {iv) Triterpenes, C30H48

(v) Tetraterpenes, C40H64 (these are the carotenoids).(vi) Polyterpenes, (C5H8)n

In addition to the terpene hydrocarbons, there are the oxygenated derivatives of each class which also occur naturally, and these are mainly alcohols, aldehydes or ketones.

The term terpene was originally reserved for those hydrocarbons of molecular formula C10H16, but by common usage, the term now includes all compounds of the formula (C5H8)n. There is, however, a tendency to call the whole group terpenoids instead of terpenes, and to restrict the name terpene to the compounds C10H16.

The thermal decomposition of almost all terpenes gives isoprene as one of the products, and this led to the suggestion that the skeleton structures of all naturally occurring terpenes can be built up of isoprene units; this is known as the isoprene rule, and was first pointed out by Wallach (1887). Thus the divisibility into isoprene units may be regarded as a necessary condition to be satisfied by the structure of any plant-synthesised terpene. Furthermore, Ingold (1925) pointed out that the isoprene units in natural terpenes were joined “head to tail” (the head being the branched end of isoprene). This divisibility into isoprene units, and their head to tail union, may conveniently be referred to as the special isoprene rule. It should be noted, however, that this rule, which has proved very useful, can only be used as a guiding principle and not as a fixed rule. Several exceptions to it occur among the simpler terpenes, e.g., lavandulol is composed of two isoprene units which are not joined head to tail; also, the carotenoids are joined tail to tail at their centre.

CH3

CH3C=CH.CH2.CH—C

CH2=C—CH=CH2

lavandulol isopreneCH2OH

CH3

CH3CH1

The carbon skeletons of open-chain monoterpenes and sesquiterpenes are:

headhead tail

C CCCCCCCCC

C C C C C C CCCC

C C C C C

tail

NOTES



Monocyclic terpenes contain a six-membered ring, and in this connection Ingold (1921) pointed out that a gem-dialkyl group tends to render the cyclohexane ring unstable. Hence, in closing the open chain to a cyclo hexane ring, use of this “gem-dialkyl rule” limits the number of possible structures. Thus the monoterpene open chain can give rise to only one possibility for a monocyclic monoterpene, viz., the p-cyrnene structure. This is shown in the following structures, the acyclic structure being written in the conventional “ ring shape “.

CC

C

CC

CCC

C

C

CC

C

CC

C

C

CCC

acyclic structure p-cymene structureAll natural monocyclic monoterpenes are derivatives of p-cyrnene. Bicyclic monoterpenes

contain a six-membered ring and a three-, four or five-membered ring. Ingold (1921) also pointed out that cyclopropane and cyclobutane rings require the introduction of a gem-dimethyl group to render them sufficiently stable to be capable of occurrence in nature. Thus closure of the C10 open chain gives three possible bicyclic structures; all three types are known.

CC

CC

CC

C CC

C CC C C

C C CCC

CC

C

C C CC

C

CCC

If we use these ideas with the sesquiterpene acyclic structure, then we find that only three monocyclic and three bicyclic structures are possible (not all are known; see the sesquiterpenes).

CC

C

C

C

C

CC

C C C C CC

CC

CC

C

C

C

C

CC

C CC

C

C

CC

C

C

CC

C C

C CC

C CC

C C

C

C

C

C

C

C CC

C CC

C

C

C C

C

C

C

C

C CC

CC

C

CC

C C

C

CCC

CCCC

CC

C

CCC

C

C

NOTES



Recently some furano-terpenes have been isolated, e.g., dendrolasin, which is believed to have the following structure (Quilico et al., 1957); it contains three isoprene units joined head to tail.

CH2.CH2.CH=C.CH2.CH2.CH=CMe2

MeOThe carotenoids are yellow or orange pigments which are widely distributed in plants

and animals. Chlorophyll is always associated with the carotenoids carotene and lutein; the carotenoids act as photo sensitisers in conjunction with chlorophyll. When chlorophyll is absent, e.g., in fungi, then the carotenoids are mainly responsible for colour. Cltro tenoids are also known as lipochromes or chromolipids because they are fat-soluble pigments. They give a deep blue colour with concentrated sulphuric acid and with a chloroform solution of antimony trichloride (the Carr-Price reaction); this Carr-Price reaction is the basis of one method of the quantitative estimation of carotenoids. Some carotenoids are hydro carbons; these are known as the carotenes, other carotenoids are oxygenated derivatives of the carotenes; these are the xanthophylls. There are also acids, the carotenoid acids, and esters, the xanthophyll esters.

Chemically, the carotenoids are polyenes, and almost all the carotenoid hydrocarbons have the molecular formula C40H56. Also, since the carbon skeleton of these compounds has a polyisoprene structure, they may be regarded as tetraterpenes.

In most of the carotenoids, the central portion of the molecule is composed of a long conjugated chain comprised of four isoprene units, the centre two of which are joined tail to tail. The ends of the chain may be two open chain structures, or one open-chain structure and one ring, or two rings. The colour of the carotenoids is attributed to the extended conjugation of the central chain. X-ray analysis has shown that in the majority of natural carotenoids, the double bonds are in the trans-position; a few natural carotenoids are cis-. Thus, if we represent the ends of the chain by R (where R may be an open-chain structure or a ring system), trans-carotenes may be written:

H

H H H H H H H

H H H H H HCCCCCCCCC

C C C C C C C C C R

CH3

CH3CH3

CH3

R

If we use the conventional formulae of terpenes, the above formula will be the following (the reader should write out in this way the various formulae given in the text; see §6 for an example):

RR

1.3 ISOLATION OF MONOTERPENES ANDSESQUITERPENESPlants containing essential oils usually have the greatest concentration at some particular time, e.g., jasmine at sunset. In general, there are four methods of extraction of the terpenes:

NOTES



(i) expression; (ii) steam distillation; (iii) extraction by means of volatile solvents; (iv) adsorption in purified fats (enfleurage). Method (ii) is the one most widely used; the plant is macerated and then steam distilled. If the compound decomposes under these conditions, it may be extracted with light petrol at 50°, and the solvent then removed by distillation under reduced pressure. Alternatively, the method of ad sorption in fats is used. The fat is warmed to about 50°, and then the flower petals are spread on the surface of the fat until the latter is saturated. The fat is now digested with ethanol, any fat that dissolves being removed by cooling to 20°. The essential oils so obtained usually contain a number of terpenes, and these are separated by fractional distillation. The terpene hydrocarbons distil first, and these are followed by the oxygenated de rivatives. Distillation of the residue under reduced pressure gives the sesquiterpenes, and these are separated by fractional distillation.

1.4 GENERAL METHODS OF DETERMINING STRUCTUREThe following brief account gives an indication of the various methods used in elucidating the structures of the terpenes.

(i) A pure specimen is obtained, and the molecular formula is ascertained by the usual methods. If the terpene is optically active, its specific rotation is measured. Optical activity may be used as a means of distinguishing structures.

(ii) If oxygen is present in the molecule, its functional nature is ascertained, i.e., whether it is present as hydroxyl, aldehyde, ketone, etc.

(iii) The presence of olefinic bonds is ascertained by means of bromine, and the number of double bonds is determined by analysis of the bromide, or by quantitative hydrogenation, or by titration with monoperphthalic acid. These facts lead to the molecular formula of the parent hydrocarbon, from which the number of rings present in the structure may be deduced.

(iv) The preparation of nitrosochlorides and a study of their behaviour.

(v) Dehydrogenation of terpenes with sulphur or selenium, and an exami nation of the products thereby obtained.

(vi) Measurement of the refractive index leads to a value for the molecular refractivity. From this may be deduced the nature of the carbon skeleton. Also, optical exaltation indicates the presence of double bonds in conjugation.

(vii) Measurement of the ultraviolet, infrared and Raman spectra. More recently X-ray analysis of crystals has also been used.

(viii) Degradative oxidation. The usual reagents used for this purpose are ozone, acid or alkaline permanganate, chromic acid and sodium hypo bromite. In general, degradative oxidation is the most powerful tool for elucidating the structures of the terpenes.

(ix) After the analytical evidence has led to a tentative structure (or structures), the final proof of structure depends on synthesis. In terpene chemistry, many of the syntheses are ambiguous, and in such cases analytical evidence is used in conjunction with the synthesis. Many terpenes have not yet been synthesised.

NOTES



1.5 MONOTERPENESThe monoterpenes may be subdivided into three groups: acyclic, mono cyclic and bicyclic. This classification affords a convenient means of study of the monoterpenes.

Acyclic Monoterpenes

Myrcene, C10H 16, is an acyclic monoterpene hydrocarbon which occurs in verbena and bay oils. It is a liquid, b.p. 166-168°. Catalytic hydro genation (platinum) converts myrcene into a decane, C10H22; thus myrcene contains three double bonds, and is an open-chain compound. Furthermore, since myrcene forms an adduct with maleic anhydride, two of the double bonds are conjugated. This conjugation is supported by evidence obtained from the ultraviolet spectrum of myrcene. These facts, i.e., that myrcene contains three double bonds, two of which are in conjugation, had been established by earlier investigators (e.g., Semmler, 1901) Ozonolysis of myrcene produces acetone, formaldehyde and a ketodialdehyde, C5H6O3, and the latter, on oxidation with chromic acid, gives succinic acid and carbon dioxide (Ruzicka et al., 1924). These results can be explained by assigning structure I to myrcene. In terpene chemistry it has become customary to use conventional formulae rather than those of the type I. In these conventional formulae only lines are used; carbon atoms are at the junctions of pairs of lines or at the end of a line, and instauration is indicated by double bonds. Furthermore, the carbon skeleton is usually drawn in a ring fashion (the cyclohexane ring).

ICH3

C=CH—CH2—CH2—C—CH=CH2

CH3CH2

II

Thus myrcene may be represented as II, and this type of structural formula will, in general, be used in this book. Thus the process of ozonolysis and oxidation of the ketodialdehyde may be written:

CHO

CHO

CHOO

O

CHO

CO2CH2

CH2

CO2H

O

+ 2CH2O

acetone formuldehyde

ketodialdehyde

CO2H

This structure for myrcene is supported by the fact that on hydration (under the influence of sulphuric acid), myrcene forms an alcohol which, on oxidation, gives citral. The structure of this compound is known, and its formation is in accord with the structure given to myrcene.

NOTES



Ocimene, C10H16, b.p. 81°/30 mm. When catalytically hydro genated, ocimene adds on three molecules of hydrogen to form a decane. Thus ocimene is an acyclic compound which contains three double bonds. Further more, since ocimene forms an adduct with maleic anhydride, two of the double bonds are conjugated. On ozonolysis, ocimene produces formaldehyde, methylglyoxal, lrevulaldehyde, acetic and malonic acids, and some acetone. All of these products, except acetone, are accounted for by structure I for ocimene (this has an isopropenyl end-group).

CO

CHO

I

CH3

CO CHO + CH2O

CH3.CO2H + CH2

CO2H

CO2HIn order to account for the appearance of acetone in the oxidation products, ocimene

is also believed to exist in the isopropylidene form, II, i.e., ocimene is a mixture of I and II, with I predominating.

II

CO2H

CO2HCH2

CH3CH3CO

+ CH3.CO.CHO + CH2O

citrAl, c10H16oThis is the most important member of the acyclic monoterpenes, since the structures of most of the other compounds· in this group are based on that of citral. Citral is widely distributed and occurs to an extent of 60-80 per cent in lemon grass oil. Citral is a liquid which has the smell of lemons.

CCC

C

C C

CCC

CI

C

C

IICH3

CH

CHCH

CHCH3CH3

CH

Citral was shown to contain an oxo group, e.g., it forms an oxime, etc. On heating with potassium hydrogen sulphate, citral forms p-cymene, II (Semmler, 1891). This reaction was used by Semmler to determine the posi tions of the methyl and isopropyl groups in citral;

NOTES



Semmler realised that the citral molecule was acyclic, and gave it the skeleton structure, I (two isoprene units joined head to tail). Citral can be reduced by sodium amalgam to an alcohol, geraniol,C10H16O, and is oxidised by silver oxide to geranic acid, C10H16O2; since there is no loss of carbon on oxidation to the acid, the oxo group in citral is therefore an aldehyde group (Semmler, 1890). Oxidation of citral with alkaline permanganate, followed by chromic acid, gives acetone, oxalic and laevulic acids (Tiemann and Semmler, 1895). Thus, if citral has structure III, the formation of these oxidation products may be accounted for.

This structure is supported by the work of Verley (1897), who found that aqueous potassium carbonate converted citral into 6-methyl hept-5-en-2-one, IV, and acetaldehyde. The formation of these products is readily explained by assuming III undergoes cleavage at the a: b-double bond; this cleavage by alkaline reagents is a general reaction of a : b-unsaturated oxo compounds .Furthermore, methylheptenone it self is also oxidised to acetone and laevulic acid; this is again in accord with structure III.

The structure of methylheptenone was already known from its synthesis by Barbier and Bouveault (1896). These workers condensed 2 : 4-dibromo-2-methylbutane with sodio-acetylacetone, and heated the re sulting compound with concentrated sodium hydroxide solution.

Barbier and Bouveault (1896) then converted methylheptenone into geranic ester, V, by means of the Reformatsky reaction, using zinc and ethyl iodoacetate.

NOTES



The synthesis of citral was completed by Tiemann (1898) by distilling a mixture of the calcium salts of geranic and formic acids (ca represents “half an atom of calcium”):

A more recent synthesis of citralis that of Arens and van Dorp (1948). Methylheptenone was first prepared as follows:

Then the methylheptenone was treated with ethoxyacetylene-magnesium bromide, the product reduced and then de-alkylated. It should be noted that an allylic rearrangement occurs in both parts of this synthesis. Ethoxyacetylenemagnesium bromide may conveniently be prepared from chloroacetaldehyde diethyl acetal as follows (Jones et al., 1954):

NOTES



Examination of the formula of citral shows that two geometrical isomers are possible:

Both isomers occur in natural citral, e.g., two semicarbazones are formed by citral; both forms of citral itself have also been obtained: citral-a (also known as geranial) has a b.p. 118-119°/20 mm., and citral-b (also known as neral) has a b.p. 117-118°/20 mm. The configurations of these two forms have been determined from a consideration of the ring closures of the corresponding alcohols.

The problem of the structure of citral is further complicated for the following reasons. Ozonolysis of citral gives acetone, lrevulaldehyde and glyoxal (Harries, 1903, 1907); these products are to be expected from structure III. On the other hand, Grignard et al. (1924) also isolated a small amount of formaldehyde from the products of ozonolysis; this points towards structure VI, which has an isopropenyl end-group. Thus citral has been regarded

as a mixture of four substances, two geranials and two nerals. Assuming, then, that both the isopropylidene and isopropenyl forms are present, it is possible that these two structures form a three-carbon tautomeric system:

NOTES



Recent work, however, has cast doubt on the existence of these two forms in citral. According to infra-red spectroscopic studies, it appears that naturally occurring acyclic monoterpenes as a class possess only the iso propylidene end-group structure (Barnard, Bateman et al., 1950). Accord ing to these authors, during oxidative degradation, partial rearrangement from the isopropylidene to the isopropenyl structure occurs, and so this method of determining fine structure is unreliable. Oliver (1961) has developed a chemical together with a chromatographic method for separating a mixture of isopropylidene and isopropenyl isomers. This should be of value in the studies of natural terpenes.

ionones

When citral is condensed with acetone in the presence of barium hydroxide, -ionone is formed and this, on heating with dilute sulphuric acid in the presence of glycerol, forms a mixture of a and b-ionones (Tiemann and Kriiger, 1893). The proportion of a to b varies with the nature of the cyclising agent used, e.g., with sulphuric acid, b-ionone is the main product; with phosphoric acid, a-ionone is the main product. Both ionones have been obtained from natural sources; the b-isomer is optically inactive, whereas the a-isomer can

exist in optically active forms since it contains one asymmetric carbon atom. Actually, the (+)-, (–) and (±)-forms of a-ionone occur naturally. Very dilute ethanolic solutions of b-ionone have the odour of violets.

The structures of the ionones were established by a study of the oxidation products produced by potassium permanganate (Tiemann, 1898, 1900);

NOTES



b-ionone gave geronic acid, I, a : a-dimethyladipic acid, II, and a : a-dimethylsuccinic acid, III. On the other hand, a-ionone gave a mixture of isogeronic acid, IV, b: b-dimethyladipic acid, V, and a: a-dimethylglutaric acid, VI.

Theimer et al. (1962) have isolated g-ionone (by vapour-phase chromatography) from the mixture of ionones obtained above (this ionone corresponds to the g-irone; see below).

,The ionones are related to irone, C14H22O; this occurs in the oil obtained from the

orris root. The structure of irone was established by Ruzicka et al. (1947), who showed that on ozonolysis, irone gives formaldehyde and b : b : g-trimethylpimelic acid, VIII; also, reduction of irone with hydriodic acid and red phosphorus, followed by dehydrogenation with selenium, gives 1:2: 6-trimethylnaphthalene, IX. Ruzicka therefore proposed structure

VII for irone. Ruzicka {1947) further showed that irone was a mixture of three isomers (VII is g-irone):

GerAniol

C10H18O, b.p. 229-230°/757 mm. This is found in many essential oils, particularly rose oil. Geraniol was shown to be a primary alcohol, e.g., on oxidation it gives an aldehyde (citral a); and since it forms a tetrabromide, geraniol therefore contains two double bonds. Reduction of citral produces geraniol, but at the same time some nerol is formed. The structural

NOTES



identity of geraniol and nerol is shown by the following facts. Both add on two molecules of hydrogen when hydrogenated catalytically; thus both contain two double bonds. Both give the same saturated alcohol, C10H22O. Also, on oxidation, geraniol and nerol give the same oxidation products which, at the same time, show the positions of the double bonds to be 2 and 7 (cf. citral, §5). Thus geraniol and nerol are geometrical iso mers. Geraniol has been assigned the trans configuration and nerol the cis on the fact that cyclisation to IX-terpineol (§ll) by means of dilute sulphuric acid takes place about 9 times as fast with nerol as it does with geraniol this faster rate with nerol is due to the proximity of the alcoholic group to the carbon (*) which is involved in the ring formation. Thus:

)Nerol also occurs naturally in various essential oils, e.g., oil of neroli, berga mot, etc.;

its b.p. is 225-226°.

Knights et al. (1955) have found that, on ozonolysis, geranyl acetate gives less than 3 per cent. of formaldehyde, and have concluded that the acetate and geraniol itself have predominantly the isopropylidene structure (cf. citral, §5).

linAlool

C10H18O, b.p. 198-199°. This is an optically active compound; the (–)-form occurs in rose oil and the (+)-form in orange oil. It was shown to be a tertiary alcohol, and since it adds on two molecules of hydrogen on catalytic hydrogenation, it must contain two double bonds. When heated with acetic anhydride, linalool is converted into geranyl acetate; and the latter is converted into the former by heating with steam at 200° under pressure. Also, heating linalool with hydrogen chloride in toluene solution at 100° produces geranyl chloride, and this, when treated with moist silver oxide in benzene solution, is reconverted into linalool. These reactions are parallel to those which occur when crotyl alcohol is treated with hydrogen bromide; a mixture of crotyl bromide and methyl vinylcarbinyl bromide is obtained. When either of these products is treated with moist silver oxide, a mixture of crotyl alcohol and methylvinylcarbinol is obtained.

Thus the elucidation of the structure of linalool is complicated by the ease with which the allylic rearrangement occurs. Since the structure of geraniol is known, a possible structure for linalool is obtained on the basis of this allylic rearrangement.

NOTES



This structure has been confirmed by synthesis of linalool (Ruzicka et al., 1919); 6-methylhept-5-en-2-one was treated as follows:

Normant (1955) has synthesised linalool in one step by the action of vinyl magnesium bromide on methylheptenone.

citronellAl c10H18oThis is an optically active compound which occurs in citronella oil. Citronellal is an aldehyde; reduction with sodium amalgam converts it into the alcohol citronellol, C10H90O, and oxidation gives citronellic acid, C10H18O2. Now there is another aldehyde, rhodinal, which is isomeric with citronellal, and on reduction, rhodinal gives the alcohol, rhodinol, which is isomeric with citronellol. Furthermore, reduc tion of ethyl geranate with sodium and ethanol gives rhodinol (Bouveault et al., 1900).

Oxidation of citronellal with chromic acid gives b-methyladipic acid and acetone (Tiemann et al., 1896, 1897). Rhodinal also gives the same products on oxidation. Thus structure I would fit the facts for both citronellal and rhodinal. On the other hand, ozonolysis of citronellal gives b-methyladipic acid, acetone and some formaldehyde (Harries et al., 1908). These results point towards structure II for citronellal, as well as I. Thus citronellal appears to be a mixture of I (isopropylidene end-group) and II (isopropenyl end-group). Furthermore,

NOTES



a detailed study of rhodinal has shown that this compound is identical with citronellal, but consists of a mixture of the two forms in different proportions (but cf. citral, §5).

Citronellol and RhodinolC10H20O. (–)-Citronellol occurs in rose and geranium oils, and is a mixture of the two forms:

The (+)-form of citronellol is made commercially by reduction of citronellal with sodium or aluminium amalgam; it also occurs in Java citronella oil. Rhodinol is identical with citronellol, but the proportions of the two forms are different from those which occur in citronellol; the identity of citronellol and rhodinol is shown by the products of ozonolysis.

1.6 MONOCYCLIC MONOTERPENESnoMenclAture

For the purposes of nomenclature of the mono cyclic monoterpenes, the fully saturated compound p-methylisopropylcyclo hexane, hexahydro-p-cymene or p-menthane, C10H20, is used as the parent substance; it is a synthetic compound, b.p. 170°. p-Menthane is I, and II is a conventional method of drawing formula I. The positions of sub stituents and double bonds are indicated by numbers, the method of numbering being shown in I (and II). When a

compound derived from p-menthane contains one or more double bonds, ambiguity may arise as to the position of a double bond when this is indicated in the usual way by a number which locates the first carbon atom joined by the double bond. To prevent am biguity, the second carbon atom joined to the double bond is also shown, but is placed in parentheses.

NOTES



The previous examples illustrate the method of nomenclature; in the first example, all the types of methods of nomenclature have been given; in the second and third examples, only the nomen clature that will be used in this book is given.

IX-Terpineol This is an optically active monoterpene that occurs naturally in the ( +)-, (–)-and (±)-forms; it is a solid, m.p. (of the racemic modification) 35°. The molecular formula of a-terpineol is C10H18O, and the oxygen atom is present as a tertiary alcoholic group (as shown by the reactions of a-terpineol). Since a-terpineol adds on two bromine atoms, it therefore contains one double bond. Thus the parent (saturated) hydro carbon of a-terpineol has the molecular formula C210H20. This corresponds to CnH2n, the general formula of the (monocyclic) cycloalkanes, and so it follows that a-terpineol is a monocyclic compound.

When heated with sulphuric acid, a-terpineol forms some p-cymene. Taking this in conjunction with the tentative proposal that a-terpineol is monocyclic, it is reasonable to infer that a-terpineol contains the p-cymene skeleton. Thus we may conclude that a-terpineol is probably p-menthane with one double bond and a tertiary alcoholic group. The positions of these functional groups were ascertained by Wallach (1893, 1895) by means of graded oxidation. The following chart gives the results of Wallach’s work; only the carbon content is indicated to show the fate of these carbon atoms (the formulae are given in the text).

NOTES



Oxidation of a-terpineol, I, with lper cent. alkaline potassium permanganate hydroxylates the double bond to produce the trihydroxy compound II, C10H20O3. This, on oxidation with chromic acid (chromium trioxide in acetic acid), produces a compound with the molecular formula C10H16O3 (IV). This compound was shown to contain a ketonic group, and that it was neutral, e.g., it gave no reaction with sodium carbonate solution. When, however, IV was refluxed with excess of standard sodium hydroxide solution, and then back titrated, it was found that alkali had been consumed, the amount corresponding to the presence of one carboxyl group. Thus compound IV appears to be the lactone of a monocarboxylic acid. Furthermore, since it is the lactone that is isolated and not the hydroxy acid, this spon taneous lactonisation may be interpreted as being produced from a g-hydroxy acid, i.e., IV is a g-lactone, and therefore III is a g-hydroxyacid. It is possible, however, for d-hydroxyacids to spontaneously lactonise, and so whether IV is a g- or d-lactone is uncertain at this stage of the evidence.

Now, since IV is formed from II by scission of the glycol bond, and since there is no loss of carbon atoms in the process, the double bond must there fore be in the ring in I. On warming with alkaline permanganate, IV gave acetic acid and a compound C18H12O4 (V). The formation of acetic acid suggests that IV is a methyl ketone, i.e., ·a CH3·CO group is present. Thus IV is a methyl ketone and a lactone; it is known as homoterpenyl methyl ketone, and the structure assigned to it has been confirmed by synthesis (Simonsen et al., 1932). A study of the properties of terpenylic acid, V, showed that it was the lactone of a monohydroxydicarboxylic acid. Further oxidation of terpenylic acid gives terebic acid C7H10O4 (VI), which is also the lactone of a monohydroxydicarboxylic acid.

The above reactions can be formulated as shown, assuming I (p-menth l-en-8-ol) as the structure of a-terpineol. These reactions were formulated by Wallach, who adopted formula I which had been proposed by Wagner (1894). The structures of terpenylic (V) and terebic (VI) acids were established by synthesis, e.g., those of Simonsen (1907).

NOTES



Terebic acid, m.p. 175°.

Terpenylic acid, m.p. 90°.

It is of interest to note here that Sandberg (1957) has prepared the b-acetotricarballylate in one step from acetoacetic ester and ethyl bromo acetate in the presence of sodium hydride (in benzene solution).

These syntheses strengthen the evidence for the structure assigned to a-terpineol, but final proof rests with a synthesis of a-terpineol itself. This has been carried out by Perkin, junior (1904), and by Perkin, junior, with Meldrum and Fisher (1908). Only the second synthesis is given here; this starts with p-toluic acid.

NOTES



Compound VII was also resolved with strychnine, each enantiomorph treated as shown above (esterified, etc.), and thereby resulted in the formation of (+)- and (–)-terpineol. It should be noted that in the above synthesis the removal of a molecule of hydrogen bromide from 3-bromo-4-methyl cyclohexane-1-carboxylic acid to give VII is an ambiguous step; instead of VII, compound VIII could have been formed. That VII and not VIII is formed rests on the analytical evidence for the position of this double bond; VIII cannot give the products of oxidation that are actually obtained from ex-terpineol.

A much simpler synthesis of ex-terpineol has been carried out by Alder and Vogt (1949); this makes use of the Diels-Alder reaction, using isoprene and methyl vinyl ketone as the starting materials.

Two other terpineols are also known, viz., b-terpineol and g-terpineol; both occur naturally.

NOTES



Carvone, C10H14O, b.p. 230°/755 mm. This occurs in various essential oils, e.g., spearmint and caraway oils, in optically active forms and also as the racemic modification.

Carvone behaves as a ketone and, since it adds on four bromine atoms, it therefore contains two double bonds. Thus the parent hydrocarbon is C10H20, and since this corresponds to the general formula CnH2n, carvone is monocyclic. When heated with phosphoric acid, carvone forms carvacrol; this suggests that carvone probably contains the p-cymene structure, and that the keto group is in the ring in the ortho-position with respect to the methyl group.

The structure of carvone is largely based on the fact that carvone may be prepared from IX-terpineol as follows:

The addition of nitrosyl chloride to a-terpineol, I, produces a-terpineol nitrosochloride, II, the addition occurring according to Markownikoff’s rule. This nitrosochloride rearranges spontaneously to the oximino compound, III (it might be noted that this rearrangement proves the orientation of the addition of the nitrosyl chloride to the double bond; addition the other way could not give an oxime, since there is no hydrogen atom at position 1 in a-terpineol). Removal of a molecule of hydrogen chloride from III by means of sodium ethoxide produces IV, and this, on warming with dilute sulphuric acid, loses a molecule of water with simultaneous hydrolysis of the oxime to form carvone, V. Thus, according to this interpretation of the reactions, carvone is p-menth-6: 8-dien-2-one. Actually, these reactions show that carvone has the same carbon skeleton as a-terpineol, and also confirm the position of the keto group. They do not prove conclusively the positions of the two double bonds; instead of position 6 (in IV), the double bond could have been 1(7), and instead of position 8 (as in V), the double bond could have been 4(8). Thus the above reactions constitute an ambiguous synthesis of carvone (a-terpineol has already been synthesised). The exact positions of these two double bonds have been determined analytically as follows.

The double bond in the 8-position. The following reactions were carried out by Tiemann and Semmler (1895).

NOTES



Reduction of carvone, V, with sodium and ethanol gives dihydrocarveol, C10H18O (VI); this is a secondary alcohol and contains one double bond, i.e., the keto group and one of the two double bonds in carvone have been reduced. Hydroxylation of the double bond in dihydrocarveol by means of I per cent. alkaline permanganate produces the trihydroxy compound C10H20O3 (VII). Oxidation of VII with chromic acid causes scission of the glycol bond to produce a compound C9H16O2 (VIII); this was shown to contain a keto group and a hydroxyl (alcoholic) group. The action of sodium hypobromite on VIII caused the loss of one carbon atom to produce the compound C8H16O3 (IX) ; this was shown to be a hydroxymonocarboxylic acid, and since one carbon is lost in its formation, its precursor VIII must therefore be a methyl ketone. Finally, dehydrogenation of IX by heating with bromine-water at 190° under pressure produced m-hydroxy-p-toluic acid, X (a known compound). Tiemann and Semmler explained these reactions on the assumption that one double bond in carvone is in the 8-position. Thus:

Had the double bond been in the 4(8)-position (structure Va), then compound VIII, and consequently X, could not have been obtained, since three carbon atoms would have been lost during the oxidation.

It might be noted in passing that V contains an asymmetric carbon atom, whereas Va is a symmetrical molecule and so cannot exhibit optical activity. Since carvone is known in optically active forms, structure Va must be rejected on these grounds.

NOTES



The double bond in the 6-position. Carvone adds on one molecule of hydrogen bromide to form carvone hydrobromide, C10H15OBr (XI), and this, on treatment with zinc dust and methanol, is converted into carvo tanacetone, C10H16O (XII), by replacement of the bromine atom by hydro gen. Thus the final result of these reactions is to saturate one of the two double bonds in carvone. Carvotanacetone, on oxidation with perman ganate, gives isopropylsuccinic acid, XIII, and pyruvic acid, XIV (Semmler, 1900). These products are obtainable only if the ring contains the double bond in the 6-position. Had the double bond been in the 1(7)-position, formic acid and not pyruvic acid would have been obtained. Further support for the 6-position is provided by the work of Simonsen et al. (1922), who obtained b-isopropylglutaric acid and acetic acid on oxidation of carvo tanacetone with . permanganate.

liMonene

C10H16, b.p. 175·5-176·5°. This is optically active; the (+)-form occurs in lemon and orange oils, the (–)-form in peppermint oil, and the (±)-form in turpentine oil. The racemic modification is also pro duced by racemisation of the optically active forms at about 250°. The racemic modification is also known as dipentene; this name was given to the inactive form before its relation to the active form (limonene) was known.

Since limonene adds on four bromine atoms, it therefore contains two double bonds. ( +)-Limonene may be prepared by dehydrating ( +)-a- terpineol with potassium hydrogen sulphate, and limonene (or dipentene) may be converted into a-terpineol on shaking with dilute sulphuric acid.

Thus the carbon skeleton and the position of one double bond in limonene are known. The position of the other double bond, however, remains un certain from this preparation; I or II is possible.

NOTES



Proof for position 8. Structure I contains an asymmetric carbon atom (C4), and hence can exhibit optical activity. II is a symmetrical molecule and so cannot be optically active. Therefore I must be limonene.

Chemical proof for position 8 is afforded by the following reactions:

Since the structure of carvoxime is known, it therefore follows that I must have one double bond in position 8; thus the above reactions may be written:

The connection between limonene and dipentene is shown by the fact that ( +)- or (–)-limonene adds on two molecules of hydrogen chloride in the presence of moisture to form limonene dihydrochloride, and this is identical with dipentene dihydrochloride.

Limonene dihydrochloride no longer contains an asymmetric carbon atom, and so is optically inactive. It can, however, exhibit geometrical isomer ism; the cis-form is produced from limonene, and the trans-form from cineole.

Dipentene can be regenerated by heating the dihydrochloride with sodium acetate in acetic acid, or boilding with aniline. On the other hand, when limonene dihydrochloride is heated with silver acetate in acetic acid, and then hydrolysing the ester with sodium hydroxide, 1: 8-terpin is formed; the direct action of sodium hydroxide on the dihydrochloride regenerates dipentene.

NOTES



8-Terpin exists in two geometrical isomeric forms, corresponding to the cis and trans dipentene dihydrochlorides. cis-1 : 8-Terpin is the common form, m.p. 105°, and readily combines with one molecule of water to form terpin hydrate. The trans-form, m.p. 158-159°, does not form a hydrate.

There is also a 1: 4-terpin; this was originally prepared by the action of dilute alkali on terpinene dihydrochloride.

Terpinenes, C10H16There are three isomeric terpinenes, and all give the same terpinene dihydrochloride with hydrogen chloride.

All three occur naturally.

Terpinolene, C10H16, b.p. 67-68°/10 mm. This occurs naturally. It is not optically active, and since it may be prepared by dehydrating ex-terpineol with oxalic acid, its structure is known (it is II, the alternative formula offered for limonene). Terpinolene adds on two molecules of hydrogen chloride to form dipentene dihydrochloride.

NOTES



Phellandrenes, C10H16. There are two phellandrenes, both of which are optically active, and all the enantiomorphs occur naturally.

1: 8-Cineole, C10H18O, b.p. 174·4°. This occurs in eucalyptus oils. It is isomeric with a-terpineol, but contains neither a hydroxyl group nor a double bond. The oxygen atom in cineole is inert, e.g., it is not attacked by sodium or by the usual reducing agents. This inertness suggests that the oxygen atom is of the ether type. Support for this is obtained from the fact that dehydration of cis-1: 8-terpin gives I: 8-cineole; at the same time, this reaction suggests that the structure of cineole is I.

Further support for this structure is afforded by a study of the products obtained by oxidation (Wallach et al., 1888, 1890, I892). When oxidised with potassium permanganate, cineole forms cineolic acid, II, and this, on distillation with acetic anhydride, forms cineolic anhydride, III. When distilled at atmospheric pressure, cineolic anhydride forms 6-methylhept- 5-en-2-one, IV, a known compound (§5). These reactions were interpreted by Wallach as follows:

Further work on the structure of cineolic acid has confirmed the above sequence of reactions.

It seems most probable that the I : 8-terpins have chair conformations, but when they form 1 : 8-cineole, the latter possesses the boat conformation; thus:

NOTES



There is also a 1 : 4-cineole; this occurs naturally.

Ascaridole, C10H16O1, b.p. 96-97°/8 mm. The cineoles are oxides; ascaridole, however, is a peroxide, the only known terpene peroxide, and it occurs naturally in, e.g., chenopodium oil. When heated to 130-150°, ascaridole decomposes with explosive violence. When reduced catalytically, ascari dole forms 1 : 4-terpin (Wallach, 1912}, and this led to the suggestion that

ascaridole is V. This structure has been confirmed by further analytical work. Ascaridole has been synthesised by Ziegler et al. (1944} by the irradiation of a.-terpinene in dilute solution in the presence of chlorophyll.

Sylvestrene, C10H16, b.p. 175-178°. This compound exists in(+)-, (–)– and (±)– forms; the racemic modification is also known as carvestrene (cf. limonene and dipentene, §13). The (+)– form of sylvestrene was first obtained from Swedish pine needle oil (Attenberg, 1877), and was shown to contain the m-cymene carbon skeleton (Baeyer et al., 1898). Thus sylvestrene appeared to be the only monocyclic monoterpene which did not have the p-cymene structure and was obtainable from natural sources. Although the m-cymene structure can be divided into two isoprene units (Wallach’s isoprene rule), these two units are not joined head to tail.

Subsequent work, however, showed that sylvestrene does not occur in pine oil. In the extraction of sylvestrene, the pine oil is heated with hydrogen chloride to give sylvestrene dihydrochloride. This compound was shown by Simonsen et al. (1923, 1925) to be produced by the action of hydrogen chloride on car-3-ene, i.e., these workers showed conclusively that

NOTES



the terpene originally present in Swedish pine oil is car-3-ene. Sylvestrene may be obtained from its dihydrochloride by heating the latter with aniline; removal of hydrogen chloride

from the ring can give rise to two possible positions for the ring double bond. Analytical work has shown that the side-chain is isopropenyl (and not isopropylidene), and that sylvestrene is a mixture of the two forms, m-mentha-1 : 8-diene and m-mentha-6 : 8-diene. Further more, it has been shown that car-4-ene is also present in pine oil; both of these carenes are readily converted into sylvestrene, and so it appears that the precursor of sylvestrene (itself a mixture) is a mixture of the two carenes.

The enantiomorphs of sylvestrene have been synthesised (Perkin, junior, et al., 1913), and it has also been shown that an equimolecular mixture of the dihydrochlorides of ( +)– and (–)– sylvestrene is identical with car vestrene dihydrochloride.

§16. Menthol and menthone. Menthol, C10H20O, is an optically active compound, but only the (–)– form occurs naturally, e.g., in peppermint oils. (–)– Menthol, m.p. 34°, is a saturated compound, and the functional nature of the oxygen atom is alcoholic, as shown by its reactions, e.g., menthol forms esters. Furthermore, since oxidation converts menthol into men thone, a ketone, the alcoholic group in menthol is therefore secondary. Also, since reduction with hydrogen iodide gives p-menthane, menthol most prob ably contains this carbon skeleton. Finally, since (+)– pulegone gives men thol on reduction, and since the structure of pulegone is known to be I, it therefore follows that menthol must be II. This structure,

p-menth-3-ol, for menthol has been confirmed by consideration of the oxidation products of menthone, and also by the synthesis of menthol. Examination of the menthol structure shows that three dissimilar asymmetric carbon atoms (1, 3 and 4) are present; thus eight optically active forms (four racemic modifications) are possible theoretically. All

NOTES



eight enantiomorphs are known and their configurations are as follows (the hori zontal lines represent the plane of the cyclohexane ring) :

These configurations have been assigned from a study of chemical and optical relationships and the Auwers-Skita rule. More recently the application of conformational analysis has confirmed these results. Eliel (1953) applied the principle that the esterification of an axial hydroxyl group occurs less readily than with an equatorial one. Furthermore, Eliel postu lated that the reaction proceeds via the conformation of the molecule in which the reactive hydroxyl group is equatorial, and that the rate differences should be attributed to that energy necessary to place the other substituents, if necessary, into the axial conformation. On this basis, the rates of esterification of the isomeric menthols will be:

menthol > iso- > neoiso- > neo-.These are the orders of rates actually obtained by Read et al. (1934). The following

conformations have been assigned by Eliel from chemical studies, and are supported by Cole et al. (1956) from their infra-red spectra and conformation studies.

In menthol, all of the substituents are equatorial, and in the rest one is axial. It should also be noted that the larger of the two alkyl groups (iso propyl) is always equatorial.

Menthone, C10H18O, b.p. 204°/750 mm. (–)-Menthone occurs in pepper mint oil, and it may readily be prepared by the oxidation of (–)-menthol with chromic acid. Menthone is a saturated compound which has the characteristic properties of a ketone. When heated with hydriodic acid and red phosphorus, menthone is reduced to p-menthane; thus this skeleton is present in menthone. Oxidation of menthone with potassium perman ganate produces a compound C10H18O3 ; this compound was shown to contain a keto-group and one carboxyl

NOTES



group, and is known as ketomenthylic acid (IV). Ketomenthylic acid itself is very readily oxidised by perman ganate to P-methyladipic acid (V) and some other acids (Arth, 1886; Manasse et al., 1894). The foregoing oxidative reactions may be formulated as follows, on the assumption that III is the structure of menthone.

This structure for menthone has been confirmed by synthesis, e.g., Kotz and Schwarz (1907) obtained menthone by the distillation of the calcium salt of {J’-methyl-cx-isopropylpimelic acid, which was prepared as follows. 3-Methyl cyclohexanone, VI, was condensed with ethyl oxalate in the presence of sodium, and the product VII then heated under reduced pressure; this gave the ethyl ester of 4-methylcyclohexan-2-one-1.:carboxylic acid, VIII. VIII, on treatment with sodium ethoxide followed by isopropyl iodide, gave IX, and this when boiled with ethanolic sodium ethoxide and the product then acidified, gave {J’-methyl-cx-isopropylpimelic acid, X (note the acetoacetic ester fragment in VIII).

Structure III contains two dissimilar asymmetric carbon atoms (I and 4), and so four optically active forms (and two racemic modifications) are possible. All are known, and correspond to the menthones and isomen thones; these are geometrical isomers, each one existing as a pair of enantio morphs. The configurations have been assigned on physical evidence; the cis-isomer has the higher refractive index and density.

NOTES



(±)– Pule one, C10H16O, b.p. 221-222°. This occurs in pennyroyal oils. Pulegone contains one double bond, and behaves as a ketone. On reduction, pulegone first gives menthone and this, on further reduction, gives menthol. When oxidised with permanganate, pulegone forms acetone and {J-methyladipic acid (Semmler, 1892); when boiled with aqueous ethan a ic potassium hydroxide, acetone and 3-methylcyclohexanone are obtained (Wallach, 1896). These reactions show that pulegone is p-menth-4(8)-en-

This structure has been confirmed by synthesis, starting from 3-methyl cyclohexanone (Black et al., 1956: cj. menthone, §16).

iso Pulegone can be isomerised to pulegone by alkaline reagents (Kon et al., 1927}, and Black et al. found that, on treating their mixture with sodium ethoxide, the resulting compound was pure pulegone.

(–)-Piperitone, C10H16O, b.p. 232-233°/768 mm. This occurs in eucalyptus oils, and is a valuable source of menthone and thymol. Piperi tone contains one double bond, and behaves as a ketone. Piperitone, on catalytic hydrogenation (nickel), gives menthone in almost quantitative yield; on oxidation with ferric chloride, thymol is obtained (Smith et al., 1920). These reactions show that piperitone is p-menthene-3-one, but do not show the position

NOTES



of the double bond. This had been shown by Schim mel (1910), who found that on oxidation with alkaline pennanganate, piperi tone gave a-hydroxy-a-methyl-a’-isopropyladipic acid, II, y-acetyl-a-iso propylbutyric acid, III, and a-isopropylglutaric acid, IV. These results can be explained only if piperitone is p-menth-1-en-3-one, I. This struc ture for piperitone has been confirmed by various syntheses (e.g., Henecka, 1948; Birch et al., 1949). Bergmann et al. (1959) have shown that piperitone is formed directly by the condensation of mesityl oxide with methyl vinyl ketone.

1.7 BICYCLIC MONOTERPENESThe bicyclic monoterpenes may be divided into three classes according to the size of the second ring, the first being a six membered ring in each class.

Class I (6– + 3-membered ring).

Class II (6- + 4-membered ring).

Class III (6- + 5-membered ring).

NOTES



It is important to note that the two rings do not lie in one plane, but are almost perpendicular to each other.

Thujone and its DThe members of this group which occur naturally are the following:

Carane and its DerivativesIt appears that only three carane derivatives occur naturally:

Car-3-ene occurs in Swedish pine needle oil. It is a liquid, b.p. 170°; when treated with hydrogen chloride it forms a mixture of sylvestrene dihydrochloride and dipentene dihydrochloride (§13).

(+)–Car-4-ene, b.p. 165·5-167°/707 mm., occurs in various essential oils. It forms sylvestrene dihydrochloride on treatment with hydrogen chloride (§15).

Car-3-ene-5: 6-epoxide, b.p. 83-85°/14 mm., occurs in certain essential oils.

NOTES



Carone, b.p. 99-100°/15 mm., is a synthetic compound, and is of some importance because of its relationship to carane. It was first prepared by

Baeyer et al. (1894} by the action of hydrogen bromide on dihydrocarvone, which was then treated with ethanolic potassium hydroxide, whereupon carone was obtained.

The structure of carone was established by Baeyer et al. (1896), who obtained caronic acid on oxidation of carone with pennanganate. Baeyer suggested that caronic acid was a cyclopropane derivative, and this was confirmed by synthesis (Perkin, junior, and Thorpe, 1899), starting with ethyl b: b-dimethylacrylate and ethyl cyanpacetate.

NOTES



An interesting point about carone is that its ultraviolet absorption spec trum shows similarities to that of a: /)-unsaturated ketones (Klotz, 1941).

Pinane and its DerivativesPinane, the parent compound of this group, is a synthetic substance which may be prepared by the catalytic hydrogenation (nickel or platinum) of either ex- or fJ-pinene. Pinane exists

in two geometrical isomeric forms, cis and tran§, and each of these exists as a pair of enantiomorphs. ·

PineneThis is the most important member of the pinane class. It occurs in both the (+)– and (–)–forms in all turpentine oils; it is a liquid, b.p. 156°.

The analytical evidence for the structure of a-pinene may conveniently be divided into two sections, each section leading independently to the structure, and the two taken together giving very powerful evidence for the structure assigned.

Method: The molecular formula of a-pinene is C10H16, and since a-pinene adds on two bromine atoms, one double bond is present in the molecule. Thus the parent hydrocarbon is Cl0H18 and since this corresponds to the general formula C,.H2n-2 the general formula of compounds containing two rings, it therefore follows that a-pinene is bicyclic (Wallach, 1887- 1891). In the preparation of a-pinene nitrosochloride (by the action of nitrosyl chloride on a-pinene) the by-products which were formed were steam distilled, and the compound pinol, C10H16O, was thereby obtained. Pinol adds on one molecule of bromine to form pinol dibromide, and so pinol contains one double bond. Furthermore, the action of lead hydroxide on pinol dibromide converts the latter into pinol glycol, C10H16O(OH)1, and this, on oxidation, gives terpenylic acid (Wallach et al., 1889). Pinol (III) is also obtained by the action of sodium ethoxide on a-terpineol dibromide, II (Wallach, 1893). Wagner (1894) showed that the oxidation of pinol with permanganate gives pinol glycol (IV), which is further oxidised to terpenylic acid (V). All these facts can be explained as follows, based on I being the structure of a-terpineol.

NOTES



Support for the structure given for pinol (III) is obtained from the fact that oxidation of sobrerol (pinol hydrate) produces a tetrahydric alcohol, sobrerythritol. Sobrerol itself is readily prepared by the action of hydrogen bromide on pinol, followed by sodium hydroxide. These reactions may thus be formulated:

Thus, if the formula for a-pinene is VI, then the formation of the above substances can be explained. This structure also accounts for other re actions of a-pinene, e.g., its ready hydration to a-terpineol.

Although the Wagner formula (VI) for a-pinene readily explains all the facts, there is no direct evidence for the existence of the cyclobutane ring. Such evidence was supplied by Baeyer (1896). This is described in method 2.

Method 2. As in method 1, a-pinene was shown to be bicyclic. When treated with ethanolic sulphuric acid, a.-pinene is converted into a-terpineol (Flavitzky, 1879). Therefore a-pinene contains a six-membered ring and another ring {since it is bicyclic), the carbon skeleton of pinene being such as to give a-terpineol when this second ring opens. Since, in the formation of a-terpineol, one molecule of water is taken up and the hydroxyl group becomes attached to C8, this suggests that the C8 of a-terpineol is involved in forming the

NOTES



second ring in a-pinene. There are three possible points of union for this C8, resulting in two three-membered and one four-membered ring (see VII); at the same time the position of the double bond in a-pinene is also shown by the conversion into a-terpineol {1).

A point of interest here is that there are actually four possible points of union for C8, the three shown in VII and the fourth being at the double bond to form a four-membered ring (VIla). This one, however, was rejected on the grounds of Bredt’s rule (1924) which states that a double bond cannot be formed by a carbon atom occupying the bridge-head (of a bicyclic system). The explanation for this rule is that structures such as VIla have a large amount of strain.

This second ring was shown to be four-membered by Baeyer (1896), who carried out the following series of reactions.

Pinene glycol, C1oH18(0H)11, is produced by hydroxylation of the double bond in a-pinene, and pinonic acid, C10H1603, is produced by scission of the , glycol bond. Pinonic acid was shown to be a saturated keto-monocarboxylic acid. The formation of pinic acid, C9H1404, and bromoform, indicates the presence of an acetyl group in pinonic acid. Pinic acid, which was shown to be a saturated dicarboxylic acid, on treatment with bromine, then barium hydroxide, and finally the product oxidised with chromic acid, gives cis norpinic acid, C8H12O4. This was shown to be a saturated dicarboxylic acid, and so its formula may be written C6H10(CO2H)2. Furthermore, since a-pinene contains two methyl groups attached to a carbon atom in the second ring (see VII), and it is the other ring (the six-membered one containing the double bond) that has been opened by the above oxidation, then norpinic acid (with this second ring intact) contains these two methyl groups. Thus the formula for norpinic acid may be written (CH3)11C4H4(CO11H)2. Hence, regarding the methyl and carboxyl groups as substituents, the parent (saturated) hydrocarbon (from which norpinic acid is derived) is C4H8. This corresponds to cyclobutane, and so norpinic acid is (probably) a dimethyl cyclobutanedicarboxylic acid. On this basis, pinic acid could therefore be a cyclobutane derivative with one side-chain of –CH11,CO2H.

NOTES



Baeyer therefore assumed that pinic and norpinic acids contained a cyclo butane ring, and so suggested the following structures to account for the above reactions, accepting structure VI for a-pinene, the structure already proposed by Wagner (1894).

The synthesis of norpinic acid (to confirm the above reactions) proved to be a very difficult problem, and it was not carried out until 1929, when Kerr succeeded with the following ingenious method (apparently the presence of the gem dimethyl group prevents closure to form the cyclobutane ring). The norpinic acid obtained was the trans-isomer; this is readily converted into the cis-isomer (the isomer obtained from the oxidation of a-pinene) by heating the trans acid with acetic anhydride, whereupon the cis anhydride is formed and this, on hydrolysis, gives the cis acid (Simonsen et at., 1929).

The total synthesis of a-pinene has now been carried out in the following way. Guha et al. (1937) synthesised pinic acid from norpinic acid, and Rao (1943) synthesised pinonic acid from synthetic pinic acid.

Ruzicka et al. (1920--1924) had already synthesised a-pinene starting from pinonk. acid (obtained by the oxidation of a-pinene). Thus we now have a total synthesis of a-pinene.

NOTES



Ruzicka’s synthesis makes use of the Darzens glycidic ester synthesis the steps are:

The final step gives a mixture of two compounds, a-and CJ-pinene. The former was identified by the preparation of the nitrosochloride; this proves that one of the products

NOTES



is a-pinene, but does not prove which is a and which is (J. These are differentiated by consideration of the analytical evidence; the following evidence also supports the structure given for a-pinene. This evidence is based on the fact that diazoacetic ester combines with compounds containing a double bond to form pyrazoline derivatives, and these, on heating alone or with copper powder, decompose to produce cyclopropane derivatives.

When the two pinenes were subjected to this treatment, and the resulting compounds oxidised, a-pinene gave 1-methyl cyclopropane-l : 2: 3-tricarboxylic acid, and (J-pinene cyclopropane-1 : 2: 3- tricarboxylic acid. These products are in accord with the structures assigned to a- and d-pinene.

Examination of the a-pinene structure shows that two dissimilar asymmetric carbon atoms are present; thus two pairs of enantiomorphs are possible. In practice, however, only one pair is known. This is due to the fact that the four-membered ring can only be fused to the six-membered one in the cis-position; trans fusion is impossible. Thus only the enantio morphs of the cis-isomer are known.

Isomeric with a-pinene are b- and d-pinene; the former occurs naturally, the latter is synthetic (see Ruzicka’s synthesis). Crowley (1962) has obtained a small amount of {J-pinene by irradiating a one per cent ethereal solution of myrcene (§4) with ultraviolet light. This is of some interest in connection with the biosynthesis of terpenes.

Camphane and its DerivativesCamphane, C10H18, is a synthetic compound, and may be prepared from camphor, e.g.,

(i) By reduction of camphor to a mixture of borneols (§23b), these then converted to the bornyl iodides which are finally reduced to camphane (Aschan, 1900).

NOTES



(ii) Camphor may also be converted into camphane by means of the Wolff-Kishner reduction (see also Vol. 1).

Camphane is a solid, m.p. 156°; it is optically inactive.

CamphorThis occurs in nature in the camphor tree of Formosa and Japan. It is a solid, m.p. 179°, and is optically active; the (+)-and (–)-forms occur naturally, and so does racemic camphor, which is the usual form of synthetic camphor (from a-pinene; see later).

A tremendous amount of work was done before the structure of camphor was successfully elucidated; in the following account only a small part of the work is described, but it is sufficient to justify the structure assigned to camphor.

The molecular formula of camphor is C10H16O, and the general reactions and molecular refractivity of camphor show that it is saturated. The functional nature of the oxygen atom was shown to be oxo by the fact that camphor formed an oxime, etc., and that it was a keto group was deduced from the fact that oxidation of camphor gives a dicarboxylic acid containing 10 carbon atoms; a monocarboxylic acid containing 10 carbon atoms cannot be obtained (this type of acid would be expected if camphor contained an aldehyde group). From the foregoing facts it can be seen that the parent hydrocarbon of camphor has the molecular formula C10H18; this corresponds to CnH2n_2, and so camphor is therefore bicyclic. Camphor contains a -CH2·CO-group, since it forms an oxime with nitrous acid (isoamyl nitrite and hydrogen chloride). Finally, distillation of camphor with zinc chloride or phosphorus pentoxide produces p-cymene.

Bredt (1893) was the first to assign the correct formula to camphor (over 30 have been proposed). Bredt based his formula on the above facts and also on the facts that (a) oxidation of camphor with nitric acid gives cam phoric acid, C10H16O4 (Malaguti, 1837); (b) oxidation of camphoric acid (or camphor) with nitric acid gives camphoronic acid, C 9H14O6 (Bredt, 1893).

Since camphoric acid contains the same number of carbon atoms as camphor, the keto group must be in one of the rings in camphor. Camphoric acid is a dicarboxylic acid, and its molecular refractivity showed that it is saturated. Thus, in the formation of camphoric acid from camphor, the ring containing the keto group is opened, and consequently camphoric acid must be a monocyclic compound.

NOTES



Camphoronic acid was shown to be a saturated tricarboxylic acid, and on distillation at atmospheric pressure, it gave isobutyric acid, II, trimethyl succinic acid, III, carbon dioxide and carbon (and a small amount of some other products). Bredt (1893) therefore suggested that camphoronic acid is a : a : b-trimethyltricarballylic acid, I, since this structure would give the required decomposition products. In the following equations, the left-hand side molecule is imagined to break up as shown; one molecule of carbon dioxide and two molecules of isobutyric acid are produced (but there is a shortage of two hydrogen atoms). The right-hand-side molecule breaks up to form one molecule of trimethylsuccinic acid, one molecule of carbon dioxide, one atom of carbon and two atoms of hydrogen which now make up the shortage of the left-hand-side molecule. Thus:

Hence, if camphoronic acid has structure I, then camphoric acid (and cam phor) must contain three methyl groups. On this basis, the formula of camphoric acid, C10H16O4 , can be written as (CH3)3C6H6 (CO2H)s· The parent (saturated) hydrocarbon of this is C5H10, which corresponds to CnH2n, i.e., camphoric acid is a cyclopentane derivative (this agrees with the previous evidence that camphoric acid is monocyclic). Thus the oxidation of camphoric acid to camphoronic acid may be written:

This skeleton, plus one carbon atom, arranged with two carboxyl groups, will therefore be the structure of camphoric acid. Now camphoric an hydride forms only one monobromo derivative (bromine and phosphorus); therefore there is only one IX-hydrogen atom in camphoric acid. Thus the carbon atom of one carboxyl group must be 1C (this is the only carbon atom joined to a tertiary carbon atom). Furthermore, 1C must be the carbon of the keto or methylene group in camphor, since it is these two groups which produce the two carboxyl groups in camphoric acid. The problem is now to find the position of the other

NOTES



carboxyl group in camphoric acid. Its position must be such that when the cyclopentane ring is opened to give camphoronic acid, one carbon atom is readily lost.

Using this as a working hypothesis, then there are only two reasonable structures for camphoric acid, IV and V. IV may be rewritten as IVa, and since the two carboxyl groups are produced from the –CH2•CO-group in camphor, the precursor of IVa (i.e., camphor) will contain a six-membered ring with a gem-dimethyl group. This Structure cannot account for the conversion of camphor into p-cymene. On the other hand, V accounts for all the facts given in the foregoing discussion. Bredt therefore assumed that V was the structure of camphoric acid, and that VI was the structure of camphor, and proposed the following reactions to show the relationships between camphor, cam phoric acid and camphoronic acid.

Bredt, however, realised that if camphor had structure VII, then all the foregoing facts would be equally satisfied, but he rejected VII in favour of VI for a number of reasons. One simple fact that may be used here for rejection of VII is that camphor gives carvacrol, VIII, when distilled with iodine. The formation of this compound can be expected from VI but not from VII.

NOTES



Formula VI for camphor was accepted with reserve at the time when Bredt proposed it (in 1893), but by 1903 all the deductions of Bredt were confirmed by the syntheses of camphoronic acid, camphoric acid and camphor.

Synthesis of (±)-camphoronic acid (Perkin, junior, and Thorpe, 1897).

Synthesis of (±)-camphoric acid (Komppa, 1903). Komppa {1899) first synthesised {3: {3-dimethylglutaric ester as follows, starting with mesityl oxide and ethyl malonate.

The product obtained was 6 : 6-dimethylcyclo hexane-2: 4-dione-1-carboxylic ester (this is produced first by a Michael condensation, followed by a Dieckmann reaction). On hydrolysis, followed by oxidation with sodium hypobromite, b : b-dimethylglutaric acid was obtained (cf. carone, §21).

Komppa {1903} then prepared camphoric acid as follows:

The structure given for camphoric acid can exist in two geometrical isomeric forms, cis and trans, neither of which has any elements of symmetry. Thus four optically active

NOTES



forms are possible; all are known, and correspond to the (+)– and (–)– forms of camphoric acid and isocamphoric acid. Since camphoric acid forms an anhydride, and isocamphoric acid does not, the former is the cis-isomer, and the latter.

Synthesis of camphor (Haller, 1896). Haller started with camphoric acid prepared by the oxidation of camphor, but since the acid was syn thesised later by Komppa, we now have a total synthesis of camphor.

This is not an unambiguous synthesis, since the campholide obtained might have had the structure IX (this is actually p-campholide).

In this case, homocamphoric acid would have had structure X, and this would have given camphor with structure VII which, as we have seen, was rejected. Sauers (1959) has now oxidised camphor directly to a-cam pholide by means of peracetic acid. It is also of interest to note that avos et al. (1960) have shown, using labelled -CH2·C*O2H (14C), that in the pyrolysis of the calcium salt of homocamphoric acid to camphor, it is the labelled carboxyl group that is lost.

Stereochemistry of camphor. Camphor has two dissimilar asymmetric carbon atoms {the same two as in camphoric acid), but only one pair of enantiomorphs is known. This is due to the fact that only the cis-form is possible; trans fusion of the gem-dimethylmethylene

NOTES



bridge to the cyclo hexane ring is impossible. Thus only the enantiomorphs of the cis-isomer are known (cf. a-pinene, §22a).

Camphor and its derivatives exist in the boat conformation. Since the gem-dimethyl bridge must be cis, the cyclohexane ring must have the boat form (for the usual way of drawing these conformations; the viewing point is different):

Some derivatives of camphor. The positions of substituent groups in camphor are indicated by numbers or by the Greek letters a (= 3), b or w (= 10) and :p {= 8 or 9). When (+)–camphor is heated with bromine at 100°, a-bromo-( +)–camphor is produced. This, on warming with sulphuric acid, is converted into a-bromo-( + )–camphor-p-sulphonic acid which, on reduction, forms (+)–camphor-p-sulphonic acid. {±}–Camphor-p-sulphonic acid is obtained by the sulphonation of (+)–camphor with fuming sulphuric acid; under these conditions, (+)–camphor is racemised.

On the other hand, sulphonation of (+)–camphor with sulphuric acid in acetic anhydride solution produces (+)–camphor-{J-sulphonic acid. These various ( + )–camphorsulphonic acids are very valuable reagents for resolving racemic bases.

Commercial preparation of camphor. Synthetic camphor is usually obtained as the racemic modification. The starting material is a-pinene, and the formation of camphor involves the Wagner-Meerwein rearrange ments. Scheme (i) is the earlier method, and (ii) is the one that is mainly used now.

Borncola, C10H18OThere are two stercoisomeric compounds of the formula C10H18O; these correspond to borneol and isoborneol, and both are known in the ( and (–)-forms. The borneols occur

NOTES



widely distributed in essential oils, but it appears that the isoborneols have been isolated from only one essential oil.

Borneol and isoborneol are secondary alcohols, and the evidence now appears to be conclusive that borneol has the endo-configuration in which the gem-dimethyl bridge is above the plane of the cyclohexane ring and the hydroxyl group is below the plane. iso Borneol has the exo-configuration in which the bridge and the hydroxyl group are both above the plane of the cyclohexane ring.

Kwart et al. (1956) have now obtained direct evidence on the configuration of bornyl chloride. Bornyl dichloride (1), the structure of which has been established by Kwart (1953), is converted into bornyl chloride (II) by sodium amalgam and ethanol, and into camphane (III) by sodium and ethanol.

Both borneol and isoborneol are produced when camphor is reduced, but the relative amounts of each are influenced by the nature of the reducing agent used, e.g., electrolytic reduction gives mainly borneol, whereas catalytic hydrogenation (platinum) gives mainly isoborneol; isoborneol is also the main product when aluminium isopropoxide is used as the reducing agent. Borneol is converted into a mixture of bornyl and isobornyl chlorides by the action of phosphorus pentachloride. Borneol and isoborneol are both dehydrated to camphene (§23c), but the dehydration occurs more readily with isoborneols than with borneol. Both alcohols are oxidised to camphor, but whereas borneol can be dehydrogenated to camphor by means of a copper catalyst, isoborneol cannot.

Camphene and BornyleneCamphene, C10H16, m.p. 51-52°, occurs naturally in the (+)–, (–)–and (±}–forms. It may be prepared by the removal of a molecule of hydrogen chloride from bornyl and isobornyl chlorides by means of sodium acetate, or by the dehydration of the borneols with potassium hydrogen sulphate. These methods of preparation suggest that camphene contains a double bond, and this is supported by the fact that camphene adds on one molecule of bromine or one molecule of hydrogen chloride. Oxidation of camphene with dilute nitric acid

NOTES



produces carboxy apocamphoric acid, C10H14O6, and apocamphoric acid, C9H14O4 (Marsh et at., 1891). The formation of the former acid, which contains the same number of carbon atoms as camphene, implies that the double bond in camphene is in a ring; and the fact that carboxyapocamphoric acid is converted into apocamphoric acid when heated above its melting point implies that the former contains two carboxyl groups attached to the same carbon atom (cf. malonic ester syntheses).

These facts were explained by giving cam phene the formula shown (I). The structure of apocamphoric acid was later proved by synthesis (Komppa, 1901; cf. camphoric acid, §23a).

This structure for camphene, however, was opposed by Wagner. The oxidation of camphene with dilute permanganate gives camphene glycol, C10H16(OH}2 [Wagner, 1890]. This glycol is saturated, and so camphene is a bicyclic compound (so, of course, is structure I). On further oxidation of camphene glycol, Wagner (1896, 1897) obtained camphenic acid, C10H16O4 (a dibasic acid), and camphenylic acid, C10H16O3 (a hydroxy-monobasic acid), which, on oxidation with lead dioxide, gave camphenilone, C9H14O (a ketone}. According to Wagner, it was difficult to explain the formation of these compounds if camphene had structure I. Wagner (1899) therefore suggested that camphene is formed by a molecular rearrangement when the borneols or bornyl chlorides are converted into camphene, and proposed structure II for camphene.

With this formula, the formation of camphene glycol, camphenylic acid and camphenilone could be explained as follows:

Although it was easy to explain the formation of III, IV and V, it was difficult to explain the formation of VII. The formation of VII was explained by later workers, who suggested it was produced via carbocamphenil one, VI. Another difficulty of the camphene formula, II, is that it does not explain the formation of apocamphoric acid when camphene is oxidised with nitric acid. The course of its formation has been suggested by Komppa (1908, 19ll), who proposed a mechanism involving a Wagner rearrangement.

NOTES



Structure II for camphene is supported by the fact that treatment of bornyl iodide with ethanolic potassium hydroxide at 170° gives bornylene, C10H18 (m.p. 98°), as well as camphene (Wagner et al., 1899). Bornylene is readily oxidised by permanganate to camphoric acid; it therefore follows that bornylene has the structure I, the structure originally assigned to camphene; no rearrangement occurs in the formation of bornylene.

Ozonolysis of camphene gives camphenilone and formaldehyde (Harries et al., 1910); these products are in keeping with the Wagner formula for camphene.

Further support for this structure for camphene is afforded by the work of Buchner et al. (1913). These workers showed that camphene reacts with diazoacetic ester, and when

the product is hydrolysed and then oxidised, cyclopropane-! : I : 2-tricarboxylic acid, VIII, is produced. VIII is to be expected from structure II, but not from I; I (bornylene) would give cyclo propane-1: 2: 3-tricarboxylic acid, IX.

NOTES



Lipp (1914) has synthesised camphenic acid (VII), and showed that it has the structure assigned to it by Wagner. Finally, camphene has been synthesised as follows (Diels and Alder, 1928-1931).

Wagner-Meerwein RearrangementsWagner, as we have seen, proposed a molecular rearrangement to explain the formation of camphene from the borneols and bornyl chlorides. Wagner also recognised that a molecular rearrangement occurred when a-pinene was converted into bornyl chloride. Many other investigations concerning rearrangements in the terpene field were carried out by Meerwein and his co-workers, e.g., when a-pinene is treated in ethereal solution at –20° with hydrogen chloride, the product is pinene liydrochloride. This is unstable, and if the temperature is allowed to rise to about 10°, the pinene hydrochloride rearranges to bornyl chloride (Meerwein et al., 1922). Rearrangements such as these which occur with bicyclic monoterpenes are known as Wagner-Meerwein rearrangements. Furthermore, Meerwein extended the range of these re arrangements to compounds outside bicyclic terpenes; these compounds were monocyclic. Finally, the range was extended to acyclic compounds, the classical example being that of neopentyl into t-pentyl compounds.

All of these rearrangements conform to a common pattern, ionisation to a carbonium ion followed by rearrangement. Most rearrangements in the terpene field involve a change in ring structure, and in a few cases the migration of a methyl group. All of these rearrangements are examples of the 1,2-shifts.

NOTES



The following are examples, and the details of the mechanisms are dis cussed later.

(i) The conversion of a-pinene hydrochloride into bornyl chloride.

(ii) The conversion of camphene hydrochloride into isobornyl chloride.

(i) and (ii) are of particular interest since both appear to proceed through the same carbonium ion. Why the epimers should be obtained is not certain (but see later).

(iii) The dehydration of borneol to camphene (with acids).

(iv) The racemisation of camphene hydrochloride.

(v) Rearrangements in the neopentyl system; e.g., the action of hydrobromic acid on neopentyl alcohol to give t-pentyl bromide.

Evidence for the intermediate formation of a carbonium ion in the Wagner-Meerwein rearrangement. Meerwein et al. {1922), in their detailed investigation of the

NOTES



reversible conversion of camphene hydrochloride into iso bornyl chloride (example ii), concluded that the first step was ionisation, and this was then followed by rearrangement of the carbonium ion:

Their evidence for this mechanism was that the rate of the rearrangement was first order, and that the rate depended on the nature of the solvent, the rate being faster the greater the ionising power of the solvent. The order observed for some solvents was:

5O2 > MeNO2 > MeCN > PhOMe > PhBr > PhH > Et2O

This dependence of rate on solvent was more clearly shown by also studying the solvolysis rates of triphenylmethyl chloride in the same solvents. It was found that the rate of the rearrangement of camphene hydrochloride was faster in those solvents in which triphenylmethyl chloride undergoes solvolysis more readily. Meerwein also found that the rearrangement was strongly catalysed by Lewis acids such as stannic chloride, ferric chloride, etc. All of these form complexes with triphenylmethyl chloride. Furthermore, halides such as phos phorus trichloride and silicon tetrachloride, which do not form complexes with triphenylmethyl chloride, did not catalyse the rearrangement. Further evidence by Meerwein et at. (1927) and by Ingold (1928) also supports the mechanism given above.

Meerwein, however, recognised a difficulty in his proposed mechanism. The carbonium ion formed in the rearrangement of camphene hydrochloride would presumably be the same as that formed in the rearrangement of pinene hydro chloride to bornyl chloride (example (i) The reason why the epimers are obtained is not certain; one possibility is that the ions are not the same, and as we shall see later, the ions are not identical if we assume there is neighbouring group participation producing a non-classical carbonium ion.

Bartlett et at. (1937, 1938) showed that the rearrangement of camphene hydro chloride in non-hydroxylic solvents is strongly catalysed by hydrogen chloride, and pointed out that the formation of isobornyl chloride requires a Walden inversion at the new asymmetric carbon atom. According to these authors, the function of the hydrochloric acid is to help the ionisation of the chloride ion (from the camphene hydrochloride). Evidence for this is that phenols have a catalytic effect on the rearrangement rate of camphene hydrochloride, and that the order of this catalytic activity of substituted phenols is the same as the order of the increase in acid strength of hydrogen chloride which phenols promote in dioxan as solvent. These catalytic effects were explained by Bartlett et al. (1941) as being due to hydrogen bonding between the phenolic hydroxyl group and the receding chloride ion.

Nevell et al. (1939) suggested that the type of resonance hybrid Z is involved in the rearrangement. Thus the hydrogen chloride-catalysed reaction in the inert solvents used

NOTES



would produce an ion-pair [Z+][HC12–] (§2e. III). z+ can now react with HC12

– at position 1 to regenerate camphene hydrochloride, or at position 2 to give isobornyl chloride. This interpretation is supported by experimental work.

(i) Nevell et al. found that the rate of radioactive chlorine (36Cl) exchange between HCl* and camphene hydrochloride is 15 times faster than the rate of rearrangement to isobornyl chloride. It therefore follows that the rate-deter mining step of the rearrangement is not the ionisation step, but is the reaction of the bridged-ion with HC12

– at position 2. It also follows, from the principle of microscopic reversibility (Vol. 1), that the rate-determining step of the re arrangement of isobornyl chloride back to camphene hydrochloride is the reaction with hydrogen chloride to produce the ion-pair directly.

(ii) On the basis of the bridged-ion being an intermediate in the rearrangement in inert solvents and also for solvolytic reactions of both camphene hydrochloride and isobornyl chloride, then both isomers should give the same products Meer wein et at. (1922) found that methanolysis, in the cold, of camphene hydrochloride gave at first the t-methyl ether (attack at position 1) and this, on long standing, gave isobornyl methyl ether isoBornyl chloride also gave isobornyl methyl ether, but in this case the reaction was slower. These results can be explained by the presence of the liberated hydrogen chloride which would make the methan olysis reversible.

(iii) Neighbouring group participation in solvolytic reactions of camphene hydrochloride would be expected to accelerate these reactions (anchimeric assistance) as compared with the formation of a classical carbonium ion inter mediate. This will be so because the formation of the bridge will assist the expulsion of the chloride ion. Hughes, Ingold et al. (1951) have found that the ethanolysis of camphene hydrochloride is 6000 times faster (at 0°) than the cor responding reaction with t-butyl chloride. Also, from the reaction rates of the solvolysis of 1-chloro-1-methylcyclopentane, it followed that camphene hydro chloride is 370 times more reactive than this cyclopentyl derivative. Purely on the basis of ring strain, the camphene compound should have been less reactive. Thus the high reactivity of the camphene compound is very strong evidence for neighbouring group participation.

The relative rates of solvolysis of cyclopentyl chloride, bornyl chloride, and isobornyl chloride (in 80 per cent ethanol at 85°) are respectively 9·4, 1·0 and 36,000 (Roberts et al., 1949; Winstein et al., 1952). This very large difference between the behaviour of bornyl and isobornyl chlorides is readily explained by neighbouring group participation. In isobornyl chloride the methylene group that forms the bridged ion is trans to the chloride ion ejected

NOTES



and so can readily attackthe C+ (of the C-Cl) at the rear, thereby assisting ionisation; this neigh bouring group participation cannot occur with bornyl chloride. Various representations of this bridged-ion are possible; I has been proposed by Winstein et al. (1952).

Very strong evidence for the participation of a neighbouring saturated hydro carbon radical has been obtained by Winstein et al. (1952) in their detailed examination of some reactions of the parent norbornyl systems.

These authors showed that the relative rates of acetolysis of the brosylates (p-bromobenzenesulphonates) of exofendo norbornyl alcohols in acetic acid at 25° are 350/l. The explanation offered for the large relative rate of the exo isomer acetolysis was neighbouring group participation to form the non-classical carbonium ion (Ia). As the OBs- ion is leaving from the front, the neighbouring group (group C8) can attack from the rear to form the bridged-ion.

This sequence is not possible as such for the endo-compound, and so the latter reacts far more slowly. Further support for the formation of (Ia) is as follows. This ion has a plane of symmetry (see Ib) and hence is optically inactive. It has been shown that solvolysis of exo-norbornyl brosylate in aqueous acetone, ethanol or acetic acid gives only exo-products, but in these products the carbon atoms have become “shuffied” (see below). Winstein et al. (1952) also showed that acetolysis of optically active exo•norbornyl brosylate gave racemic exo norbornyl acetate. Attack must be from the back of the CH2 bridge and so this results in the exo-product; also, since positions 1 and 2 are equivalent, equal amounts of the enantiomorphs (i.e., racemate) will be produced.

When endo-norbornyl brosylate undergoes acetolysis, ionisation of the CBs group leaves the endo-norbornyl carbonium ion. This is probably originally the lassical carbonium ion, but it then rearranges to the more stable exo-bridged Ion.

NOTES



The formation of the latter is shown by the fact that acetolysis of the optically active endo-brosylate produces racemic exo-acetate.

The structure of the bridged carbonium ion, however, appears to be more complicated than that shown by formula (Ia). Examination of (Ib) shows the equivalence of positions 1 and 2, and of positions 3 and 7. Thus labelling the brosylate with 14C at positions 2 and 3 should give products equally labelled at positions 1, 2, 3 and 7. Roberts et al. (1954) carried out the acetolysis of this labelled exo-brosylate, and the tracer atom was found at 1, 2, 3 and 7, but posi tions 5 and 6 also contained labelled carbon (15 per cent: of the total radio activity). These results can be explained on the basis that there is also a 1,3- hydride shift from position 2 to position 6. Thus positions 1, 2 and 6 become shuffled to a certain extent, and there is also the same amount of interchange among positions 3, 5 and 7.

This raises the question as to whether some ions have both carbon and hydrogen bridging. Winstein (1955) has pointed out that the “extra” carbon shuffling (to positions 5 and 6) depends on the nucleophilic activity of the solvent, and is zero for very reactive solvents in which the life of the carbonium ion is short. This suggests that the hydrogen shift competes with the solvent attack and so occurs after the formation of the purely carbon bridged:-ion.

Correlation of Configurations of TerpenesThis has been made possible by the work of Fredga on quasi-racemic compounds. This author has established the following configurations:

By means of these configurations, combined with various interrelations obtained by oxidative degradations and by molecular rearrangements, it has been possible to correlate the configurations of many mono- and bicyclic terpenes with L-glyceraldehyde, e.g.,

NOTES



Fenchane and its DerivativesThe most important natural terpene of this group is fenchone; this occurs in oil of fennel. It is a liquid, b.p. 192--193°, and is optically active, both enantiomorphs occurring naturally.

The molecular formula of fenchone is C10H16O, and the compound behaves as a ketone. When fenchone (I) is reduced with sodium and ethanol, fenchyl alcohol, C10H18O (II), is produced, and this, on dehydration under the influence of acids, gives a-fenchene, C10H18 (III). On ozonolysis, a fenchene is converted into a-fenchocamphorone, C9H14O (IV), which, on oxidation with nitric acid, forms apocamphoric acid, V, a compound of known structure. This work was carried out by Wallach et al. (1890-1898), but it was Semmler (1905) who was the first to assign the correct structure to fenchone; the foregoing reactions may be formulated:

NOTES



It should be noted that the dehydration of fenchyl alcohol, II, to a-fenchene, III, occurs via a Wagner-Meerwein rearrangement; the mechanism for this reaction may thus be written (cf. §23d):

The structure of fenchone has been confirmed by synthesis (Ruzicka, 1917).

1.8 SESQUITERPENESThe sesquiterpenes, in general, form the higher boiling fraction of the essential oils; this provides their chief source. Wallach (1887) was the first to suggest that the sesquiterpene structure is built up of three isoprene units; this has been shown to be the case for the majority of the known sesquiterpenes, but there are some exceptions.

NOTES



The sesquiterpenes are classified into four groups according to the number of rings present in the structure. If we use the isoprene rule, then when three isoprene units are linked (head to tail) to form an acyclic sesquiterpene hydrocarbon, the latter will contain jour double bonds. Each isoprene unit contains two double bonds, but one disappears for each pair that is con nected:

When this open-chain compound is converted into a monocyclic structure, another double bond is utilised in the process, and so monocyclic sesqui terpene hydrocarbons contain three double bonds. In a similar manner, it will be found that a bicyclic structure contains two double bonds, and a tricyclic one. Thus the nature of the sesquiterpene skeleton is also characterised by the number of double bonds present in the molecule. The sesqui terpene hydrocarbon structures may also be distinguished by the calculation of the molecular refractivities for the various types of structures, and then using these values to help elucidate the structures of new sesquiterpenes; e.g., zingiberene (§27a).

Class of sesquiterpene Number of double bonds Molecular refractivityAcyclic 4 69.5Monocyclic 3 67.8Bicyclic 2 66.1Tricyclic 4 64.4

This type of information can also be used with the monoterpenes, but in this case it has not been so useful as in the sesquiterpenes. It might be noted here that the non-acyclic members of the sesquiterpenoid group may have rings of various sizes: 4, 5, 6, 7, 9, 10 and 11; and in many of these the rings are fused.

1.9 ACYCLIC SESQUITERPENESFarnesene, C15H24, b.p. 128-130°/12 mm., is obtained by the de hydration of farnesol with potassium hydrogen sulphate (Harries et al., cx-farnesene {1-farnesene 1913). This compound is the a-isomer, and it has now been shown that the P-isomer occurs naturally (in oil of hops}, and Sorm et al. (1949, 1950) have assigned it the structure shown. P-Famesene is also obtained by the dehydration of nerolidol.

NOTES



Farnesol, C15H26O, b.p. 120°/0·3 mm., occurs in the oil of ambrette seeds, etc. Its structure was elucidated by Kerschbaum (1913) as follows. When oxidised with chromic acid, famesol is converted into famesal, C15H24O, a compound which behaves as an aldehyde. Thus famesol is a primary alcohol. Conversion of famesal into its oxime, followed by de hydration with acetic anhydride, produces a cyanide which, on hydrolysis with alkali, forms famesenic acid, C15H24O2, and a ketone, C13H22O. This ketone was then found to be dihydro-pseudo-ionane (geranylac€tone). In the formation of this ketone, two carbon atoms are removed from its precursor. This reaction is characteristic of a: b-unsaturated carbonyl compounds, and so it is inferred that the precursor, famesenic acid (or its nitrile), is an a: b-unsaturated compound. Thus the foregoing facts may be formulated as follows, on the basis of the known structure of geranyl acetone.

Kerschbaum’s formula has been confirmed by Harries et al. (1913}, who obtained acetone, lrevulaldehyde and glycolaldehyde on the ozonolysis of farnesol.

Ozonolysis, however, also gave some formaldehyde, thus indicating the presence of the isopropenyl end-group as well as the isopropylidene end group (but cf. citral, §5). Ruzicka (1923) synthesised famesol (with the isopropylidene end-group) by the action of acetic anhydride on synthetic nerolidol (cf. linalool, §8).

NOTES



Nerolidol, C15H26O, b.p. 125-127°/4·5 mm., occurs in the oil of neroli, etc., in the (+)-form. Nerolidol is isomeric with famesol, and Ruzicka (1923) showed that the relationship between the two is the same as that between linalool and geraniol. Ruzicka (1923) confirmed the structure of nerolidol by synthesis.

1.10 MONOCYCLIC SESQUITERPENESBisabolene, C15H24, b.p. 133-134°/12 mm., occurs in the oil of myrrh and in other essential oils. The structure of bisabolene was deter mined by Ruzicka et al. (1925). Bisabolene adds on three molecules of hydrogen chloride to form bisabolene trihydrochloride, and this regenerates bisabolene when heated with sodium acetate in acetic acid solution. Thus bisabolene contains three double bonds and is therefore monocyclic. Nerolidol may be dehydrated to a mixture of a- and b-famesenes (cf. §26). This mixture, on treatment with formic acid, forms a monocyclic sesqui terpene (or possibly a mixture) which combines with hydrogen chloride to form bisabolene trihydrochloride. Removal of these three molecules of hydrogen chloride (by means of sodium acetate in acetic acid) produces bisabolene; thus bisabolene could be I, II or III, since all three would give the same bisabolene trihydrochloride.

NOTES



Ruzicka et al. (1929) showed that synthetic and natural bisabolene con sisted mainly of the y-isomer (III), since on ozonolysis of bisabolene, the products were acetone, laevulic acid and a small amount of succinic acid. These products are readily accounted for by III; and this structure has been confirmed by synthesis (Ruzicka et al., 1932).

Zingiberene, C15H24, b.p. 134°/14 mm., occurs in the (–)-form in ginger oil. It forms a dihydrochloride with hydrogen chloride, and thus apparently contains two double bonds. The molecular refractivity, how ever, indicates the presence of three double bonds and, if this be the case, zingiberene is monocyclic. The presence of these three double bonds is conclusively shown by the fact that catalytic hydrogenation (platinum) converts zingiberene into hexahydrozingiberene, C15H3O. Zingiberene can be reduced by means of sodium and ethanol to dihydrozingiberene, C15H26; this indicates that two of the double bonds are probably conjugated (Semmler et al., 1913). Further evidence for this conjugation is afforded by the fact that zingiberene shows optical exaltation; whereas dihydro zingiberene does not. The absorption spectrum of zingiberene also shows the presence of conjugated double bonds (Gillam et al., 1940).

Ozonolysis of zingiberene gives acetone, laevulic acid and succinic acid (Ruzicka et al., 1929). Since these products are also obtained from bis abolene (§27), it appears probable that zingiberene and bisabolene have the same carbon skeleton. Oxidation of dihydrozingiberene, I, with perman ganate gives a keto-dicarboxylic acid, C12H20O5 (II), which, on oxidation with sodium hypobromite, forms a tricarboxylic acid, C11H18O6 (III). Thus II must contain a methyl ketone group (CH3.CO–), and so, if I be assumed as the structure of dihydrozingiberene, the foregoing oxidation reactions may be formulated:

NOTES



Thus I, with another double bond in conjugation with one already present, will be (probably) the structure of zingiberene. The position of this third double bond was shown as follows (Eschenmoser et al., 1950). Zingiberene forms an adduct with methyl acetylenedicarboxylate, and this adduct (which was not isolated), on pyrolysis, gives 2: 6-d.imethylocta-2: 7-diene and methyl4-methylphthalate. These reactions can be explained on the assumption that zingiberene has the structure shown below.

Humulene (a-caryophyllene), C16Hu, b.p. 264°, is an eleven membered ring compound which contains three double bonds. Its structure is very closely related to that of caryophyllene (§28c).

Pyrethrosin is also a monocyclic sesquiterpene; it is a y-lactone which con tains a ten-membered ring.

1.11 BICYCLIC SESQUITERPENESCadinene, C15H24, b.p. 134-136°/11 mm., occurs in the (–)-form I in oil of cubebs, etc. Catalytic hydrogenation converts cadinene into tetra hydrocadinene, C15H28. Thus cadinene contains two double bonds and is .I bicyclic. On dehydrogenation with sulphur, cadinene forms cadalene, C15H18 (Ruzicka et al., 1921). Cadalene does not add on bromine, and forms a picrate. This led to the belief that cadalene was an aromatic compound, and its structure was deduced as follows. Ruzicka assumed that the relationship of famesol (§26a) to cadinene

NOTES



was analogous to that of geraniol to dipentene. Furthermore, since dipentene gives p-cymene when dehydrogenated with sulphur, then cadalene should be, if the analogy is correct, 1 : 6-dimethyl-4-isopropylnaphthalene; thus:

1 : 6-Dimethyl-4-isopropylnaphthalene was synthesised by Ruzicka et al. (1922), and was found to be identical with cadalene.

Thus cadinene has the carbon skeleton assumed. The only remammg problem is to ascertain the positions of the two double bonds in cadinene. Since the molecular refractivity

NOTES



shows no optical exaltation, the two double bonds are not conjugated (§ll. I); this is supported by the fact that cadinene is not reduced by sodium and amyl alcohol. Ozonolysis of cadinene pro duces a compound containing the same number of carbon atoms as cadinene. The two double bonds are therefore in ring systems, but they cannot be in the same ring, since in this case carbon would have been lost on ozonolysis. Ruzicka et al. (1924) were thus led to suggest I (a or b) for the structure of cadinene, basing it on the relationship of cadinene to copaene, which had been given structure II by Semmler (1914). I was proposed mainly on the

fact that copaene adds two molecules of hydrogen chloride to form copaene dihydrochloride, which is identical with cadinene dihydrochloride (both the a and b structures of I would give the same dihydrochloride as II). Struc ture I (a or b) was accepted for cadinene until 1942, when Campbell and Soffer re-investigated the problem. These authors converted cadinene into its monoxide and dioxide by means of perbenzoic acid, treated these oxides with excess of methylmagnesium chloride, and then dehydrogenated the product with selenium. By this means, Campbell and Soffer obtained a monomethylcadalene from cadinene monoxide, and a dimethylcadalene from cadinene dioxide. Now the introduction of a methyl group via the oxide takes place according to the following scheme :

Thus the positions of the additional methyl groups show the positions of the double bonds in cadinene. The Ruzicka formula for cadinene would give dimethylcadalene III (from the a isomer) or IV (from the b), and the monomethylcadalenes would be V (from a or b), VI (from a) and VII (from b). Campbell and Soffer oxidised their dimethylcadalene, first with chromic acid and then with nitric acid, and thereby obtained pyromellitic acid (benzene-1: 2:4: 5-tetracarboxylic acid), VIII. The formation of VIII therefore rules out III as the structure of dimethylcadalene, but IV, with the two methyl groups at positions 6 and 7 in ring B, could give VIII. Therefore the double bond in cadinene in ring B is 6 : 7. From this it follows that VI is also eliminated. If the double bond in ring A is as in structure I, then dimethylcadalene is IV, and monomethylcadalene is V or VII. Campbell and Soffer synthesised IV and VII, and found that each was different from the methylcadalenes

NOTES



they had obtained from cadinene. Thus IV and VII are incorrect; consequently the double bond in ring A cannot be 3 : 4. The only other dimethylcadalene which could give VIII on oxidation is IX. This was synthesised, and was found to be identical with the dimethylcadalene from cadinene. Cadinene must therefore be X, and the introduction of one or two methyl groups may thus be formulated as follows:

X could give two monoxides (oxidation of ring A or B), and one of these (ring B oxidised) would give VII. This, as pointed out above, was different from the monomethylcadalene actually obtained. Therefore, if X is the structure of cadinene, the monomethylcadalene obtained from cadinene must be XI. XI was synthesised, and was found to be identical with the compound obtained from cadinene. Thus X is the structure of cadinene.

It should be noted, in passing, that this new structure for cadinene has necessitated revision of the structure of copaene. Briggs and Taylor (1947), using a technique similar to that of Campbell and Soffer, have assigned the following structure to copaene.

NOTES



The absolute configurations of the cadinenes (and cadinols) have now been established (Motl et at., 1958; Soffer et at., 1958).

Selinenes, Cl5H24. Selinene occurs in celery oil; when treated with hydrogen chloride, it forms a dihydrochloride which, when warmed with aniline, is converted into the compound C15H24. This is isomeric with selinene, and the natural compound was called b-selinene, and the synthetic isomer a.-selinene (Semmler et al., 1912). Semmler showed that the catalytic hydrogenation of the two selinenes gives the same tetrahydroselinene, C15H28. Thus they each contain two double bonds, and are bicyclic. Ozonolysis of b-selinene produces a diketone (I) with the loss of two carbon atoms, and oxidation of I with sodium hypobromite gives a tricarboxylic acid (II), with the loss of one carbon atom. From this it follows that I contains a CH3.CO- group. Ozonolysis of a-selinene gives a diketo-monocarboxylic acid (III) with loss of one carbon atom, and III, on oxidation with sodium hypobromite, loses two carbon atoms to form II. Thus III contains two CH3•CO- groups (Semmler et al., 1912). Ruzicka et al. (1922) distilled p-selinene with sulphur, and thereby obtained eudalene.. If we use the isoprene rule, all the foregoing facts are explained by giving the selinenes the following structures (Ruzicka et al., 1922). The relationship of the selinenes to eudesmol (§28b) confirms the nature of the carbon skeleton given to the selinenes.

Eudesmol, C16H2 uO, occurs in eucalyptus oil. Catalytic hydro genation converts eudesmol into dihydroeudesmol, C15H28O. Thus one double bond is present in the molecule, and since eudesmol behaves as a tertiary alcohol, the parent hydrocarbon is C15H2 g= CnH2n– 2; eudesmol is therefore bicyclic. When dehydrogenated with sulphur, eudesmol forms eudalene, C14H16, and methanethiol (Ruzicka et al., 1922). Eudalene be haved as an aromatic compound (cf. cadalene, §28), and its structure was deduced as follows. Since eudalene was a naphthalene derivative, and since it contained one carbon atom less than cadalene, it was thought to be an apocadalene, i.e., cadalene minus one methyl group. Thus eudalene is either 1-methyl-4-isopropylnaphthalene (Ila) or 7-methyl-1-isopropyl naphthalene (Ia). To test this hypothesis, Ruzicka oxidised cadalene with chromic acid, and thereby obtained a naphthoic acid, C15H16O2, which must be I or II. Distillation of this acid with soda-lime gives a methylisopropyl naphthalene which must be Ia or Ila. Ila was synthesised from carvone (the synthesis is the same as for cadalene except that ethyl malonate is used instead of

NOTES



ethyl methylmalonate). The synthetic compound (Ila) was found to be different from the hydrocarbon obtained by the distillation of the naphthoic acid from cadalene. Thus the apocadalene obtained must be Ia, i.e., 7-methyl-1-isopropylnaphthalene.

Ruzicka now found that eudalene was not identical with either Ia or Ila. On oxidation, however, eudalene gives the same naphthalenedicarboxylic acid as that which is obtained by the oxidation of Ia. This is only possible if in eudalene the two side-chains in Ia are interchanged, i.e., eudalene is 1-methyl-7-isopropylnaphthalene; thus:

This structure for eudalene was proved by synthesis (Ruzicka et al., 1922).

NOTES



To develop the sesquiterpene carbon skeleton from that of eudalene, it is necessary to introduce one carbon atom in such a position that it is eliminated as methanethiol during the sulphur dehydrogenation. If we use the isoprene rule with the units joined head to tail, then there is only one possible structure that fits the requirements, viz., III (cf. §1).

Now b-selinene combines with hydrogen chloride to form selinene dihydro chloride, which is also obtained by the action of hydrogen chloride on eudesmol (Ruzicka et al., 1927, 1931). Since eudesmol contains one double bond and a tertiary alcoholic group, it follows that the double bond must be in the side-chain, and the hydroxyl group in the ring, or vice versa, i.e., IV, V or VI is-the structure of eudesmol.

Hydrogenation of eudesmol forms dihydroeudesmol, VII, and this, on treatment with hydrogen chloride followed by boiling with aniline (to remove a molecule of hydrogen chloride), gives dihydroeudesmene, VIII. VIII, on ozonolysis, forms 3-acetyl-5 : 9-dimethyldecalin, IX, with the elimination of one carbon atom. These results are explained if IV or V is the structure of eudesmol, but not by VI. Thus the hydroxyl group is in the isopropyl side-chain.

The final problem is to ascertain the position of the double bond in eudesmol, i.e., Is the structure IV or V? Ozonolysis of eudesmol showed that eudesmol is a mixture of IV {oc-eudesmol) and V (p-eudesmol), since two products are obtained: a hydroxyketo-acid X, with no loss of carbon, and a hydroxy ketone XI, with the loss of one carbon atom (but cf. citral, §5).

NOTES



The proportions of these two isomers vary with the source, and McQuillin et al. {1956) have succeeded in separating them (via their 3: 5-dinitrobenzo ates), and at the same time have characterised a third, synthetic y-isomer.

Caryophyllene, C15H24, b.p. 123-125°/10 mm., is a bicyclic sesqui terpene containing a fused system of a four- and a nine-membered ring. The main source of this compound is the sesquiterpene fraction of oil of cloves, and three isomeric hydrocarbons have been isolated. These were originally called

a, b-, and g-caryophyllene, but it has now been shown that the a.-isomer is identical with humulene (§27b); the b-isomer (the main hydrocarbon) is called caryophyllene; and the g-isomer (which is believed to be produced by thermal isomerisation) is known as isocaryophyllene.

Santonin is a lactone sesquiterpene of the decalin type (cf pyrethrosin, §27b).

Acorone is a most interesting bicyclic sesquiterpene in that it is a carbo cyclic spiran, the first example of such a compound to be found in nature.

NOTES



AzulenesMany essential oils contain blue or violet compounds, or may form such compounds after distillation at atmospheric pressure or dehydrogenation with sulphur, selenium or palladium-charcoal (Ruzicka et al., 1923). These coloured compounds may be extracted by shaking an ethereal solution of the essential oil with phosphoric acid (Sherndal. 1915). These coloured substances are known as azulenes. Their molecular formula is C15H18, and they are sesquiterpenes, the parent substance being azulene, C10Hl8, which contains a seven-membered ring fused to a five-membered one. Azulene has been synthesised as follows (Plattner et al., 1936).

Azulene is a deep blue solid, m.p. 99°; its systematic name is bicyclo[5 : 3 : 0] decane. Two sesquiterpenes containing this bicyclodecane skeleton are

Azulene is a non-benzenoid aromatic compound in which n = 2 (aromatics

contain (4n + 2) n-electrons in a “circular” system; see Vol. I, Ch. XX). It undergoes many typical aromatic substitution reactions.

1.12 DITERPENESPhytol, C20H40O, b.p. 145°/0·03 mm., is an acyclic diterpene; it is produced from the hydrolysis of chlorophyll (§6. XIX), and it also forms part of the molecules of vitamins E and K (see Ch. XVII). The reactions of phytol showed that it is a primary alcohol (Willstatter

NOTES



et al., 1907), and since on catalytic reduction phytol forms dihydrophytol, C20H42O, it there fore follows that phytol contains one double bond. Thus the parent hydro carbon is C20H42 (:=CnH2n+2). and so phytol is acyclic. Ozonolysis of phytol gives glycolaldehyde and a saturated ketone, C18H36O (F. Fischer et al., 1928). Thus this reaction may be written:

The formula of phytol led to the suggestion that it was composed of four reduced isoprene units. If this were so, and assuming that the units are joined head to tail, the structure of the saturated ketone would be:

. This structure was proved to be correct by the synthesis of the ketone from farnesol (F.

Fischer et al., 1928). The catalytic hydrogenation of farnesol, I, produces hexahydrofarnesol, II, which, on treatment with phosphorus tribromide, gives hexahydrofarnesyl bromide, III. III,

on treatment with sodio-acetoacetic ester, followed by ketonic hydrolysis, forms the saturated ketone, IV. This ketone (IV) was then converted into phytol as follows (F. Fischer et al., 1929); it should be noted that the last step involves an allylic rearrangement.

It appears that natural phytol has a very small optical rotation; Karrer et al. (194) have isolated a (+)–form from nettles.

NOTES



Abietic acid, C20H30O2, m.p. 170-174o, is a tricyclic diterpene. The non-steam volatile residue from turpentine is known as rosin (or colo phony), and consists of a mixture of resin acids which are derived from the diterpenes. Abietic acid is one of the most useful of these acids.

A great amount of work was done before the structure of abietic acid was elucidated. For our purpose it is useful to have the structure of abietic acid as a reference, and then describe the evidence that led to this structure. I is the structure of abietic acid; the system of numbering is shown, and also the four isoprene units comprising it. This way of numbering abietic acid follows the phenanthrene numbering. There has been recently, how ever, a tendency to bring the numbering of all diterpenes in line with the steroids ; this is shown in Ia. In the following discussion I has been used (the reader should work out the change-over for himself).

The general reactions of abietic acid showed that it was a monocarboxylic acid. On dehydrogenation with sulphur, abietic acid gives retene (Vester berg, 1903); better yields of retene are obtained by dehydrogenating with selenium (Diels et al., 1927), or with palladised charcoal (Ruzicka et al., 1933). Retene, C18H18, m.p. 99°, was shown by oxidative degradation to be 1-methyl-7-isopropylphenanthrene (Bucher, 1910), and this structure was later confirmed by synthesis, e.g., that of Haworth et al. (1932).

NOTES



Hence we may assume that this carbon skeleton is present in abietic acid. Thus:

Now it is known that in sulphur dehydrogenations, carboxyl groups and angular methyl groups can be eliminated. It is therefore possible that the two carbon atoms lost may have been originally the carb oxyl group (in abietic acid) and an angular methyl group.

Abietic acid is very difficult to esterify, and since this is characteristic of a carboxyl group attached to a tertiary carbon atom, it suggests that abietic acid contains a carboxyl group in this state. This is supported by the fact that abietic acid evolves carbon monoxide when warmed with concentrated sulphuric acid; this reaction is also characteristic of a carboxyl group attached to a tertiary carbon atom.

NOTES



Catalytic hydrogenation of abietic acid gives tetrahydroabietic acid, C20H34O2. Thus abietic acid contains two double bonds; also, since the parent hydrocarbon is C19H34 (regarding the carboxyl group as a substituent group), abietic acid is tricyclic (parent corresponds to CnH2n– 4), which agrees with the evidence already given.

Oxidation of abietic acid with potassium permanganate gives a mixture of products, among which are two tricarboxylic acids, C11H16O6 (II), and C12H18O6 (III) [Ruzicka et al., 1925, 1931]. II, on dehydrogenation with selenium, forms m-xylene, and III forms hemimellitene (1 : 2 : 3-trimethyl benzene) [Ruzicka et al., 1931]. In both cases there is a loss of three carbon atoms, and if we assume that these were the three carboxyl groups, then two methyl groups in II and III must be in the meta-position. Further more, since II and III each contain the methyl group originally present in abietic acid (position 1), acids II and III must contain ring A of abietic acid. This suggests, therefore, that there is an angular methyl group at position 12, since it can be expected to be eliminated from this position in sulphur dehydrogenations of abietic acid (this 12-methyl group is meta to the !-methyl group). Vocke (1932) showed that acid II evolves two molecules of carbon monoxide when warmed with concentrated sulphuric acid; this indicates that II contains two carboxyl groups attached to tertiary carbon atoms. These results can be explained by assuming that one carboxyl group in II is that in abietic acid, and since in both cases this carboxyl group is attached to a tertiary carbon atom, the most likely position of this group is I (in abietic acid). Accepting these assumptions, the oxidation of abietic acid may be formulated as follows, also assuming IV

as the carbon skeleton of abietic acid. Vocke subjected II to oxidative degradation, and obtained a dicarboxylic acid (V) which, on further oxidation, gave ex-methyl glutaric acid (VI). Vocke assumed that II had the structure shown, and formulated the reactions as below, assuming structure V as the best way of explaining the results.

Structure V (assumed by Vocke) has been confirmed by synthesis (Rydon, 1937).

NOTES



The position of the carboxyl group at position 1 in abietic acid (assumed above) has been confirmed by Ruzicka et al. (1922). Methyl abietate, C19H29·CO2CH3, on reduction with sodium and ethanol, forms abietinol, C19H29·CH2OH, which, on treatment with phosphorus pentachloride, loses a molecule of water to form “ methylabietin”, C20H30. This, on distillation with sulphur, forms homoretene, C19H20. Homoretene contains one CH2 group more than retene, and on oxidation with alkaline potassium terri cyanide, gives phenanthrene-1: 7-dicarboxylic acid, the identical product obtained from the oxidation of retene under similar conditions (Ruzicka et al., 1932). These results can only be explained by assuming that homo retene has an ethyl group at position 1 (instead of the methyl group in retene), i.e., homoretene is 1-ethyl-7-isopropylphenanthrene. This has been confirmed by synthesis (Haworth et al., 1932; ethylmagnesium iodide was used instead of methylmagnesium iodide in the synthesis of retene). The formation of an ethyl group in homoretene can be explained by assuming that abietinol undergoes a Wagner-Meerwein rearrangement on dehydration. Thus:

It has already been pointed out that abietic acid has two double bonds. Since abietic acid forms an adduct with maleic anhydride at above 100°, it was assumed that the two double bonds are conjugated (Ruzicka et al., 1932). It was later shown, however, that levopimaric acid also forms the same adduct at room temperature. It thus appears that abietic acid iso merises to levopimaric acid at above 100°, and then forms the adduct. Thus this reaction cannot be accepted as evidence for conjugation in abietic acid. Nevertheless, the conjugation of the double bonds in abietic acid has been shown by means of the ultraviolet spectrum, which has not only shown the conjugation, but also indicates that the two double bonds are not in the same ring (Kraft, 1935; Sandermann, 1941).

Oxidation of abietic acid with potassium permanganate gives, among other products, isobutyric acid (Ruzicka et al., 1925). This suggests that one double bond is in ring C and the 6: 7- or 7: 8-position. If the double bond is in the 6: 7-position, then the other double bond, which is con jugated with it, must also be in the same ring (5 : 13 or 8 : 14); if 7 : 8, then the other double bond could be in the same ring C, but it could also be in ring B. Since, as we have seen, the two double bonds are in different rings, their positions are probably 7 : 8 and 14: 9. Further evidence for these positions is afforded by the fact that in the oxidation of abietic acid to give acids II and III, in which ring A is intact, rings Band C are opened, and this can be readily explained only if rings B and C each have a double bond. Oxidative studies on abietic acid by Ruzicka et al. (1938-1941) have conclusively confirmed the positions 7 : 8 and 14: 9.

NOTES



The only other point that will be mentioned here is the conversion of abietic acid into levopimaric acid. Since the latter was originally believed to be the enantiomorph of (+)-pimaric acid, it was called (–)-pimaric acid or lrevopimaric acid. It is now known to be a structural isomer of dextro pimaric acid, and so it has been suggested that levopimaric acid be called sapietic acid to avoid any confusion. The following equations show the formation of the adduct of abietic acid with maleic anhydride.

TriTerpenes

Squalene, C30H50, b.p. 240-242°/4 mm., has been isolated from the liver oils of sharks. other sources are olive oil and several other vegetable oils. Squalene has also been detected in leaves. Catalytic hydrogenation (nickel) converts squalene into perhydrosqualene, C30H62 ; therefore squalene has six double bonds, and is acyclic. Ozonolysis of squalene gives, among other products, laevulic acid; this suggests that the following group is present in squalene:

Since squalene cannot be reduced by sodium and amyl alcohol, there are no conjugated double bonds present in the molecule. Perhydrosqualene was found to be identical with the product obtained by subjecting hexa hydrofarnesyl bromide to the Wurtz reaction. This led Karrer et al. (1931) to synthesise squalene itself from farnesyl bromide by a Wurtz reaction.

NOTES



It should be noted that the centre portion of the squalene molecule has the two isoprene units joined tail to tail (cf. the carotenoids, Ch. IX). Squalene forms a thiourea inclusion complex, and hence it has been inferred that it is the all-trans stereoisomer (Schiessler et al., 1952). This is supported by X-ray crystallographic studies of the thiourea inclusion complex (Nicolaides et al., 1954).

Biosynthesis of TerpenesAs more and more natural products were synthesised in the laboratory, so grew the interest in how these compounds are synthesised in the living organism (both animal and plant). The general approach to biosynthesis has been to break up the structure into units from which the compound could plausibly be derived. These units must, however, be known, or can be expected, to be available in the organism. Furthermore, this does not mean that the units chosen must necessarily be involved in the building-up of the compound. The general principle is that although a particular unit may itself be involved, it is also possible that its “ equivalent “ may act as a substitute, i.e., any compound that can readily give rise to this unit (by means of various reactions such as reduction, oxidation, etc.) may be the actual compound involved in the biosynthesis. E.g., the equivalent of formaldehyde could be formic acid, and that of acetone acetoacetic acid. One other point about the choice of units or their equivalents is to attempt to find some relationships between the various groups of natural products so that the units chosen are common precursors.

When the units have been chosen, the next problem is to consider the types of reactions whereby the natural products are synthesised in the organism. The general principle is to use reactions which have been de veloped in the laboratory. The difficulty here is that some types of labora tory reactions require conditions that cannot operate in the organism, e.g., carboxylation and decarboxylation are known biological processes, but when carried out in the laboratory, these reactions normally require elevated temperatures. Deamination is also a known biological process, but in the laboratory this reaction is usually carried out under conditions of (pH) which would be lethal to the living organism. These differences between laboratory syntheses and biosyntheses are due to the action of enzymes in the latter. According to SchOpf (1932), syntheses in plants may take place through the agency of specific or non-specific enzymes, or without enzymes at all. Chemical syntheses (these do not involve the use of enzymes) must therefore, from the point of biosynthetic studies, be carried out under conditions of pH and temperatures comparable with those operating in plants. Chemical syntheses performed in this way (with the suitable units) are said to be carried out under physiological conditions (which involve a pH of about 7 in aqueous media and ordinary temperatures).

Reactions which are commonly postulated in biosynthesis are oxidation, hydrogenation, dehydrogenation, dehydration, esterification, hydrolysis, carboxylation, decarboxylation, amination, deamination, isomerisation, con densation and polymerisation. It might be noted here that the choice of units and type of reaction are usually dependent on each other. Further more, other reactions which are known to occur in biological syntheses are 0- and

NOTES



N-methylation or acylation. These may be described as extra-skeletal processes, and can occur at any suitable stage in the postulated biosynthesis. Another extra-skeletal process is C-methylation, but this is much rarer than those mentioned above.

Now let us apply these principles to the biosynthesis of terpenes. As we have seen, according to the special isoprene rule, terpenes are built up of isoprene units joined head to tail. Assuming then that the isoprene unit is the basic unit, the problem is: How is it formed, and how do these units join to form the various types of terpenes? At present it is believed that the fundamental units used in the cell in syntheses are water, carbon dioxide, formic acid (as “active formate”), and acetic acid (as “active acetate”). These” active” compounds are acyl derivatives of coenzyme A (written as CoA-H in the following equation); e.g., acetoacetic acid is believed to be formed as follows:

Now the biosynthesis of cholesterol from acetic acid labelled with 14C in the methyl group (Cm) and in the carboxyl group (Cc) has led to the suggestion that the carbon atoms in the isoprene unit are distributed as follows:

This distribution is in agreement with a scheme in which senecioic acid (3-methylbut-2-enoic acid) is formed first, and this pathway was supported by the isolation of this acid from natural sources. Further support for the formation of this carbon skeleton is given by the fact that labelled isovaleric acid gives rise to cholesterol in which the isopropyl group and the carboxyl group have been incorporated.

Tavormina et al. (1956), however, have shown that the lactone of mevalonic acid (b-hydroxy-b-methyl-d-valerolactone) is converted almost completely into cholesterol by rat liver, and is a much better precursor than senecioic acid. The following scheme has therefore been proposed for the early stages in the biosynthesis of terpenes; it is in agreement with the distribution of the carbon atoms in cholesterol (See above):

NOTES



Three molecules of active acetate form hydroxymethylglutaric acid, HMG (Lynen et al., 1958; Rudney, 1959), and this is then converted into mevalonic acid (MVA), possibly through the intermediate mevaldic acid (Rudney et al.,1958; Lynen, 1959). Support for this sequence is afforded by the following facts. MVA has been isolated from natural sources (Wolf et al., 1957), and it is also known that HMG may be formed from leucine by the route shown (Lynen et al., 1958, 1959).

The biosynthesis of terpenes can be subdivided into three definite steps: (i) the formation of a biological isopentane unit from acetate; (ii) the con densation of this unit to form acyclic terpenes; (iii) the conversion of acyclic into cyclic terpenes.

The stages leading to MVA have been discussed above. What happens after this is uncertain. One suggestion is that MVA forms a pyrophosphate (at the primary alcoholic group), and then the carboxyl and the tertiary hydroxyl group are eliminated simultaneously to form isopentenyl pyro phosphate (I). This isomerises to the isopropylidene compound, b: b-di methylallyl pyrophosphate, which combines with (I) to form the pyro phosphate of the acyclic terpene geraniol (in the following equations P represents the pyrophosphate residue, P2O6H3):

This is supported by the following work: Stanley (1958) has shown that labelled MVA (2-14C-MVA) is incorporated into a-pinene. Park et al. (1958) have observed the incorporation of labelled MVA into rubber (§33) by an enzyme system from latex, and Lynen et al. (1961) have also demonstrated the conversion of isopentenyl pyrophosphate into rubber. Geranyl pyrophosphate has also been shown to be a precursor for farnesyl pyrophosphate, which then gives squalene.

A point of interest here is that Harley-Mason et al. (1961) have prepared phenylpropiolic acid by the action of brosyl chloride on the sodium derivative of diethyl benzoylmalonate and treating the product with sodium hydroxide in aqueous dioxan at room temperature. The reaction has been formulated as follows:

NOTES



This provides one of the mildest known methods for making an acetylenic bond, and this reaction may be regarded as support for the mechanism proposed by Jones (1961) as a possible route for the biosynthesis of acetylenic bonds:

Rubber. Rubber (caoutchouc) is obtained from latex, which is an emulsion of rubber particles in water that is obtained from the inner bark of many types of trees which grow in the tropics and sub-tropics. When the bark of the rubber tree is cut, latex slowly exudes from the cut. Addi tion of acetic acid coagulates the rubber, which is then separated from the liquor and either pressed into blocks or rolled into sheets, and finally dried in a current of warm air, or smoked.

Crude latex rubber contains, in addition to the actual rubber hydro carbons (90-95 per cent.), proteins, sugars, fatty acids and resins, the amounts of these substances depending on the source. Crude rubber is soft and sticky, becoming more so as the temperature rises. It has a low tensile strength and its elasticity is exhibited only over a narrow range of tempera ture. When treated with solvents such as benzene, ether, light petrol, a large part of the crude rubber dissolves; the rest swells but does not dis solve. This insoluble fraction apparently contains almost all of the protein impurity. On the other hand, rubber is insoluble in acetone, methanol, etc. When unstretched, rubber is amorphous; stretching or prolonged cooling causes rubber to crystallise.

Structure of rubber. The destructive distillation of rubber gives iso prene as one of the main products; this led to the suggestion that rubber is a polymer of isoprene, and therefore to the molecular formula (C5H8)n This molecular formula has been confirmed by the analysis of pure rubber. Crude rubber may be purified by fractional precipitation from benzene solution by the addition of acetone. This fractional precipitation, however, produces molecules of different sizes, as shown by the determination of the molecular weights of the various fractions by osmotic pressure, viscosity and ultra centrifuge measurements; molecular weights of the order of 300,000 have been obtained.

The halogens and the halogen acids readily add on to rubber, e.g., bromine gives an addition product of formula (C5H8Br2)n. and hydrogen chloride the addition product (C5H9Cl)n Pure rubber has been hydrogenated to the fully saturated hydrocarbon (C5H10)n -this is known as hydrorubber-by heating with hydrogen in the presence of platinum as catalyst (Pummerer et al., 1922). Rubber also forms an ozonide of formula (C5H8O3)... All these addition reactions clearly indicate that rubber is an unsaturated compound, and the formulce of the addition products show that there is one double bond for each isoprene unit present.

Ozonolysis of rubber produces lcevulaldehyde and its peroxide, lcevulic acid and small amounts of carbon dioxide, formic acid and succinic acid (Harries, 1905-1912). Pummerer (1931) showed that the lcevulic derivatives comprised about 90 per cent. of the products formed by the ozonolysis. This observation led to the suggestion that rubber is composed of isoprene units joined head to tail. Thus, if rubber has the following structure, the formation of the products of ozonolysis can be explained:

NOTES



Some of the lrevulaldehyde is further oxidised to laevulic and succinic acids.

Gutta-percha (which is also obtained from the bark of various trees) is isomeric with rubber; their structures are the same, as shown by the methods of analysis that were used for rubber. X-ray diffraction studies (Bunn, 1942) have shown that rubber is composed of long chains built up of isoprene units arranged in the cis-form, whereas gutta-percha is the tra -form. Gutta-percha is hard and has a very low elasticity.

In rubber, the chain repeat unit is 8·10 A, whereas in gutta-percha it is 4·72 A. Both of these values are shorter than the theoretical values of the repeat distances (9·13 A and 5·04 A respectively) calculated from models. The reasons for these discrepancies are not clear, but for gutta-percha it has been explained by assuming that the isoprene units are not coplanar. The infra-red absorption spectrum of rubber has bands which are in keeping with the structure that has been proposed. Also, the linear shape of the molecule is indicated by viscosity measurements of rubber solutions. Schulz et al. have examined cyclohexane solutions of rubber by light-scattering methods, and obtained a value of 1,300,000 for the molecular weight. Their other work also supports the linear nature of the chain.

Vulcanisation of RubberWhen crude rubber is heated with a few per cent of sulphur, the rubber becomes vulcanised. Vulcanised rubber is less sticky than crude rubber, and is not so soluble and does not swell

NOTES



so much in organic solvents. Furthermore, vulcanised rubber has greater tensile strength and elasticity than crude rubber.

The mechanism of vulcanisation is still not clear. Vulcanised rubber is not so unsaturated as rubber itself, the loss of one double bond corresponding approximately to each sulphur atom introduced. It therefore appears that some sulphur atoms enter the chain, vulcanisation thus occurring through intramolecular and intermolecular cross-links; it is the latter type of reaction that is desirable in vulcanisation. It should be noted that not all the sulphur is in a combined state; some is free, and this can be readily extracted. Vulcanisation may be accelerated and carried out at lower temperatures in the presence of certain organic compounds. These compounds are con sequently known as accelerators, and all of them contain nitrogen or sulphur, or both, e.g.,

Mercaptobenzothiazole is the most widely used accelerator. Many inorganic compounds can also act as accelerators, e.g., zinc oxide. Organic accelerators are promoted by these inorganic compounds, and current practice is to vulcanise rubber with, e.g., mercaptobenzothiazole in the presence of zinc oxide.

The actual properties of vulcanised rubber depend on the amount of sulphur used, the best physical properties apparently being achieved by using about 3 per cent sulphur, 5 per cent. zinc oxide and about I per cent of the accelerator. When 30-50 per cent sulphur is used, the px:oduct is ebonite.

The elasticity of rubber is believed to be due to the existence of rubber as long-chain molecules which are highly “ kinked “ in the normal state. When subjected to a stretching force, these chains “unkink “, and return to their normal condition when the force is removed.

Synthetic rubbersThere are many synthetic rubbers in use, each type possessing certain desirable properties. A great deal of work has been done on the synthesis of natural rubber, but the difficulty has been to obtain the isoprene units in the all-cis configuration. Wilson et al. (1956) have achieved this by using stereospecific catalysts.

Buna RubbeJi”sUnder the influence of sodium, butadiene polymerises to a substance which has been used as a rubber substitute under the name of Buna. Buna N is a synthetic rubber which is produced by the copolymerisation of butadiene and vinyl cyanide. Buna S or Perbunan is a copolymer of butadiene and styrene.

NOTES



Butyl RubberCopolymerisation of isobutylene with a small amount of isoprene produces a polyisobutylene known as Butyl rubber.

NeopreneWhen passed into a solution of cuprous chloride in am monium chloride, acetylene dimerises to vinylacetylene. This dimer can add on one molecule of hydrogen chloride to form Chloroprene (2-chlorobuta- 1 : 3-diene), the addition taking place in accordance with Markownikoff’s rule (See also Vol. J).

Chloroprene readily polymerises to a rubber-like substance known as Neo prene. Actually, the nature of the polychloroprene depends on the conditions of the polymerisation. .

Silicone RubbersThese are chemically similar to the silicone resins. The chief silicone rubber is prepared by treating the hydrolysis product of dimethyldichlorosilane, (CH3)2SiC12, with various compounds capable of in creasing the molecular weight without the formation of cross-links, i.e., they produce long-chain molecules.

Silicone rubbers have very high electrical insulating properties, and do not deteriorate on exposure to light and air, and are resistant to the action of acids and alkalis.

1.13 CAROTENESCarotene was first isolated by Wackenroder (1831) from carrots (this was the origin of the name carotin, which was later changed to carotene). The molecular formula of carotene, however, was not determined until 1907, when Willstatter showed it was C40H56. Carotene was shown to be unsaturated, and when treated with a small amount of iodine, it forms a crystalline di-iodide, C40H56I2. Kuhn (1929) separated this di-iodide into two fractions by means of fractional crystallisation. Treatment of each fraction with thiosulphate regenerated the corresponding carotenes, which were designated a- and P-carotene. Kuhn et al. (1933) then found that chromatography gives a much better separation of the carotenes themselves, and in this way isolated a third isomer, which he designated y-carotene.

It appears that all three carotenes occur together in nature, but their relative proportions vary with the source, e.g., carrots contain 15 per cent. a, 85 per cent b and 0·1 per cent g. Carotenes are obtained commercially by chromatography, two of the best sources being carrots and alfalfa.

NOTES



Biosynthetic studies of the carotenes have been carried out, and the pathways are those for the terpenes (§32a. VIII). Thus Braithwaite et al. (1957) and Grob (1957) have shown that labelled mevalonic acid is incorporated into {1- carotene. Scheuer et al. (1959) have also shown that this acid is incorporated into lycopene. Furthermore, Modi et al. (1961) have isolated mevalonic acid from carrots.

b-Carotene, C40H56. When catalytically hydrogenated {platinum), b - carotene forms perhydro-p-carotene, c40H78. Thus b-carotene contains eleven double bonds, and since the formula of perhydro-p-carotene corresponds to the general formula CnH2n_ 2, it follows that the compound contains two rings. When exposed to air, b-carotene develops the odour of violets. Since this odour is characteristic of b-ionone, it was thought that this residue is present in b-carotene. This was confirmed by the fact that the oxidation of a benzene solution of b-carotene with cold aqueous potas sium permanganate gives b-ionone. Now b-ionone, I, on ozonolysis, give5, among other things, geronic acid, II (Karrer et al., 1929).

b-Carotene, on ozonolysis, gives geronic acid in an amount that corresponds to the presence of two b-ionone residues (Karrer et al., 1930). Thus a tenta tive structure for b-carotene is:

Since the colour of b-carotene is due to extended conjugation (§1), the C14 portion of the molecule will be conjugated. The presence of conjugation in this central portion is confirmed by the fact that b-carotene forms an adduct with five molecules of maleic anhydride (Nakamiya, 1936).

Geronic acid, on oxidation with cold aqueous potassium permanganate, forms a mixture of acetic acid, a : a-dimethylglutaric, III, a : a-dimethyl succinic, IV, and dimethylmalonic acids, V.

Oxidation of b-carotene in benzene solution with cold aqueous permanganate gives a mixture of b-ionone, III, IV, V, and acetic acid, the amount of acetic acid being more than can be accounted for by the presence of two b-ionone residues. Thus there must be some methyl side-chains in the central C14 portion of the molecule. Since it is essential to know

NOTES



the exact number of these methyl side-chains, this led to the development of the Kuhn-Roth methyl side-chain determination (1931). The first method used was to oxidise the carotenoid with alkaline permanganate, but later chromic acid (chromium trioxide in sulphuric acid) was found to be more reliable, the methyl group in the fragment –C(CH3)= being always oxidised to acetic acid. It was found that alkaline permanganate only oxidises the fragment =C(CH3)-CH= to acetic acid, and fragments such as =C(CH3)-CH2- are incompletely oxidised to acetic acid, or not attacked at all (Karrer et al., 1930). Since a molecule ending in an isopropylidene group also gives acetic acid on oxidation with chromic acid, this end group is determined by ozon olysis, the acetone so formed being estimated volumetrically. Application of the Kuhn-Roth methyl side-chain determination to b-carotene gave four molecules of acetic acid, thus indicating that there are four –C(CH3)= groups in the chain. The positions of two of these have already been tenta tively placed in the two end b-ionone residues, and so the problem is now to find the positions of the remaining two. This was done as follows. Distillation of carotenoids under normal conditions brings about decomposition with the formation of aromatic compounds. Thus the distillation of p-carotene produces toluene, m-xylene and 2 : 6-dimethyl naphthalene (Kuhn et al., 1933). The formation of these compounds may be explained by the cyclisation of fragments of the polyene chain, without the b-ionone rings being involved. The following types of chain fragments would give the desired aromatic products:

The following symmetrical structure for b-carotene would satisfy the requirements of (a), (b) and (c); the tail to tail union of the two isoprene units at the centre should be noted.

NOTES



This symmetrical formula for b-carotene has been confirmed by the following oxidation experiments (Kuhn et al., 1932-1935). When b-carotene is oxidised rapidly with potassium dichromate, dihydroxy-b-carotene, VI, is obtained and this, on oxidation with lead tetra-acetate, gives semi-b carotenone, VII, a diketone. Since both VI and VII contain the same number of carbon atoms as b-carotene, it follows that the double bond in one of the b-ionone rings has been oxidised; otherwise there would have been chain scission had the chain been oxidised. Oxidation of semi-b-carotenone with chromium trioxide produces b-carotenone, VIII, a tetraketone which also has the same number of carbon atoms as b-carotene. Thus, in this compound, the other b-ionone ring is opened. Now only one dihydroxy-b-carotene and one semi-b-carotenone are obtained, and this can be explained only by assuming a symmetrical structure for b-carotene. Thus the oxidations may be formulated:

This structure for b-carotene has been confirmed by synthesis, e.g., that of Karrer et al. (1950). The acetylenic carbinol IX is treated with ethyl magnesium bromide and the product is treated as shown on opposite page.

NOTES



IX has been prepared by Isler (1949) by treating b-ionone with propargyl bromide in the presence of zinc (cj. the Reformatsky reaction):

The most convenient way of preparing the diketone (oct-4-ene-2: 7-dione) starts with but-1-yn-3-ol (Inhoffen et al., 1951):

An important point to note in this synthesis is that lithium aluminium hydride will reduce a triple bond to a double bond when the former is adjacent to a propargylic hydroxyl group, i.e.,

lt is worth while at this point to consider the general aspects of carotene syntheses. All syntheses have used the union of a bifunctional unit, which forms the central part of the carotene molecule, with two molecules (identical as for, e.g., p-carotene, or not identical as for, e.g., a-carotene). The various methods have been divided into four groups according to the carbon content of the three units used in the synthesis: C19 + C2 + C19 ; C16 + C8 + C16; C14 + C12 + C14 ; C10 + C20 + C10. The second group has been used in the above synthesis of b-carotene.

An example of the synthesis of b-carotene by the third is that of Isler et al. (1957) [Rb = b-ionine ring]:

NOTES



An example of the fourth group makes use of the Wittig reaction (see crocetin, §9 for an illustration of this method).

Carotene, C40H56. This is isomeric with b-carotene, and oxidation experiments on at-carotene have led to results similar to those obtained for b-carotene, except that isogeronic acid is obtained as well as geronic acid. Since isogeronic acid is an oxidation product of at-ionone, the conclusion is that at-carotene contains one b-ionone ring and one at-ionone ring {§6. VIII) [Karrer et al., 1933].

Thus the structure of at-carotene is:

As we have seen, at-carotene is optically active (§1), and this is due to the presence of the asymmetric carbon atom (*) in the at-ionone ring. The structure given for at-carotene has

NOTES



been confirmed by synthesis (Karrer et al., 1950). The method is the same as that described for {J-carotene, except that one molecule of the acetylenic alcohol (structure IX, §3) is used together with one molecule of the corresponding at-ionone derivative:

It is interesting to note that at-carotene has been converted into the b-isomer by heating the at-compound with ethanolic sodium ethoxide and benzene at 100--ll0° for some time (Karrer et al., 1947); this is an example of three carbon prototropy.

Lycopene, C40H56, m.p. 175°, is a carotenoid that is the tomato pig ment. Since the structure of g-carotene depends on that of lycopene, the latter will be discussed here, and the former in the next section.

On catalytic hydrogenation (platinum), lycopene is converted into per hydrolycopene, C40H82. Therefore lycopene has thirteen double bonds, and is an acyclic compound (Karrer et al., 1928). Ozonolysis of lycopene gives, among other products, acetone and laevulic acid; this suggests that lycopene contains the terminal residue:

This is supported by the fact that controlled oxidation of lycopene with chromic acid produces 6-methylhept-5-en-2-one (cj. §5. VIII). Quantitative oxidation experiments (ozonolysis) indicate that this grouping occurs at each end of the molecule (Karrer et al., 1929, 1931). Also, the quantitative oxidation of lycopene with chromic acid gives six molecules of acetic acid per molecule of lycopene, thereby suggesting that there are six -C(CH 3)= groups present in the chain (cj. §3). Controlled oxidation of lycopene with chromic acid gives one molecule of methylheptenone and one molecule of lycopenal, C32H42O, and the latter may be further oxidised with chromic acid to another molecule of methylheptenone and one molecule of a dialdehyde, C24H28O2 (Kuhn et al., 1932). Thus this dialdehyde constitutes the central part of the chain, and the two molecules of methylheptenone must have been produced by the oxidation of each end of the chain in lycopene. The dialdehyde may be converted into the corresponding dioxime, and this, on dehydration to the dicyanide, followed by hydrolysis, forms the dicarboxylic acid C24H28O4, which is identical with norbixin (§9). Thus the dialdehyde must be bixindialdehyde, and so it may be inferred that the structure of lycopene is the following symmetrical one, since it accounts for all the above facts.

NOTES



The structure assigned to lycopene has been confirmed by synthesis (Karrer et al., 1950). Instead of the acetylenic carbinol IX in §3, two molecules of the following compound were used.

g-Carotene, C40H56. Catalytic hydrogenation converts g-carotene into perhydro y-carotene, C40H80. Thus there are twelve double bonds pre sent, and the compound contains one ring. Ozonolysis of g-carotene gives, among other products, acetone, laevulic acid and geronic acid. The formation of acetone and laevulic acid indicates the structural relationship of g-carotene to lycopene, and the formation of geronic acid indicates the presence of a b-ionone ring (Kuhn et al., 1933). On this evidence, and also on the fact that the growth-promoting response in rats was found to be half that of b-carotene, Kuhn suggested that g-carotene consists of half a molecule of b-carotene joined to half a molecule of lycopene; thus:

NOTES



This structure for g-carotene is supported by the fact that the absorption maximum of g-carotene in the visible region lies between that of b-carotene and that of lycopene. Final proof for this structure has been obtained by the synthesis of g-carotene (Karrer et al., 1953); the following reactions are written with the conventional formulae:

A d-carotene has also been isolated, and this has been shown to be the a-ionone analogue of g-carotene (Kargel et al., 1960).

Vitamin A, C20H30O. Vitamin A is also known as Axerophthol, and is also usually referred to as vitamin A1 since a second compound, known as vitamin A2, has been isolated.

Vitamin A1 influences growth in animals, and also apparently increases resistance to disease. Night blindness is due to vitamin A1 deficiency in the human diet, and a prolonged deficiency leads to xerophthalmia (hardening of the cornea, etc.). Vitamin A1 occurs free and as esters in fats, in fish livers and in blood. It was originally isolated as a viscous yellow oil, but later it was obtained as a crystalline solid, m.p. 63-64° (Baxter et al., 1940). Vitamin A1

is estimated by the blue colour reaction it gives with a solution of antimony trichloride in chloroform (the Carr-Price reaction; cj. §1); it is also estimated by light absorption (vitamin A1 has a maximum at 328 m,m).

Carotenoids are converted into vitamin A1 in the intestinal mucosa, and feeding experiments showed that the potency of a- and g-carotenes is half that of b-carotene. This provitamin nature of b-carotene led to the sugges tion that vitamin A1 is half the molecule of b-carotene.

NOTES



On catalytic hydrogenation, vitamin A1 is converted into perhydro vitamin A1, C20H40O; thus vitamin A1 contains five double bonds. Since vitamin A1 forms an ester with p-nitrobenzoic acid (this ester is not crystal lisable), it follows that vitamin A1 contains a hydroxyl group. Thus the parent hydrocarbon of vitamin A1 is C20H40, and consequently the molecule contains one ring. Ozonolysis of vitamin A1 produces one molecule of geronic acid (§3) per molecule of vitamin A1, and so there must be one b-ionone nucleus present {Karrer, 1931, 1932). Oxidation of vitamin A1 with permanganate produces acetic acid; this suggests that there are some–-C(CH3)= groups in the chain. All of the foregoing facts are in keeping with the suggestion that vitamin A1 is half the b-carotene structure. When heated with an ethanolic solution of hydrogen chloride, vitamin A1 is converted into some compound (II) which, on dehydrogenation with selenium forms 1 : 6-dimethylnaphthalene, III (Heilbron et al., 1932). Heilbron assumed I as the structure of vitamin A1, and explained the course of the reaction as follows:

Perhydrovitamin A1 has been synthesised from b-ionone (Karrer, 1933), and was shown to be identical with the compound obtained by reducing vitamin A1; thus there is evidence to support the structure assigned to vitamin A1. Final proof of structure must rest with a synthesis of vitamin A1 itself, and this has now been accomplished by several groups of workers.

The following synthesis is that of Isler et al. (1947). This starts with methyl vinyl ketone to produce compound IV, one stage of the reactions involving

Preparation of IV.

NOTES



Preparation of V.

an allylic rearrangement (cf. §8. VIII). Compound V is prepared from b-ionone by means of the Darzens glycidic ester reaction. The following chart shows the steps of the synthesis, and it should be noted that another allylic rearrangement is involved in one of the later steps.

NOTES



In the hydrogenation of VI to VII, barium sulphate is used to act as a poison to the catalyst to prevent hydrogenation of the double bonds. Partial acetylation of VII (primary alcoholic groups are more readily acetylated than secondary) protects the terminal group from an allylic rearrangement in the conversion of VIII to IX.

The crude vitamin A1 obtained in the above synthesis was purified via its ester with anthraquinone-2-carboxylic acid, and was thereby obtained in a crystalline form which was

NOTES



shown to be identical with natural vitamin A1. Lindlar (1952) has shown that triple bonds may be partially hydrogenated in the presence of a Pd-CaCO3 catalyst that has been partially inactivated by treatment with lead acetate; better results are obtained by the addition of quinoline. Thus the hydrogenation of VI gives VII in 86 per cent yield when the Lindlar catalyst is used.

Another method of synthesising vitamin A1 is due to van Dorp et al. (1946) who prepared vitamin A1 acid (X), which was then reduced by means of lithium aluminium hydride to vitamin A1 by Tishler (1949); b-ionone and methyl g-bromocrotonate are the starting materials.

Attenburrow et al. {1952) have also synthesised vitamin A1 starting from 2-methylcyclohexanone. .

Acid causes rearrangement of XI to XII in which all multiple bonds are in complete conjugation, and the reduction of XII to XIII by lithium aluminium hydride is possible because of the presence of the propargylic hydroxyl grouping {§3).

Synthetic vitamin A1 is now a commercial product.

Two biologically active geometrical isomers of Vitamin A1 (all-trans) have also been isolated: neovitamin a from rat liver (Robeson et al., 1947) and neo -vitamin b from the eye (Oroshnik et al., 1956). Vitamin Ax is the most active form in curing “vitamin A” deficiency.

NOTES



Vitamin A2

A second vitamin A, vitamin A2, has been isolated from natural sources, and has been synthesised by Jones et al. (1951, 1952); it is dehydrovitamin A1.

Jones et al. (1955) have also introduced a method for converting vitamin A1 into vitamin A2. Vitamin A1 may be oxidised to vitamin A1 aldehyde (retinene1) by means of manganese dioxide in acetone solution (Morton et al., 1948), and then treated as follows:

XanthophyllThe xanthophylls occur naturally, and all have the same carbon skeletons as the carotenes or lycopene (except :fiavoxanthin).

Cryptoxanthin, C40H56O, m.p. 169°, is monohydroxy-, b-carotene; it has provitamin-A activity.

NOTES



Rubixanthin, C40H58O, m.p. 160°, is monohydroxy-y-carotene, and lyco xanthin, C40H56O, m.p. 168°, appears to be monohydroxylycopene.

Rhodoxanthin, C40H52O2, m.p. 219°, is believed to be the following diketone.

Lutein, C40H56O2, m.p. 193°, was formerly known as xanthophyll; it is di hydroxy--carotene.

Zeaxanthin, m.p. 205°, and lycophyll, m.p. 179°, are the corresponding di hydroxy derivatives of p-carotene and lycopene, respectively.

Carotenoid aCids

These are compounds which do not contain 40 carbon atoms.

Bixin, C25H30O4. Natural bixin is a brown solid, m.p. 198°, and is the cis-form; it is readily converted into the more stable trans-form, m.p. 216-217°.

When boiled with potassium hydroxide solution, bixin produces one molecule of methanol and a dipotassium salt which, on acidification, gives the dibasic acid norbixin, C24H28O4. Thus bixin is a monomethyl ester, and can be esterified to give methylbixin.

On catalytic hydrogenation, bixin is converted into perhydrobixin, C25H48O4 ; thus there are 9 double bonds present in the molecule (Lieber mann et al., 1915). Perhydrobixin, on hydrolysis, forms perhydronorbixin. Oxidation of bixin with permanganate produces four molecules of acetic acid (Kuhn et al., 1929); thus there are four –C(CH3)= groups in the chain. Furthermore, since the parent hydrocarbon of perhydronorbixin, C24H46O4, is C22H46 (the two carboxyl groups are regarded as substituents), the molecule is acyclic.

NOTES



The thermal decomposition of bixin produces toluene, m-xylene, m-toluic acid and the methyl ester of this acid (Kuhn et al., 1932}. Hence the following assumptions may be made regarding the nature of the chain (cf. {J carotene, §3}.

The foregoing facts may be explained by assuming the following structure for bixin (Kuhn et al., 1932):

This structure is supported by the fact that perhydronorbixin has been synthesised, and shown to be identical with the compound obtained from the reduction of bixin (Karrer et al., 1933). Further proof is the synthesis of norbixin (Isler et al., 1957).

Jackman et al. (1960) have shown, from an examination of the NMR spectra (§19a. I) of many carotenoids, that the positions of the absorption bands resulting from the methyl groups give some indication of the molecular environment of these groups. “Natural” methylbixin is the cis-isomer of the following trans-isomer:

The methyl ester of crocetin also probably has the cis-configura tion at the corresponding 2,3-position.

Crocetin, C20H24O4. Crocetin occurs in saffron as the digentiobioside, crocin. The structure of crocetin was elucidated by Karrer et al. (1928) and Kuhn et al. (1931). Crocetin behaves as a dica:rboxylic acid and has seven double bonds (as shown by catalytic hydrogenation to perhydrocrocetin, C20H38O4). On oxidation with chromic acid, crocetin gives 3-4 molecules of acetic acid per molecule of crocetin; thus there are 3-4 methyl side chains. The structure of crocetin was finally shown by the degradation of perhydronorbixin, C24H46O4, by means of the following method:

NOTES



This set of reactions was performed twice on perhydronorbixin, thereby resulting in the loss of four carbon atoms (two from each end); the product so obtained was perhydrocrocetin, C20H38O4. On these results, crocetin is therefore:

This structure is supported by the fact that the removal of two carbon atoms from perhydrocrocetin by the above technique (one carbon atom is lost from each end) resulted in the formation of a diketone. The formation of this compound shows the presence of an a-methyl group at each end of the molecule. The structure of crocetin is further supported by the synthesis of perhydrocrocetin, and by the synthesis of crocetin diesters by Isler et al. (1957). These diesters probably have the (: -configuration at the 2,3- position between the dialdehyde and two molecules of the phosphorane (Buchta et al., 1959, 1960).

1.14 SUMMARYThe thermal decomposition of almost all terpenes gives isoprene as one of the products, and this led to the suggestion that the skeleton structures of all naturally occurring terpenes can be built up of isoprene units; this is known as the isoprene rule, and was first pointed out by Wallach (1887). Thus the divisibility into isoprene units may be regarded as a necessary condition to be satisfied by the structure of any plant-synthesised terpene. Furthermore, Ingold (1925) pointed out that the isoprene units in natural terpenes were joined “head to tail”.

In most of the carotenoids, the central portion of the molecule is composed of a long conjugated chain comprised of four isoprene units, the centre two of which are joined tail to tail. The ends of the chain may be two open chain structures, or one open-chain structure and one ring, or two rings. The colour of the carotenoids is attributed to the extended conjugation of the central chain.

The fat is now digested with ethanol, any fat that dissolves being removed by cooling to 20°. The essential oils so obtained usually contain a number of terpenes, and these are separated by fractional distillation. The terpene hydrocarbons distil first, and these are followed by

NOTES



the oxygenated de rivatives. Distillation of the residue under reduced pressure gives the sesquiterpenes, and these are separated by fractional distillation.

These facts, i.e., that myrcene contains three double bonds, two of which are in conjugation, had been established by earlier investigators (e.g., Semmler, 1901) Ozonolysis of myrcene produces acetone, formaldehyde and a ketodialdehyde, C5H6O3, and the latter, on oxidation with chromic acid, gives succinic acid and carbon dioxide (Ruzicka et al., 1924). These results can be explained by assigning structure I to myrcene. In terpene chemistry it has become customary to use conventional formulae rather than those of the type I. In these conventional formulae only lines are used; carbon atoms are at the junctions of pairs of lines or at the end of a line, and instauration is indicated by double bonds. Furthermore, the carbon skeleton is usually drawn in a ring fashion (the cyclohexane ring).

Further more, since ocimene forms an adduct with maleic anhydride, two of the double bonds are conjugated. On ozonolysis, ocimene produces formaldehyde, methylglyoxal, lrevulaldehyde, acetic and malonic acids, and some acetone. All of these products, except acetone, are accounted for by structure I for ocimene (this has an isopropenyl end-group).

The structural identity of geraniol and nerol is shown by the following facts. Both add on two molecules of hydrogen when hydrogenated catalytically; thus both contain two double bonds. Both give the same saturated alcohol, C10H22O. Also, on oxidation, geraniol and nerol give the same oxidation products which, at the same time, show the positions of the double bonds to be 2 and 7 (cf. citral, §5).

The reasons for these discrepancies are not clear, but for gutta-percha it has been explained by assuming that the isoprene units are not coplanar. The infra-red absorption spectrum of rubber has bands which are in keeping with the structure that has been proposed. Also, the linear shape of the molecule is indicated by viscosity measurements of rubber solutions. Schulz et al. have examined cyclohexane solutions of rubber by light-scattering methods, and obtained a value of 1,300,000 for the molecular weight. Their other work also supports the linear nature of the chain.

This symmetrical formula for b-carotene has been confirmed by the following oxidation experiments (Kuhn et al., 1932-1935). When b-carotene is oxidised rapidly with potassium dichromate, dihydroxy-b-carotene, VI, is obtained and this, on oxidation with lead tetra-acetate, gives semi-b carotenone, VII, a diketone. Since both VI and VII contain the same number of carbon atoms as b-carotene, it follows that the double bond in one of the b-ionone rings has been oxidised; otherwise there would have been chain scission had the chain been oxidised. Oxidation of semi-b-carotenone with chromium trioxide produces b-carotenone, VIII, a tetraketone which also has the same number of carbon atoms as b-carotene. Thus, in this compound, the other b-ionone ring is opened.

Vitamin A1 influences growth in animals, and also apparently increases resistance to disease. Night blindness is due to vitamin A1 deficiency in the human diet, and a prolonged deficiency leads to xerophthalmia (hardening of the cornea, etc.). Vitamin A1 occurs free and

NOTES



as esters in fats, in fish livers and in blood. It was originally isolated as a viscous yellow oil, but later it was obtained as a crystalline solid, m.p. 63-64° (Baxter et al., 1940).

1.15 REVIEW QUESTIONS 1. Explain the Isolation of Monoterpenes and Sesquiterpenes

2. Describe the General Methods of Determining Structure.

3. What is Monoterpenes?

4. Explain the Monocyclic Monoterpenes.

5. What is the Bicyclic Monoterpenes?

6. What is Sesiquiterpenes?

7. Explain the Acyclic Sesquiterpenes.

8. Describe the Monocyclic Sesquiterpenes.

9. What is Bicyclic Sesquiterpenes?

10. Explain the Diterpenes & Carotenes.

1.16 FURTHER READINGSzz Organic chemistry, Volume 2 : Stereochemistry and the chemistry of natural

products, fifth edition – I.L. Finar.

zz Organic chemistry – J. Calyden, Greeve, S. Warren and others (Oxford University Press) 2001.

zz Chemistry of the alkaloids, Pelletier ed., Van Nostrand Reinhold Co. (1970).

zz The plant alkaloids, Churchill (1949, 4th ed.).

NOTES

Alkaloids


CHAPTER – 2

ALKALOIDSSTRUCTURE

2.1 Learning Objectives

2.2 Introduction

2.3 Nomenclature of Alkaloids

2.4 Physiological Action of Alkaloids

2.5 Occurrence of Alkaloids

2.6 Isolation of Alkaloids

2.7 General Methods of Structure Determination/Elucidation

2.8 Methods of Degradation of Alkaloids

2.9 Detailed Study of some Alkaloids

2.10 Summary

2.11 Review Questions

2.12 Further Readings


zz To definition nomenclature of alkaloids

zz To discuss the physiological effects of alkaloids

zz The Structure of important alkaloids

zz The Stereochemical aspects of alkaloids

2.2 INTRODUCTIONThe word alkaloids refered as –

Alkal-alkali (basic), oid-like (same).

The alkaloids are isolated from plants and word alkaloid first time introduced by W. meissner. In early year no correct definition has been given for alkaloids, so in views of scientists many definition has suggested.

zz According to Koings alkaloids are pyridine ring containing organic bases.

zz According to Ladenburg alkaloids are organic bases having at least one heteroatom in ring.

NOTES



zz In keeping of above definition alkaloids may be defined as "alkaloids are optically active nitrogen containing heterocycles which possess effective physiological properties".

2.3 NOMENCLATURE OF ALKALOIDSAlkaloids have complex molecular structure. So systematic nomenclature not refered for alkaloids. So following methods are proposed for nomenclature.

Source basis nomenclature : Name of alkaloids will be according to source (plant) from which they are obtained.

Source (Plant) Name of alkaloidPapaver somniferum PapavarineBerberis vulgaris L. BerberineSubstituent basis nomenclature : During nomenclature of alkaloids such prefixes

like Iso, Neo, Nor, Pseudo etc. are used for e.g. Nor prefix is used for the absence of methyl substituent.

2.4 PHYSIOLOGICAL ACTION OF ALKALOIDSAlkaloids are obtained from natural sources. Mostly alkaloids are isolated from plants. Alkaloids can perform one/more function in plant metabolism process which are described as follow :

(a) Alkaloids may be act as food reservior for protein synthesis.

(b) Many alkaloids have poisonous effect, so it is useful for plant protection from insects and herbivores.

(c) Alkaloids can be used as nitrogen suppler.

(d) Accumulation of alkaloids in plants show toxic effect and due to this plant can be damage.

Production of alkaloids in plant can affect plant metabolic process, plant growth and reproductive process.

2.5 OCCURRENCE OF ALKALOIDSAs we have already discussed that alkaloids are isolated from natural plant and found in bark, leaves, seeds, roots etc. Alkaloids are nitrogen containing heterogeneous substances which are found in families of dicotyledons e.g. Solanaceae, Rutaceae, Rubiaceae, Papilionaceae, Papaveraceae, Apocyanaceae etc.

Due to presence of nitrogen, alkaloids are basic in nature. So mostly alkaloids are found in their salt from as salt of succinic acid, oxalic acid, tannic acid, tartonic acid etc. Some alkaloids are found as their glycoside derivatives of sugar eg. piperine found as ester derivative atropine, cocaine etc.

NOTES

Alkaloids


2.6 ISOLATION OF ALKALOIDS

Isolation of alkaloids is a very complicated process because of a plant contains a mixture of

alkaloids. So isolation of alkaloids have following procedure/methods.

Method-I(I) At first by using alkaloidal precipitating reagents e.g. Mayer's reagent (potassium

mercuric iodide), Wegner's reagent (Iodine dissolve in potassium iodide) the presence of alkaloids is confirmed in plant. Precipitation of alkaloids can be done by using of colour reagents e.g. formaldehyde (Marquis reagent) and nitric acid (Erdman's reagent).

(II) When the presence of alkaloids in plant is confirmed then separation technique is applied as small percentage by dry weight basis from plant material e.g. Atropa belladonna contains 0.2% hyocyamine, Rauwolfia Serpentina root contains 0.1-0.2% reserpine, opium contains 10% morphine.

(III) Alkaloids are separated and purified from crude extract in the last step. The alkaloids have a heteroatom so these are soluble in acidic/alkaline solution but not in organic solvent. It is the basis of Stas-otto process. There are three basis of solvent dependent method involving Stas-otto process.

4

Acidic salt of alkaloids as oxalates and tannates

Add alkaline solution of Na2CO3, K2CO3 and Ca(OH)2

Alkaloids obtained in free state

Complete extract of alkaloids

Extract with organic solvent

Shake and concentrate it via centrifugeand filter it

Alkaloidal salt as aqueous acidic

solution

Residual organic material e.g.

pigments

Residual aqueous material

Alkaloidal organic bases

SolventEvoporation

Mixture of alkaloid obtained

Method-II:

This method involve the extraction of powdered material with organic solvents like methanol, ethanol etc. and then repeat the steps of method-I.

Method (II) have some advantage over method (I).

(a) In this method alkali not used so, less number of extraction is applied then method (I).

(b) It is less hazardous for health.

Method III:

Water and alcohol are used for the extraction of alkaloids from plant. Other waste material is removed from extract. The alkaloids are precipitated by addition of alkali and separated by filteration of extract.

Method-IIThis method involve the extraction of powdered material with organic solvents like methanol, ethanol etc. and then repeat the steps of method-I.

NOTES



Method (II) have some advantage over method (I).(a) In this method alkali not used so, less number of extraction is applied then method (I).(b) It is less hazardous for health.

Method IIIWater and alcohol are used for the extraction of alkaloids from plant. Other waste material is removed from extract. The alkaloids are precipitated by addition of alkali and separated by filteration of extract.

2.7 GENERAL METHODS OF STRUCTURE DETERMINATION /ELUCIDATION

(I) When a pure sample of alkaloid is obtained then applied qualitative analysis (presence of C, H, N, O) and followed by quantitative analysis for the determination of empirical formula. Then its molecular weight is calculated and followed by determination of molecular formula.

The mostly alkaloids are unsaturated so, no. of double bond equivalents (DBE) is calculated for alkaloids as follow:

5

1.7 General methods of structure determination / elucidation:



no. of (H) atom in saturated alkane - No. of (H) atom in given compound 2

e.g. Benzene (C6H6) Complete saturated alkane (C6H14)

14 6 8 42 2

Note: DBE may be equivalent to double bond or ring.

If the alkaloids contains other atoms like N, O then following formula is used for calculation of D.B.E.

a - 12

b + 12

c + 1

C H N O

Name of alkaloid Molecular formula

Molecular structure D.B.E.

1. Hygrine C8H15NO N OCH3

21 ring

1-oxo group


5






14 6 8 42 2



a - 12

b + 12

c + 1

C H N O




21 ring

1-oxo group

Note: DBE may be equivalent to double bond or ring. If the alkaloids contains other atoms like N, O then following formula is used for

calculation of D.B.E.

5






14 6 8 42 2



a - 12

b + 12

c + 1

C H N O




21 ring

1-oxo group

5






14 6 8 42 2



a - 12

b + 12

c + 1

C H N O




21 ring

1-oxo group

6

2. Mescaline C11H17NO3 OMe

OMeMeO

CH2–CH2–NH2

43-double bond

1-ring

(II) When the alkaloids is optically active then its specific rotation is measured as.

(+) dextrorotatory (d)

(–) Leavorotatory (l)

(III) Mostly alkaloids have oxygen. So it can be present in following functional group. e.g. Hydroxyl group (alcoholic or phenolic), carboxylic acid, carbonyl and ether group. So presence of these functional group explained as –

(i) Oxygen as hydroxyl group: Presence of oxygen in hydroxyl group is confirmed by its derivative formation as ester e.g. benzoate, acetate etc.

–OH + Cl–C Ph

O

O C

O

Ph + HClBenzoate

–OH + Cl–C CH3

O

O C

O

CH3 + HCl

Acetate If the presence of hydroxyl group is confirmed then problem is that it is phenolic or alcoholic. If the hydroxyl group is soluble in NaOH and reprecipitated by carbondioxide, if colouration appear with FeCl3 then hydroxyl group will be phenolic in nature. If the hydroxyl group does not behave as phenolic then it will be alcoholic in nature. The number of hydroxyl group is confirmed by acetylation and the estimation of active hydrogen is done by Zerewitinoff method by using of methyl magnesium bromide.

(ii) Oxygen as carbonyl group: Carbonyl group may be ascertained by its derivative formation e.g. oxime, semicarbazone and phenylhydrazone.

Carbonyl group may be aldehydic or ketonic, so it is confirmed by spectral technique e.g. UV, IR, NMR etc. and chemical process (reduction and oxidation).

NOTES

Alkaloids







6

2. Mescaline C11H17NO3 OMe

OMeMeO

CH2–CH2–NH2

43-double bond

1-ring






–OH + Cl–C Ph

O

O C

O

Ph + HClBenzoate

–OH + Cl–C CH3

O

O C

O

CH3 + HCl

Acetate If the presence of hydroxyl group is confirmed then problem is that it is phenolic or alcoholic. If the hydroxyl group is soluble in NaOH and reprecipitated by carbondioxide, if colouration appear with FeCl3 then hydroxyl group will be phenolic in nature. If the hydroxyl group does not behave as phenolic then it will be alcoholic in nature. The number of hydroxyl group is confirmed by acetylation and the estimation of active hydrogen is done by Zerewitinoff method by using of methyl magnesium bromide.



If the presence of hydroxyl group is confirmed then problem is that it is phenolic or alcoholic. If the hydroxyl group is soluble in NaOH and reprecipitated by carbondioxide, if colouration appear with FeCl3 then hydroxyl group will be phenolic in nature. If the hydroxyl group does not behave as phenolic then it will be alcoholic in nature. The number of hydroxyl group is confirmed by acetylation and the estimation of active hydrogen is done by Zerewitinoff method by using of methyl magnesium bromide.



(iii) Oxygen as carboxylic group: If the alkaloid is soluble in aqueous sodium carbonate or ammonia solution then it suggest the presence of carboxylic group. Presence of carboxylic group is also confirmed by its ester formation. When presence of carboxylic group is confirmed then its number is confirmed volumetrically by titration against barium hydroxide by using of phenolphthalein as an indicator.

(iv) Oxygen as methoxy group: Presence and number of methoxyl group is determined by Zeisel method which have following procedure. The known amount of alkaloid is heated with HI at its b.p. then methoxyl group converted in to methyl iodide which is absorbed by ethanolic solution of silver nitrate then precipitate of (AgI) setteled, filter, dried and weight it.

NOTES



7

(iii) Oxygen as carboxylic group:

If the alkaloid is soluble in aqueous sodium carbonate or ammonia solution then it suggest the presence of carboxylic group. Presence of carboxylic group is also confirmed by its ester formation. When presence of carboxylic group is confirmed then its number is confirmed volumetrically by titration against barium hydroxide by using of phenolphthalein as an indicator.


–OMe + HI OH + CH3 – I

AgNO3

CH3NO3 + AgI ( )

(v) Oxygen as methylenedioxyl group (–O–CH2–O–): Its presence is confirmed by the formation of formaldehyde which is formed by the reaction of alkaloid with the hydrochloric acid or sulphuric acid.

(vi) The following functional groups are identified by their product when the alkaloid undergoes acidic or alkali hydrolysis.

C

O

NH2 + NaOHAmide C

O

ONa + NH3

C

O

OR + NaOHEster C

O

ONa + R–OH

CH

C

CH2Lactone (cyclic ester)

O

O

+ NaOH CH2

COONa

CH2

OH

CH

C

CH2Lactum (cyclic amide)

NH

O

+ NaOH CH2

COONa

CH2

NH3



7

(iii) Oxygen as carboxylic group:

If the alkaloid is soluble in aqueous sodium carbonate or ammonia solution then it suggest the presence of carboxylic group. Presence of carboxylic group is also confirmed by its ester formation. When presence of carboxylic group is confirmed then its number is confirmed volumetrically by titration against barium hydroxide by using of phenolphthalein as an indicator.


–OMe + HI OH + CH3 – I

AgNO3

CH3NO3 + AgI ( )



C

O

NH2 + NaOHAmide C

O

ONa + NH3

C

O

OR + NaOHEster C

O

ONa + R–OH

CH

C

CH2Lactone (cyclic ester)

O

O

+ NaOH CH2

COONa

CH2

OH

CH

C

CH2Lactum (cyclic amide)

NH

O

+ NaOH CH2

COONa

CH2

NH3

Nature of Functional Nitrogen in AlkaloidsNature of amine (primary, secondary and tertiary) can be confirmed by following points:

(a) Reaction with methyl iodide: Amine (primary, secondary and tertiary) react with excess amount of methyl iodide to form quartnary ammonium iodide salt. The primary amine react with three equivalent, secondary amine react with two equivalent and tertiary amine react with one equivalent of methyl iodide.

(b) Reaction with aqueous potassium hydroxide: It confirm the nature and number of alkyl groups which are attached with nitrogen atom. The product forms as methyl amine, dimethyl amine and trimethyl amine which confirm the nature of nitrogen.

(c) If the alkaloid have N-methyl group then it gives methyl amine on reaction with Sodalime.

(d) The hydrolysis of alkaloid having an amide, lactum is also useful in the determination of functional nature of nitrogen atom.

NOTES

Alkaloids


2.8 METHODS OF DEGRADATION OF ALKALOIDSFollowing methods are suggested for degradation of alkaloids.

(I) Hoffmann exhaustive methylation method(II) Emde's degradation(III) Von Braun's degradation(IV) Reduction degradation with zinc dust(V) Oxidation(I) Hoffmann exhaustive methylation method: This method involve the reaction

of alkaloid having nitrogen atom (amine with excess amount of methyl iodide to form quartnary ammonium iodide salt, which is undergoes pyrolysis at 200°C to give an olefin via b-elimination.

The above process will be continue until the nitrogen atom is eliminated from eg. (a)

9

eg. (a)

NH

(i) CH3I

(ii) AgOHN

HH3COH

–HNCH3

(i) CH3I

(ii) AgOHN

CH3H3CPiperidine

–H2O

NMeMe

(i) CH3I

(ii) AgOHN

MeMe Me

Heat

–H2O

Piperylene

OH

(b) NMe2MeO

(i) CH3I

(ii) AgOH(iii) Heat MeO

p-methoxy styreneHordenine methyl ether

(c) If the nitrogen atom is part of ring and having two -hydrogen then the

-hydrogen will eliminate which gives more conjugated olefin. In given e.g. (A) is major product.

(c) If the nitrogen atom is part of ring and having two b-hydrogen then the b-hydrogen will eliminate which gives more conjugated olefin. In given e.g. (A) is major product

10

CHN

H3CO

H3CO

Me

Me

CH

OCH3OCH3

(i) CH3I

(ii) AgOH(iii) Heat

N

H3CO

H3CO CH3

CH2

OCH3OCH3

(i) CH3I

(ii) AgOH(iii) Heat

H3CO

H3CO

CH2

OCH3OCH3

Laudonosine Minor product (B)Major product (A)

Note: If the nitrogen atom present in unsaturated ring or takes part in conjugation

or -hydrogen is absent then Hoffmann's exhaustive methylation fails. So, the Emde's degradation followed.

(II) Emde's degradation:

It involve the degradation of quartnary salt with sodium amalgam, sodium liq. NH3 or by catalytic hydrogenation.

e.g.

N H

(i) CH3I (excess)

(ii) AgOH, heat(iii) Na-Hg

+ Me3N(a)

N

(i) CH3I (excess)

(ii) AgOH(iii) Na-Hg

(b)

Me

Me2N

(III) Von Braun's method:

This method can be studied in two parts :

Part-A: It involve substitution reaction of tertiary amine (cyclic or acyclic) with cyanogen bromide to give alkyl bromide and cynamide.

NR'

R

R''CNBr N

R

R'CN + R''–Br

Note: If alkyl groups on nitrogen atom are different then smaller group goes with halide.

e.g.

NOTES



Note: If the nitrogen atom present in unsaturated ring or takes part in conjugation or b-hydrogen is absent then Hoffmann's exhaustive methylation fails. So, the Emde's degradation followed.

(II) Emde's degradation: It involve the degradation of quartnary salt with sodium amalgam, sodium liq. NH3 or by catalytic hydrogenation.

e.g.

10

CHN

H3CO

H3CO

Me

Me

CH

OCH3OCH3

(i) CH3I

(ii) AgOH(iii) Heat

N

H3CO

H3CO CH3

CH2

OCH3OCH3

(i) CH3I

(ii) AgOH(iii) Heat

H3CO

H3CO

CH2

OCH3OCH3






e.g.

N H

(i) CH3I (excess)


+ Me3N(a)

N

(i) CH3I (excess)


(b)

Me

Me2N




NR'

R

R''CNBr N

R

R'CN + R''–Br


e.g.

(III) Von Braun's method: This method can be studied in two parts :


10

CHN

H3CO

H3CO

Me

Me

CH

OCH3OCH3

(i) CH3I

(ii) AgOH(iii) Heat

N

H3CO

H3CO CH3

CH2

OCH3OCH3

(i) CH3I

(ii) AgOH(iii) Heat

H3CO

H3CO

CH2

OCH3OCH3






e.g.

N H

(i) CH3I (excess)


+ Me3N(a)

N

(i) CH3I (excess)


(b)

Me

Me2N




NR'

R

R''CNBr N

R

R'CN + R''–Br


e.g.

Note: If alkyl groups on nitrogen atom are different then smaller group goes with halide. e.g.

11

N Me

Hoffmann

method NMe2

Von-Braunsmethod

N

BrMe

CN

+BrN

(a) (b)

Me

CN

Product (b) formed as major product because it is formed by formation by benzyl carbocation.

(IV) Reductive degradation with Zn-dust and HI:

(i) Reaction with HI: It involve reduction followed by ring opening.

N

HI300°C

N

H

HI300°C

CH3–(CH2)3CH3 + NH3

Pyridine Piperidine

n-pentane

CH3–(CH2)6–CH3

NCH2–CH2–CH3

HI300°C

NCH2–CH2–CH3

H

HI300°C

NH3

+Conyrine

Coniine n-octane (ii) Zinc-duct distillation: It causes dehydrogenation and deoxygenation to

generate unsaturation.

CH–CH2–NH–CH3

OH

OH

OH

KOH

COOH

OH

OH

Adrenaline Protocatechuic acid (V) Oxidation: It is the useful method for determination of alkaloid structure.

Various products are formed which are depend on the nature of oxidizing agents.


NOTES

Alkaloids


(IV) Reductive degradation with Zn-dust and HI:(i) Reaction with HI: It involve reduction followed by ring opening.

11

N Me

Hoffmann

method NMe2

Von-Braunsmethod

N

BrMe

CN

+BrN

(a) (b)

Me

CN




N

HI300°C

N

H

HI300°C


Pyridine Piperidine

n-pentane

CH3–(CH2)6–CH3

NCH2–CH2–CH3

HI300°C

NCH2–CH2–CH3

H

HI300°C

NH3

+Conyrine



CH–CH2–NH–CH3

OH

OH

OH

KOH

COOH

OH

OH



(ii) Zinc-duct distillation: It causes dehydrogenation and deoxygenation to generate unsaturation.

11

N Me

Hoffmann

method NMe2

Von-Braunsmethod

N

BrMe

CN

+BrN

(a) (b)

Me

CN




N

HI300°C

N

H

HI300°C


Pyridine Piperidine

n-pentane

CH3–(CH2)6–CH3

NCH2–CH2–CH3

HI300°C

NCH2–CH2–CH3

H

HI300°C

NH3

+Conyrine



CH–CH2–NH–CH3

OH

OH

OH

KOH

COOH

OH

OH



(V) Oxidation: It is the useful method for determination of alkaloid structure. Various products are formed which are depend on the nature of oxidizing agents.

12

Oxidising agents

Mild oxidising agent Moderate oxidising agent Strong/vigrousoxidising agent

eg. H2O2, O3, I2 inethanolic solution

Acidic/alkaline KMnO4

CrO3 in acetic acid

K2Cr2O7–H2SO4

CrO3–H2SO4MnO2–H2SO4conc. HNO3

e.g. Nicotine undergoes oxidation with K2Cr2O7-H2SO4 then it gives nicotinic acid, it confirm that nicotine have pyridine ring.

N

N

CH3

K2Cr2O7

H2SO4N

COOH

Nicotine Nicotinic acid Now a days some spectral techniques also useful in the determination of alkaloid structure which are listed below:

(a) UV-spectroscopy (b) IR-spectroscopy (c) NMR-spectroscopy (d) Mass-spectrometry (e) Optical rotatory dispersion (ORD) and circular dichroism (CD)

1.9 Detailed study of some alkaloids:

1.9.1 Nicotine:

It is a pyridine-pyrrolidine alkaloid. The name nicotine given in the honour of J. Nicot. Nicotine occurs in tobacco plant (Nicotiana tobacum) and other Nicotiana species in the form of salt of acid. Tobacco is the commercial source of nicotine and mainly found in the leaves of plant.

(I) Molecular formula: C10H14N2

(II) Molecular structure: N

N

CH3


12

Oxidising agents




CrO3 in acetic acid

K2Cr2O7–H2SO4



N

N

CH3

K2Cr2O7

H2SO4N

COOH




1.9.1 Nicotine:




N

CH3

NOTES



Now a days some spectral techniques also useful in the determination of alkaloid structure which are listed below:

(a) UV-spectroscopy

(b) IR-spectroscopy

(c) NMR-spectroscopy

(d) Mass-spectrometry

(e) Optical rotatory dispersion (ORD) and circular dichroism (CD)

2.9 DETAILED STUDY OF SOME ALKALOIDS

nicotine



(II) Molecular structure:

12

Oxidising agents




CrO3 in acetic acid

K2Cr2O7–H2SO4



N

N

CH3

K2Cr2O7

H2SO4N

COOH




1.9.1 Nicotine:




N

CH3

(III) Presence of two tertiary N-atom: It is confirmed by the reaction of Nicotine with methyl iodine, that it will react with two equivalent of methyl iodide.

(IV) If the oxidation of nicotine is done by KMnO4 or HNO3 then it yields nicotinic acid (pyridine-3-carboxylic acid).

13



N

N

CH3 N

COOHKMnO4

Nicotine Nicotinic acid

(V) Attachment of pyrrolidine ring with pyridine nucleus: Pyrrolidine ring in nicotine which is attached with pyridine ring can be attached in two ways as follows:

N

N

CH3

-linkage -linkage

N N

CH3(I) (II)

There are such chemical reaction which are support and attachement of pyrrolidine ring.

(a) When the nicotine zinc chloride is distilled then yields pyridine, pyrrole and methylamine. This reaction confirm that side chain is pyrrole ring derivative.

(b) Secondary if the nicotine is heated with HI at 150°C then methyl iodide is obtained. This confirm that nicotine side chain possess N-methyl group.

(c) When the nicotine isomethiodide is undergoes reaction with CrO3 or K3[Fe(CN)6] then it gives hygrinic acid.


13



N

N

CH3 N

COOHKMnO4

Nicotine Nicotinic acid


N

N

CH3

-linkage -linkage

N N

CH3(I) (II)







NOTES

Alkaloids




14

N

N

CH3

K3[Fe(CN)6]

CH3

N

N

CH3

CH3

O

Nicotine isomethiodide Nicotone

CrO3

N

CH3

C

O

HO

Hygrinic acid Further the correct structure of nicotine is supported by the synthesis.

(I) Craig's synthesis:

Further the correct structure of nicotine is supported by the synthesis.(I) Craig's synthesis:

15

Hygrinic acid

N

CN

+ BrMg.CH2CH2CH2–O–C2H5

-ethoxy magnesium bromide

H2O

N

C

H2C CH2

CH2

OC2H5

O

–H2O NH2OH

N

C

H2C CH2

CH2

OC2H5N.OHZn/CH3COOH

N

H2C CH2

CH2

OC2H5NH2

HBr

150°C

N

H2C CH2

CH2

BrNH2

N

N

H

(i) CH3I

(ii) NaOHN

N

CH3

Nornicotine Nicotine 1.9.2 Atropine:

Atropine is known as Solanaceous alkaloid. Atropine is mostly found in Atropa belladonna, Datura stramonium, and other plants of Solanaceae family with the hyocyamine. Atropine is optically active that is exist in form of (±) – hyocyamine.

(I) Molecular formula: C17H23NO3

(II) Molecular structure: N Me O C

O

CHC6H5

CH2OH

When the atropine is treated with barium hydroxide solution then it yields racemic (±) tropic acid and optically inactive tropine alcohol (tropanol). So, we can say that atropine is a tropane ester of tropic acid.

Atropine


NOTES





15

Hygrinic acid

N

CN

+ BrMg.CH2CH2CH2–O–C2H5

-ethoxy magnesium bromide

H2O

N

C

H2C CH2

CH2

OC2H5

O

–H2O NH2OH

N

C

H2C CH2

CH2

OC2H5N.OHZn/CH3COOH

N

H2C CH2

CH2

OC2H5NH2

HBr

150°C

N

H2C CH2

CH2

BrNH2

N

N

H

(i) CH3I

(ii) NaOHN

N

CH3

Nornicotine Nicotine 1.9.2 Atropine:



(II) Molecular structure: N Me O C

O

CHC6H5

CH2OH



16

N Me O C

O

CHC6H5

CH2–OH

Ba(OH)2N Me OH

+

CHC

O

HOC6H5

CH2–OH

(±) - Tropic acid For the structure elucidation of atropine we have to study separately tropic acid and tropine.

Tropic acid: Molecular formula: C9H10O3

Molecular structure: CHC

O

HOC6H5

CH2–OH

Following points should be studied for the explanation of structure of tropic acid:

(a) It has monocarboxylic acid: Tropic acid react with one equivalent of alkali and forms monoester with alcohol.

(b) It has one primary hydroxyl group: Tropic acid react with one equivalent of acetic acid to form monoacetate and with one equivalent of benzoic acid to give monobenzoate. When the tropc acid is heated then removal of water molecule and yields an unsaturated carboxylic acid (atropic acid). When the atropic acid react with H2O then it gives two products.

C

CH2

COOHH2O

C

CH2OH

COOH

H

+ C

OH

COOH

CH3

Atropic acid (a) (b)

So, the correct structure of atropic acid is confirmed by its synthesis.

(I) Mackenzie and Wood synthesis:

For the structure elucidation of atropine we have to study separately tropic acid and tropine.Tropic acid: Molecular formula: C9H10O3

Molecular structure:

16

N Me O C

O

CHC6H5

CH2–OH

Ba(OH)2N Me OH

+

CHC

O

HOC6H5

CH2–OH




O

HOC6H5

CH2–OH




C

CH2

COOHH2O

C

CH2OH

COOH

H

+ C

OH

COOH

CH3




Following points should be studied for the explanation of structure of tropic acid:(a) It has monocarboxylic acid: Tropic acid react with one equivalent of alkali and

forms monoester with alcohol.(b) It has one primary hydroxyl group: Tropic acid react with one equivalent of acetic

acid to form monoacetate and with one equivalent of benzoic acid to give monobenzoate. When the tropc acid is heated then removal of water molecule and yields an unsaturated carboxylic acid (atropic acid). When the atropic acid react with H2O then it gives two products.

16

N Me O C

O

CHC6H5

CH2–OH

Ba(OH)2N Me OH

+

CHC

O

HOC6H5

CH2–OH




O

HOC6H5

CH2–OH




C

CH2

COOHH2O

C

CH2OH

COOH

H

+ C

OH

COOH

CH3




So, the correct structure of atropic acid is confirmed by its synthesis.(I) Mackenzie and Wood synthesis:

17

PhC

H3CO

HCNPh

CH3C

OH

CN

H2OPh

CH3C

OH

COOH

–H2O

H

CPh COOHCH2OH

KOHH

CPh COOHCH2Cl

HClether

CPh COOHCH2

Tropic acid Tropine (Tropanol): Molecular formula: C8H15NO

Molecular structure: N Me OH

Following points should be studied for structure determination of tropine.

(a) Presence of tertiary N-atom: Tropine react with one equyivalent of methyl iodide to give quartnary ammonium Iodide salt.

(b) Presence of alcoholic group: Tropine react with one equivalent of acetic acid to give monoacetate derivative. The secondary nature of (OH) group is confirmed by that tropine yields tropinone on oxidation which further undergoes oxidation to give tropinic acid (i.e. dicarboxylic acid).

N Me OH(O)

CrO3N Me O

(O)N–Me

COOH

COOHTropanol Tropinone Tropinic acid

When the tropinic acid undergoes Hoffmann exhaustive methylation then it yields pimelic acid.

N–Me

COOH

COOH

H.E.M. COOH

COOH

4H COOH

COOH

Pinelic acid The original structure of tropine is confirmed by its synthesis.

NOTES

Alkaloids


Tropine (Tropanol): Molecular formula: C8H15NO

Molecular structure:

17

PhC

H3CO

HCNPh

CH3C

OH

CN

H2OPh

CH3C

OH

COOH

–H2O

H

CPh COOHCH2OH

KOHH

CPh COOHCH2Cl

HClether

CPh COOHCH2






N Me OH(O)

CrO3N Me O

(O)N–Me

COOH



N–Me

COOH

COOH

H.E.M. COOH

COOH

4H COOH

COOH





17

PhC

H3CO

HCNPh

CH3C

OH

CN

H2OPh

CH3C

OH

COOH

–H2O

H

CPh COOHCH2OH

KOHH

CPh COOHCH2Cl

HClether

CPh COOHCH2






N Me OH(O)

CrO3N Me O

(O)N–Me

COOH



N–Me

COOH

COOH

H.E.M. COOH

COOH

4H COOH

COOH



17

PhC

H3CO

HCNPh

CH3C

OH

CN

H2OPh

CH3C

OH

COOH

–H2O

H

CPh COOHCH2OH

KOHH

CPh COOHCH2Cl

HClether

CPh COOHCH2






N Me OH(O)

CrO3N Me O

(O)N–Me

COOH



N–Me

COOH

COOH

H.E.M. COOH

COOH

4H COOH

COOH


The original structure of tropine is confirmed by its synthesis.Robinson synthesis:

18

Robinson synthesis:

CHO

CHO

+ CH3NH2 +

OC

C O

O

O

N Me O

COO

COO

–2CO2

N Me ONaBH4

N Me OH

Finally tropic acid and tropanol are heated in presence of HCl to give atropine.

N Me OH + HOC

O

CHC6H5

CH2OH

HClN Me OC

O

CHC6H5

CH2OH

Tropanol Tropic acid Atropine Streochemistry of tropine: When the tropinone undergoes reduction then it yields

mixture of two alcohols (a) Tropine (b) -tropine (Pseudotropine). The formation of product depend on the nature of reducing agent which are explain as follow:

NOTES




18

Robinson synthesis:

CHO

CHO

+ CH3NH2 +

OC

C O

O

O

N Me O

COO

COO

–2CO2

N Me ONaBH4

N Me OH


N Me OH + HOC

O

CHC6H5

CH2OH

HClN Me OC

O

CHC6H5

CH2OH

Tropanol Tropic acid Atropine Streochemistry of tropine: When the tropinone undergoes reduction then it yields

mixture of two alcohols (a) Tropine (b) -tropine (Pseudotropine). The formation of product depend on the nature of reducing agent which are explain as follow:

Streochemistry of tropine: When the tropinone undergoes reduction then it yields mixture of two alcohols (a) Tropine (b) -tropine (Pseudotropine). The formation of product depend on the nature of reducing agent which are explain as follow:

19

N Me O

Tropinone

N HOH

Me

N HOH

Me

-tropine (syn) Tropine (anti)

Reducing agents Reducing agents

(a) Catalytic hydrogenation (Pt, H2) (a) Na-Hg (b) Electrolytic reduction (b) Na/C2H5OH (c) Zinc dust / HI (c) LiAlH4/NaBH4 (d) LiAlH4/NaBH4

The tropine and -tropine both are epimer of each other in which one have N-methyl and (OH) insame side and another have in opposite side. The both forms possess plane of symmetry, so both are optically inactive (due to internal compensation).

Bose et al. suggested the chair conformation of tropine and -tropine in which

(OH) group is equatorial in -tropine and at axial in tropine.

NMe

H

OH

NMe

OH

H

-tropine Tropine For tropine the predominant conformation is chair form but it also exist in minor amount in boat form.

Reducing agents Reducing agents (a) Catalytic hydrogenation (Pt, H2) (a) Na-Hg (b) Electrolytic reduction (b) Na/C2H5OH (c) Zinc dust / HI (c) LiAlH4/NaBH4

(d) LiAlH4/NaBH4

The tropine and -tropine both are epimer of each other in which one have Nmethyl and (OH) insame side and another have in opposite side. The both forms possess plane of symmetry, so both are optically inactive (due to internal compensation).

Bose et al. suggested the chair conformation of tropine and -tropine in which (OH) group is equatorial in -tropine and at axial in tropine.

19

N Me O

Tropinone

N HOH

Me

N HOH

Me

-tropine (syn) Tropine (anti)

Reducing agents Reducing agents

(a) Catalytic hydrogenation (Pt, H2) (a) Na-Hg (b) Electrolytic reduction (b) Na/C2H5OH (c) Zinc dust / HI (c) LiAlH4/NaBH4 (d) LiAlH4/NaBH4

The tropine and -tropine both are epimer of each other in which one have N-methyl and (OH) insame side and another have in opposite side. The both forms possess plane of symmetry, so both are optically inactive (due to internal compensation).

Bose et al. suggested the chair conformation of tropine and -tropine in which

(OH) group is equatorial in -tropine and at axial in tropine.

NMe

H

OH

NMe

OH

H

-tropine Tropine For tropine the predominant conformation is chair form but it also exist in minor amount in boat form.

For tropine the predominant conformation is chair form but it also exist in minor amount in boat form.

NOTES

Alkaloids


20

NMe

H

OH

NMe

OH

H

NMe

H

OH

-tropine Tropine chair form Boat form 1.9.3 Morphine:

Morphine is an opium alkaloid which contain phenanthrene nucleus. So morphine also called phenanthrene alkaloid.


(II) Molecular structure: ON Me

HO

HO

(III) Nature of N-atom: Morphine react with one equivalent of methyl iodide, so it is confirmed that nature of N-atom is tertiarly. Morphine also undergoes Hoffmann exhaustive methylation.

(IV) Nature of oxygen atom: When Morphine react with acetyl chloride and benzoyl chloride then it yields diacetyl (heroin) and dibenzoyl derivative. It confirm the presence of two hydroxyl group. Now it is not clear that (OH) group is phenolic or alcoholic. Morphine gives monosodium salt and also produce colour with FeCl3, the both reaction reveals that one of the (OH) group is phenolic in nature. When the another (OH) group is treated with halo acid then yields halogen derivative and also react with methyl iodide to form methoxyl derivative (codeine). The methoxyl derivative undergoes oxidation to give a ketone, means it is secondary in nature. The third oxygen in morphine is unreactive so, it is present as ether linkage.

(V) Presence of benzene ring: When morphine undergoes bromination then it gives monobrominated product not addition product.

MorpHine




20

NMe

H

OH

NMe

OH

H

NMe

H

OH

-tropine Tropine chair form Boat form 1.9.3 Morphine:



(II) Molecular structure: ON Me

HO

HO



(V) Presence of benzene ring: When morphine undergoes bromination then it gives monobrominated product not addition product.



(V) Presence of benzene ring: When morphine undergoes bromination then itgives monobrominated product not addition product.

(VI) Presence of phenanthrene nucleus: When the morphine distilled with zinc dust then yield phenanthrene. It can be confirmed on the basis of following facts:

21

(VI) Presence of phenanthrene nucleus: When the morphine distilled with zinc dust then yield phenanthrene. It can be confirmed on the basis of following facts:

When codeine methiodide is boiled with NaOH then -methyl morphinethine is obtained which further heated with acetic anhydride to give methylmorphol and ethanol dimethyl amine.

Morphine undergoes a chemical change with concentrated HCl to produce apomorphine via rearrangement which takes place through dehydration.

ON Me

HO

HO

conc. HCl

HO

NMe

HO

Morphine Apomorphine

Synthesis of morphine:

Gates et al. synthesis:

When codeine methiodide is boiled with NaOH then a-methyl morphinethine is obtained which further heated with acetic anhydride to give methylmorphol and ethanol dimethyl amine.

NOTES



Morphine undergoes a chemical change with concentrated HCl to produce apomorphine via rearrangement which takes place through dehydration.

Synthesis of morphine:Gates et al. synthesis:

22

HO

OH

(i) Ph CO

Cl

(ii) C5H5N(iii) NaNO2/AcOH(iv) H2/Pd-C

HO

OBz

H2N(i) FeCl3

(ii) SO2/MeOH

MeO

OBz

MeO

(i) KOH/HCl(ii) NaNO2/AcOH(iii) H2/Pd-C(iv) FeCl3

MeO

O

MeO O(i) CH2CN–COOEt

(ii) K3[Fe(CN)6]

(iii) KOH/MeOH–H2O

(iv) HCl

MeO

O

MeO O

CN

CH2=CH–CH=CH2

MeO

OH

MeO O

CN(i) H2/CuCrO2

(ii) W.K.R.

(iii) LiAlH4

N Me

MeO

MeO (i) H2SO4

(ii) KOHN Me

MeO

HO

HO

(i) Oppenaur oxi.(ii) Br2/AcOH

N Me

MeO

HO

O

Br

Br

(i) DNP / H

(ii) H2, PtN Me

MeO

HO

O

Br

Br2

AcOHN Me

MeO

HO

O

Br

Br

Br

(iii) Me2SO4

ON Me

MeO

HOCodeine

(i) DNP

(ii) H(iii) LiAlH4/THF

C5H5N.HCl

220°CMorphine

1.10 Summary

NOTES

Alkaloids


2.10 SUMMARYAlkaloids are obtained from natural sources mostly from plants. The separation of alkaloid from plant involve many chemical process, so it is a time consuming process. When the alkaloidal extract is obtained then it will be purified by many extraction process. The alkaloids may affect many plant activities like growth, metabolism and reproductive process. This chapter covers the study of alkaloids e.g. nicotine, atropine and morphine under molecular structure, separation, chemical reaction, stereochemistry and synthesis.

2.11 REVIEW QUESTIONS 1. What are alkaloids ? Give the details of nomenclature of alkaloids

2. Discuss the separation techniques of alkaloids

3. Write short note on following points

(i) Hoffman exhaustive methylation method

(ii) Emde's degradation

(iii) Von-Braun's method

4. Give the details of general methods of structure elucidation of alkaloids.

5. Explain the following in nicotine

(a) Presence and nature of nitrogen atom

(b) Presence of pyridine-pyrrolidine nucleus

(c) Presence of benzene ring

(d) Synthesis of nicotine

6. Explain the occurrence, general structure and stereochemistry of atropine

7. Discuss the presence of following in morphine –

(a) Presence of hydroxyl group

(b) Presence of ethylenic bond

(c) Presence and nature of N-atom

(d) Presence of phenanthrene nucleus

2.12 FURTHRE READINGSzz Organic chemistry, Volume 2 : Stereochemistry and the chemistry of natural


zz Organic chemistry – J. Calyden, Greeve, S. Warren and Others (Oxford University Press) 2001.

zz Chemistry of the alkaloids, Pelletier ed., Van Nostrand Reinhold Co. (1970).

zz The plant alkaloids, Churchill (1949, 4th ed.).

NOTES



CHAPTER – 3

VITAMINSSTRUCTURE

3.1 Learning Objectives

3.2 Introduction

3.3 General Functions of Vitamines

3.4 General Properties of Vitamins

3.5 Classification of Vitamins

3.6 Summary




zz To definition the general functions of vitamines

zz To discuss general properties of vitamins

zz The understand the classification of vitamins

3.2 INTRODUCTIONVitamins are organic substances which are necessary for the proper functioning of our body or an organic chemical compound is called a vitamin when the organism cannot synthesize the compound in sufficient quantities, and must be supplied through the diet. For example, ascorbic acid (vitamin C) is a vitamin for humans, but not for most other animal organisms. Vitamins are important nutrients found to be essential for life. Unlike other classes of nutrients, vitamins serve no structural function nor provide significant energy. Common food forms of most vitamins require some metabolic activation into a functional form. Vitamines have closely related chemical or functional similarities. Some vitamins function as coenzymes, others function as antioxidants, although some vitamins (A and D), act as hormones. In total, you need 13 vitamins for good health, and they were initially named in the alphabetical order that they were discovered. Since their initial discovery, this order has gone through some revisions, and the vitamins got somewhat shuffled around and classified into two main groups. So, the purpose of this lesson is to sort through the alphabet soup of vitamins and provide some help to remember how they are classified. The term vitamin

NOTES

Vitamins


neither includes the essential nutrients, such as dietary minerals essential fatty acids, or essential amino acids (which are needed in greater amounts than vitamins) nor the great number of other nutrients that promote health, and are required less often to maintain the health of the organism. Universally thirteen vitamins are recognized at present. Vitamins are classified on the basis of their biological and chemical properties, not their structure. Thus, each "vitamin" refers to a number of vitamer compounds that all show the biological activity associated with a particular vitamin. So vitamins are grouped under an alphabetized vitamin "generic descriptor" title, such as "vitamin A", which includes the compounds retinal, retinol, and four known carotenoids. Some of the vitamines are listed below with their discovery dates sources:

Year of discovery Vitamin Food source

1913 Vitamin A (Retinol) Cod liver oil

1913 Vitamin B1 (Thiamine) Rice bran

1920 Vitamin C (Ascorbic acid) Citrus, most fresh foods

1920 Vitamin D (Calciferol) Cod liver oil

1920 Vitamin B2 (Riboflavin) Meat, dairy products, eggs

1922 (Vitamin E) (Tocopherol) Wheat germ oi l , unref ined vegetable oils

1926 Vitamin B12 (Cobalamins) Liver, eggs, animal products

1939 Vitamin K1 (Phylloquinone) Leafy green vegetables

1931 Vitamin B5 (Pantothenic acid) Meat, whole grains, in many foods1931 Vitamin B7 (Biotin) Meat, dairy products, eggs

1934 Vitamin B6 (Pyridoxine) Meat, dairy products

1936 Vitamin B3 (Niacin) Meat, grains

1942 Vitamin B9 (Folic acid) Leafy green vegetables

3.3 GENERAL FUNCTIONS OF VITAMINESVitamins perform many diverse physiochemical and biochemical functions. For eg. vitamin D, have hormone-like functions which act as regulators of mineral metabolism, or regulators of cell and tissues growth. Vitamin E and sometimes vitamin C used as antioxidants. The largest number of vitamins, the B complex vitamins, function as precursors for enzyme cofactors, that help enzymes in their work as catalysts in metabolism. Vitamins may be tightly bound to enzymes as part of prosthetic groups, For eg. biotin is part of enzymes involved in making fatty acids. Vitamines may be less tightly bound to enzyme catalysts as coenzymes,easily detachable molecules that function to carry chemical groups or electrons between molecules. For example, folic acid may carry methyl, formyl, and methylene groups in the cell. Vitamines also role in the assisting of enzymesubstrate reactions.

NOTES



Vitamins are essential for the normal growth and development of a multicellular rganism. For eg. a fetus begins to develop, at the moment of conception, from the nutrients it absorbs. It requires certain vitamins and minerals for growth. These nutrients facilitate the chemical reactions that produce among other body parts, skin, bones and muscle. If there is serious deficiency in one or more of these nutrients, a child may develop a deficiency disease. Even minor deficiencies may cause permanent damage of the organs. Once growth and development are completed, vitamins remain essential nutrients for the healthy maintenance of the cells, tissues, and organs that make up a multicellular organism. Dietary supplements contain vitamins, but may also include other ingredients, such as minerals, herbs. Scientific evidence supports the benefits of dietary supplements for persons with certain health conditions. In some cases, vitamin supplements may have unwanted effects, especially if taken before surgery, with other dietary supplements or medicines, or if the person taking them has certain health conditions. They may also contain levels of vitamins many times higher, and in different forms, than one may ingest through food.Naming of some vitamins:

Previous name Chemical name Cause of name changing

Vitamin B4 Adenine DNA metabolite; synthesized in body

Vitamin B8 Adenylic acid DNA metabolite; synthesized in body

Vitamin F Essential fatty acids Needed in large quantities (doesnot fit the definition of a vitamin).

Vitamin G Riboflavin Reclassified as Vitamin B2

Vitamin H Biotin Reclassified as Vitamin B7

Vitamin J Catechol, Flavin Catechol nonessential; flavin reclassifiedas Vitamin B2

Vitamin L1 Anthranilic acid Non essential

Vitamin L2 Adenylthiomethylpentose RNA metabolite; synthesized in body

Vitamin M Folic acid Reclassified as Vitamin B9

Vitamin O Carnitine Synthesized in body

Vitamin P Flavonoids No longer classified as a vitamin

Vitamin PP Niacin Reclassified as Vitamin B3

Vitamin S Salicylic acid Proposed inclusion of salicylate as anessential micronutrient

Vitamin U S-Methylmethionine Protein metabolite; synthesized in body

NOTES

Vitamins


3.4 GENERAL PROPERTIES OF VITAMINSVitamin A is a thermally stable in oxygen-free environment and can tolrate the heat of 60, 100 and 120°C. But, at the air at higher temperatures (about 60°C) it decomposes rapidly, especially under acidic conditions. Sunlight also promotes vitamin A decomposition.

94

Previous name

Chemical name Cause of name changing

Vitamin L2 Adenylthiomethylpentose RNA metabolite; synthesized in body

Vitamin M Folic acid Reclassified as Vitamin B9

Vitamin O Carnitine Synthesized in body

Vitamin P Flavonoids No longer classified as a vitamin

Vitamin PP Niacin Reclassified as Vitamin B3

Vitamin S Salicylic acid

Proposed inclusion of salicylate as an essential micronutrient

Vitamin U S-Methylmethionine Protein metabolite; synthesized in body

5.3 General properties of vitamins

Vitamin A is a thermally stable in oxygen-free environment and can tolrate the heat of 60, 100 and 120°C. But, at the air at higher temperatures (about 60°C) it decomposes rapidly, especially under acidic conditions. Sunlight also promotes vitamin A decomposition.

Vitamin A

This chapter introduce with the fundamental chemistry of the vitamins i.e. water soluble and fat soluble e.g. tocopherols and tocotrienols relevant to their

antioxidant action. The general agreement that α-tocopherol is the most efficient antioxidant and vitamin E homologue in vivo, there was always a considerable discrepancy in its absolute and relative antioxidant effectiveness in vitro, especially

when compared to γ-tocopherol. Many chemical, physical, biochemical, physicochemical, and other factors seem responsible for the observed discrepancy between the relative antioxidant potencies of the tocopherols in vivo and in vitro.

Vitamin A

This chapter introduce with the fundamental chemistry of the vitamins i.e. water soluble and fat soluble e.g. tocopherols and tocotrienols relevant to their antioxidant action. The general agreement that α-tocopherol is the most efficient antioxidant and vitamin E homologue in vivo, there was always a considerable discrepancy in its absolute and relative antioxidant effectiveness in vitro, especially when compared to γ-tocopherol. Many chemical, physical, biochemical, physicochemical, and other factors seem responsible for the observed discrepancy between the relative antioxidant potencies of the tocopherols in vivo and in vitro.

This paper aims at highlighting some possible reasons for the observed differences between the tocopherols (α-, β-, γ-, and δ-) in relation to their interactions with the important chemical species involved in lipid peroxidation, specifically trace metal ions, singlet oxygen, nitrogen oxides, and antioxidant synergists. Although literature reports related to the chemistry of the tocotrienols are quite known, they also were included in the discussion in virtue of their structural and functional resemblance to the tocopherols.

Solubilities of eight different species of the fat-soluble vitamins A, D, E, and K in supercritical carbon dioxide were measured atdifferent range of temperature and pressure. Solubilities have been determined by an analytical method using the direct coupling of an equilibrium cell to a supercritical fluid chromatographic system with UV detection.

3.5 CLASSIFICATION OF VITAMINSFourteen substances are in nature now generally recognized as vitamins. Vitamins are described according to their solubility, they may be fat or water soluble. This method of classification and their discovery as labeled by McCollum as "fatsoluble A" and "water-soluble B." Our body needs to consume 14 different vitamins to maintain normal health The important vitamins are the vitamin B complex (folate, B12, B6, biotin, pantothenic acid, niacin, riboflavin and thiamine) as well as vitamins A, C, D, E and K. The each vitamin is essential for different functions of the animal and human body. In most cases, people are able to get sufficient vitamins simply from consuming a diet that is well-balanced. The main classification for vitamins is based on solubility as some are soluble in water while others are soluble in fat. The vitamins which are soluble in fat are stored by the body and therefore can accumulate. On the other hand, the kidneys flush out water soluble vitamins. Another way

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that some people classify vitamins is based on how they were obtained: either from food or naturally from food. This method, however, can become complicated because many of the foods we consume on a daily basis are vitamin fortified. On the basis of their soloubilities vitamines can be classified in to two categories.

FAt Soluble VitAMines

Vitamins are classified on the basis of their solubility or in other words, the vitamin's ability to dissolve into another substance (solvents). For eg. fat-soluble vitamins are the vitamins which dissolve in fat. Because fat is easily stored in our body, fat-soluble vitamins can be stored within your fat. This means they can accumulate and be saved for later use. Because the body stores fat-soluble vitamins in its cells, they are not flushed out as simply as the water-soluble vitamins. This means that they do not require as frequent ingestion as water-soluble vitamins but you still need sufficient amounts. It is important to remember that consuming too much of fat-soluble vitamins can cause toxicity. We are particularly sensitive to high levels of vitamin D as well as high levels of vitamin A specifically from animal sources. Simply consuming a balanced diet should provide sufficient fatsoluble vitamins.

The fat-soluble vitamins are A, D, E and K. Now these four letters represent four different vitamins, but if you try to pronounce them like they spell a word, then you might pronounce them as 'attic.' So, a great little memory jogger for recalling the fat-soluble vitamins is 'The fat cat is in the ADEK (attic).' These vitamins are important for the normal functioning of your body. For example, carrots are so important for eyes, so you could see better at night. This is because carrots contain vitamin A, which helps with vision. Playing in sunshine may be helpful for skin.This was also good advice because exposure to the sun helps your body make vitamin D, which is a vitamin that helps calcium absorption for healthy bones. Vitamin E helps with your 'immunity-E' because it works as an antioxidant protecting your cells from free radicals. And vitamin K is needed for blood clotting, or would it help you recall this fact if you spelled clotting with a 'K' and thought of vitamin K as the blood 'K-lotting' vitamin.

Vitamin Name Absorbed Function

Vitamin A Vitamin A helps with healthy mucous membranes and skin, vision, tooth and bone growth and the health of the immune system.

From retinol (animal sources): liver, eggs, fortified margarine, butter, cream, cheese, fortified milk.From plant sources (beta-carotene): dark orange vegetables (pumpkin, sweet potatoes, winter squash, carrots) and fruits (cantaloupe, apricots); dark green leafy vegetables.

Vitamin K Vitamin K is required for correct blood clotting.

Vegetables from the cabbage family, leafy green vegetables, milk; it is also produced in the intestinal tract by the bacteria.

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Vitamins


Vitamin E Vitamin E is an antioxidant and helps protect the cell walls.

Nuts and seeds, egg yolks, liver, wholegrain products, wheat germ, leafy green vegetables and polyunsaturated plant oils (safflower, cottonseed, corn, soybean).

Vitamin D Vitamin D is stored in the bones and is required to properly absorb calcium.

Fortified margarine, fortified milk, fatty fish, liver, egg yolks; the skin can also produce vitamin D when it is exposed to sunlight.

Vitamin A: The active forms of vitamin A participate in three essential functions: visual perception, cellular differentiation, and immune function. A number of food sources are available for vitamin A. Preformed vitamin A is abundant in animal foods and provitamin A carotenoids are abundant in dark-colored fruits and vegetables.

Dietary forms of vitamin A and provitamin A carotenoids

Consumed Absorbed BioconvertedDietary or supplemental Vitamin A Retinol RetinolSupplemental beta-carotene beta-carotene Retinol

Dietary beta-carotene beta-carotene Retinol

Dietary alpha-carotene or betacryptoxanthin

alpha-carotene or betacryptoxanthin

Retinol

Relative carotene concentration increases when consumed with oil or associated with plant matrix material. That is part of the plant vitamin source, not separated out as a supplement. The presence of dietary fat stimulates the secretion of bile acids and improves the absorption of carotenoids.

The richest sources of vitamin A are fish oil, liver, and other organ meats. Whole milk, butter, and fortified margarine and low-fat milks are also rich in the vitamin

In the United States carrots, fortified spreads, and dairy products are the leading sources of vitamin A to the diet.

Vitamin D. Vitamin D is essential for life in human beings and animals. It is one of the most important regulators of calcium homeostasis and was historically considered the "anitrachitic" factor. The biological effects of vitamin D are achieved only by its hormonal metabolites, including two key kidney-produced metabolites: 1,25(OH)2 vitamin D and 24,25(OH) vitamin D. In addition to its role in calcium metabolism, research has identified that vitamin D plays an important role in cell differentiation and growth of keratinocytes and cancer cells and has shown that it participates in the process of parathyroid hormone and insulin secretion (Bouillon et al. 1995).

Vitamin D3, the naturally occurring form of the vitamin, is produced from the provitamin, 7-dehydrocholesterol, found in the skin under the stimulation of ultraviolet

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(UV) irradiation or UV light. Vitamin D2 is a synthetic form of vitamin D that is produced by irradiation of the plant steroid ergosterol. A requirement for vitamin D has never been precisely defined because vitamin D is produced in the skin after exposure to sunlight. Therefore, humans do not have a requirement for vitamin D when sufficient sunlight is available. The fact that humans wear clothes, live in cities where tall buildings block the sunlight, use synthetic sunscreens that block UV rays, and live in geographical regions of the world that do not receive adequate sunlight contributes to the inability of the skin to synthesize sufficient vitamin D.

A substantial proportion of the U.S. population is exposed to suboptimal levels of sunlight during the winter months. Under these conditions, vitamin D becomes a true vitamin and must be supplied regularly in the diet.

The use of 1,25 (OH)2 vitamin D for treatment of hypoparathyroidism, vitamin D– resistant rickets, renal osteodystrophy, osteoporosis, and psoriasis opens the door for potential toxicity because this form of the vitamin is much more toxic and the body's metabolic controls are bypassed. When this medication is being used, careful monitoring of plasma calcium concentrations is required.

Salt-water fish are good unfortified sources of vitamin D. Small quantities are derived from eggs, beef, butter, and vegetable oils. Fortification of milk, butter, margarine, cereals, and chocolate mixes help in meeting the dietary requirements. Excessive amounts of vitamin D are not available in usual dietary sources.

However, excessive amounts can be obtained through supplements that result in high plasma levels of 25(OH) vitamin D.

Vitamin E. Vitamin E (also called tocopherol) is found in cell membranes and fat depots. Because of their chemical structure, there are eight stereoisomers of each of the tocopherols. In addition to each of the stereoisomers, each occur in alpha, beta, gamma, and delta forms.

The various forms of vitamin E have different biological activity, with the natural source isomer—R,R,R,-alpha-tocopherol—being the most active. In supplements you may see this isomer called by its former name, d -alpha-tocopherol. Synthetic vitamin E is called ll -rac -alpha-tocopherol or dl -alpha-tocopherol in supplements. Because of the many forms of vitamin E in plants available synthetically, the relative activities of each form is complex. Current evidence indicates that vitamin E from natural sources has approximately twice the bioactivity in humans that the ll-rac (synthetic) vitamin does.

The tocopherol content of foods varies widely depending on storage, processing, and preparation. The best sources of vitamin E are the common vegetable oils and products made from them. However, most of the tocopherols may be removed in processing. Wheat germ and walnuts also have high amounts of tocopherols.

Vitamin K. Vitamin K was named after the first letter of the German word Koagulation. For many years blood coagulation was assumed to be the sole physiological role for vitamin

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Vitamins


K. We now know that vitamin K plays an essential role in the synthesis of proteins including prothrombin and the bone-forming protein, osteocalcin. Dietary vitamin K absorption is enhanced by dietary fat and is dependent on bile and pancreatic enzymes. The human gut contains large amount of bacterially produced vitamin K, but its contribution to the maintenance of vitamin K status has been difficult to assess. The vitamin K produced by bacteria in the gut is less biologically active even though it is stored in the liver and present in blood. Current understanding supports the view that vitamin K source may partially satisfy the human requirement but the contribution is much less than previously thought.

The drug warfarin, widely prescribed as an anticoagulant, functions through inhibition of vitamin K. As a result, alterations in vitamin K intake can influence the efficacy of warfarin. The effective dose of warfarin varies from individual to individual, as does the dietary intake of vitamin K. The best solution appears to be to establish the necessary dose of warfarin and urge patients to maintain a constant intake of foods high in vitamin K in their diets. Only a small number of food items contribute substantially to the dietary vitamin K.

Collards, spinach, and salad greens are high in vitamin K. Broccoli, Brussels sprouts, cabbage, and Bib lettuce contain about two-thirds as much, and other green vegetables contain even less. Vitamin K is also found in plant oils and N margarine, with soybean and canola oils having the highest amounts. U.S. food intake surveys indicate that spinach, collards, broccoli, and iceberg lettuce are the major contributors of vitamin K in the diet.

Vitamines and their Deficiency DiseasesVitamin A:

Properties :

zz Soluble in fat and insoluble in water

zz Viscous, colorless oil or pale yellowish substance

zz Heat stable in absence of air

Source of Vitamin A

Liver, heart, kidney, milk, codliver oils, fishliver-oils, butter, eggs, carrots, cabbage, vegetables, green leaves, mangoes, potatoes tomatoes, spinach, papaya etc.

Functions

zz Effect on reproductive processes, differentiation, and immune system

zz Essential for growth and night vision

zz Required for bone and teeth formation, influence genetic expression, Mreproduction to manufacture R.B.C etc.

zz Maintain the health and activity of epithelial tissues, and glands prevent infection, maintains nutrition and function of the nervous tissue.

zz Controls the action of bone cells and formation, helps in normal fertility and glucose synthesis.

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zz Acts as antioxidant.

zz Helps in RNA and protein metabolism.

Vitamin A Deficiency DiseasesNight-blindness, Xerophthalmia, Keratinisation of skin and mucous membrane.

Retardation of growth in children, defective growth of bone and teeth, skin lesions, Bitot's, sports etc.

Abnormalities in respiratory, GU and GI epithelium, Diarrhoea, Kidney stone, bladder disorders, infections of vagina, depression of immune reactions, anaemia, injury to brain and nerve causes paralysis, stunted skull and spine.

Vitamins D (Cholecalciferol)

Properties

Soluble in fat solvents but insoluble in water

Heat stable

White crystalline material

Ordinary boiling does not destroy it.

Source

Fish liver oils e.g., cod liver oil, halibut - liver oil etc. Butter, milk, eggs, liver. In sub coetaneous tissue, 7 dihydrocholesterol is conveted to vitamin D by UV light.

FunCtions

Control calcium and phosphorus absorption from the small intestine, concerned with calcium metabolism, helps in the bone and teeth formation.

Minimize the losses of calcium and increases phoshate excretion by the kidneys, affects insulin secretion in pancreases.

deFiCienCy disease

Causes Rickets (directive bone growth) in childless, osteomalacia in adults, disturbs calcium and phosphorous absorption.

A knowledge of the minerals, vitamins and other substances needed for the human body to function at optimum levels is very useful and worthy of deep study. It has been shown that, probably due to modern intensive farming methods, the levels of vitamins and minerals found in fresh vegetables etc is significantly lower now than they were in the past. These initial lower levels along with over cooking, processing, microwave cooking etc. means that it is a challenge in the modern age to eat healthily. Residual pesticides and chemical addictives that increase shelf life or "improve" flavor etc. further reduce the nutritional value of the food generally available. For this reason the wise person, avoiding morbid obbsession, takes an interest in the quality of food that they consume.

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Vitamins


Nutritional supplements, whilst largely unnecessary in past ages as a general rule, now have a place in the modern diet. Apart from any physical symptoms a diet lacking essential vitamins can effect the mental/psychological and emotional well being as well leading to depression, laziness, agression and poor memory etc etc. On this page I will collect a list of properties for the wide range of minerals and vitamins that are relevent to human wellbeing.

Water-Soluble VitaminsIf a vitamin is not fat-soluble, then it is classified as water-soluble vitamins. Because our body is a watery environment, these vitamins can move through our body very easily, and they can also be flushed out in urine. So, our body does not store water-soluble vitamins. Water-soluble vitamins are able to freely travel throughout the body and any unnecessory quantities are usually flushed out by the kidneys. Small doses of water-soluble vitamins are required by the body and this type of vitamin is not as likely to approach toxic levels as fat-soluble vitamins. In addition, vitamin C, choline, folate, vitamin B6 and niacin have higher consumption limits. Consumption of high levels of vitamin B6 during long periods of time can cause irreversible nerve damage.

Vitamins B-complex and vitamin C both are water-soluble vitamins. The B vitamins were initially thought to be just one vitamin, but later it was discoveredthat they were a group of vitamins with different characteristics, this is why B vitamins have numbers and different names. There are eight B vitamins, including vitamin B1, B2, B3, B5, B6, B7, B9 and B12. Some of the vitamins, such as vitamin B6 and B12, are usually referenced by their numbers, but all eight of these B vitamins have a corresponding name. In order, their names are thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folate and cobalamin. Now, remembering all of the names of the B vitamins can seem a bit like trying to figure out a riddle, so it might help you to recall the B vitamins by using the mnemonic, 'These Riddles Need Practice, Practice Builds Future Character.'

The B-complex vitamins are important for energy, so it might help you to think of the B vitamins as 'Busy bees that are full of energy.' More specifically the B vitamins convert energy from the nutrients you eat into ATP, which is the energy your body runs on. The majority of B vitamins are generally found in foods from all of the food groups; however, getting enough B12 can be a bit tricky. Vitamin B12 is lacking in grains, fruits and vegetables but found in meats and dairy products. Because of this, strict vegetarians sometimes have to plan their diets carefully to ensure that they are getting enough B12.

Most people are able to consume sufficient quantities simply by consuming a balanced diet. However some vegetarians as well as those over 50 years of age may require supplements for sufficient B12 intake.

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The B-complex vitamins are important for energy, so it might help you to think of the B vitamins as 'Busy bees that are full of energy.' More specifically the B vitamins convert energy from the nutrients you eat into ATP, which is the energy your body runs on. The majority of B vitamins are generally found in foods from all of the food groups; however, getting enough B12 can be a bit tricky. Vitamin B12 is lacking in grains, fruits and vegetables but found in meats and dairy products. Because of this, strict vegetarians sometimes have to plan their diets carefully to ensure that they are getting enough B12.

Most people are able to consume sufficient quantities simply by consuming a balanced diet. However some vegetarians as well as those over 50 years of age may require supplements for sufficient B12 intake.

Vitamin Name Benefits Dietary Sources

Ascorbic acid

(vitamin C)

Ascorbic acid is an antioxidant and it is a portion of an enzyme that is required for protein metabolism. It also helps with iron absorption and is important for the health of the immune system.

Only found in vegetables and fruits, especially: kiwifruit, mangoes, papayas, lettuce, potatoes, tomatoes, peppers, strawberries, cantaloupe and vegetables that are part of the cabbage family

Thiamine

(vitamin B1)

Thiamine is a portion of an enzyme that is required for energy metabolism and it is important for nerve function.

Found in moderate amounts in all of the nutritious foods: nuts and seeds, legumes, whole-grain/enriched cereals and breads, pork

Riboflavin

(vitamin B2)

Riboflavin is a portion of an enzyme that is required for energy metabolism. It is also important for skin health and normal vision.

Enriched, whole-grain cereals and breads, leafy green vegetables, milk products

Niacin

(vitamin B3)

Niacin is a portion of an enzyme that is required for energy metabolism. It is also important for skin health as well as the digestive and nervous systems.

Peanut butter, vegetables (particularly leafy green vegetables, asparagus and mushrooms), enriched or whole-grain cereals and breads, fish, poultry and meat

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Pantothenic Acid (vitamin B5)

Pantothenic acid is a portion of an enzyme that is required for energy metabolism

It is widespread in foods.

Pyridoxine

(vitamin B6)

Pyridoxine is a portion of an enzyme that is required for protein metabolism. It also helps with the production of red blood cells.

Fruits, vegetables, poultry, fish, meat

Folic Acid

(vitamin B9)

Folic acid is a portion of an enzyme that is required for creating new cells (particularly red blood cells) and DNA.

Liver, orange juice, seeds, legumes, leafy green vegetables. It is now added to many refined grains.

Cobalamin

(vitamin B12)

Cobalamin is a portion of an enzyme required for the production of new cells and it is important to the function of nerves.

Milk, milk products, eggs, seafood, fish, poultry, meat. It is not present in plant foods.

Biotin Biotin is a portion of any enzyme that is required for energy metabolism.

It is widespread in foods and can be produced by bacteria in the intestinal tract.

Properties of some water soluble vitamines are discussed below:

Thiamin. Thiamin was the first vitamin to be identified. In modern times, thiamin deficiency is seen most commonly in association with chronic alcoholism. Only a small percentage of large doses are absorbed, and elevated serum levels result in its active urinary excretion. After an oral dose of the vitamin, peak excretion occurs in about two hours.

There are no reports of adverse effects from the consumption of excess thiamin consumed in food or supplements. No upper level can be set due to the lack of reported findings associated with adverse effects.

Food sources from which most of thiamin is derived include enriched, fortified, or whole-grain products, such as bread, bread products, mixed foods that contain grain, and ready-to-eat cereals. Foods that are especially rich in thiamin include

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Vitamins


104

Pantothenic Acid (vitamin B5)

Pantothenic acid is a portion of an enzyme that is required for energy metabolism

It is widespread in foods.

Pyridoxine

(vitamin B6)

Pyridoxine is a portion of an enzyme that is required for protein metabolism. It also helps with the production of red blood cells.

Fruits, vegetables, poultry, fish, meat

Folic Acid

(vitamin B9)

Folic acid is a portion of an enzyme that is required for creating new cells (particularly red blood cells) and DNA.

Liver, orange juice, seeds, legumes, leafy green vegetables. It is now added to many refined grains.

Cobalamin

(vitamin B12)

Cobalamin is a portion of an enzyme required for the production of new cells and it is important to the function of nerves.

Milk, milk products, eggs, seafood, fish, poultry, meat. It is not present in plant foods.

Biotin Biotin is a portion of any enzyme that is required for energy metabolism.

It is widespread in foods and can be produced by bacteria in the intestinal tract.

Properties of some water soluble vitamines are discussed below:

Thiamin. Thiamin was the first vitamin to be identified. In modern times, thiamin deficiency is seen most commonly in association with chronic alcoholism. Only a small percentage of large doses are absorbed, and elevated serum levels result in its active urinary excretion. After an oral dose of the vitamin, peak excretion occurs in about two hours.


Food sources from which most of thiamin is derived include enriched, fortified, or whole-grain products, such as bread, bread products, mixed foods that contain grain, and ready-to-eat cereals. Foods that are especially rich in thiamin include

Properties of some water soluble vitamines are discussed below:Thiamin. Thiamin was the first vitamin to be identified. In modern times, thiamin

deficiency is seen most commonly in association with chronic alcoholism. Only a small percentage of large doses are absorbed, and elevated serum levels result in its active urinary excretion. After an oral dose of the vitamin, peak excretion occurs in about two hours.


Food sources from which most of thiamin is derived include enriched, fortified, or whole-grain products, such as bread, bread products, mixed foods that contain grain, and ready-to-eat cereals. Foods that are especially rich in thiamin include yeast, lean pork, and legumes. Thiamin is absent from fats, oils, and refined sugars while Milk, milk products, seafood, fruits, and vegetables are not good sources for thiamin.

Riboflavin. The second vitamin was named vitamin B2 or riboflavin. Most dietary riboflavin is consumed as a complex of food protein. Signs of riboflavin deficiency are sore throat, redness, and edema of the throat and oral mucous membranes, cheilosis (cracking of the skin around the mouth), and glossitis (red tongue). Vitamin B2 deficiency most often occurs in combination with other nutrient deficiencies. The B vitamins are quite interrelated; for example, niacin requires riboflavin for its formation from the amino acid tryptophan, and vitamin B6 requires riboflavin for conversion to the active coenzyme form. When riboflavin is absorbed in excess amount, then its little amount is stored in the body tissues. No adverse effects associated with riboflavin consumption from food or supplements have been reported. The greatest contribution of riboflavin from the diet comes from milk and milk drinks, followed by bread products and fortified cereals. Especially good food sources of riboflavin are eggs, lean meats, milk, broccoli, and enriched breads and cereals.

Niacin. The term "niacin" refers to nicotinamide and nicotinic acid. The coenzymes, the active form of niacin in the body, are synthesized in all tissues of the body. The amount of niacin in the body is the result of absorbed nicotinic acid and nicotinamide, as well as conversion of the amino acid tryptophan. Excess niacin is excreted through the urine.

Pellagra is the classical manifestation of niacin deficiency. Pellagra has been seen in areas where corn (low in niacin and tryptophan) is the dietary staple.

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Niacin, given as nicotinic acid in doses from 4 to 6 g/day, is one of the oldest drugs used in the treatment of hyperlipidemia, which consists of elevated blood levels of triglycerides and cholesterol. Niacin lowers low-density lipoprotein (LDL) cholesterol and triglyceride concentration. This therapeutic effect is not seen with nicotinamide. Nicotinic acid in therapeutic doses can cause flushing and headache in some people. These side effects are not harmful.

Dietary intake of niacin comes mainly from mixed dishes containing meat, poultry, or fish, followed by enriched and whole-grain breads, and fortified cereals. Significant amounts of niacin are found in red meat, liver, legumes, milk, eggs, alfalfa, cereal grains, yeast, and fish

Vitamin B6. Vitamin B6 is a coenzyme for more than 100 enzymes involved in the metabolism of amino acids, glycogen, and nerve tissues. Microcytic anemia, reflecting decreased hemoglobin synthesis, can be seen in deficiency states. The interaction of vitamin B6 and folate has been shown to reduce the plasma concentrations of homocysteine and decrease the incidence of cardiovascular disease.

This was probably an artifact of hormonal stimulation of tryptophan catabolismrather than vitamin B6 deficiency. At the time these studies were conducted, estrogen concentrations were three to five times higher in contraceptive agents than they are today.

No adverse effects have been associated with intakes of vitamin B6 from food However, large doses of pyridoxine used to treat carpal tunnel syndrome and premenstrual syndrome have been associated with sensory neuropathy. It appears that the risk of developing sensory neuropathy decreases quite rapidly at dosages below 1 g/day. Food sources of vitamin B6 include fortified, ready-to-eat cereals; mixed foods with meat, fish, or poultry as the main ingredient: white potatoes, starchy vegetables, and noncitrus fruits. Vitamin B6 is widely distributed in foods; good sources are meats, whole-grain products, vegetables, and nuts.

Folate. Folate is a vitamin B that exists in many chemical forms. Folic acid, the most stable form of folate, occurs rarely in food, but is the form used in supplements and fortified food products. Folate coenzymes are involved in numerous reactions that involve DNA synthesis, purine synthesis, and amino acid metabolism. The most well known is the conversion of homocysteine to methionine. It is this reaction that reduces the concentration of homocysteine in the plasma, and may lower the risk of cardiovascular disease. The metabolic interrelationship between folate and vitamin B12 may explain why a single deficiency of either vitamin leads to the same hematological changes. In either folate or vitamin B12 deficiency, megaloblastic changes occur in the bone marrow and other replicating cells.

Pregnant women are at risk for developing folate deficiency because of the heightened demands imposed by increased synthesis of DNA. Low folate status is associated with poor pregnancy outcome, low birth weight, and fetal growth retardation because of the possible incidence of neural tube defects during the preconception period (that is, just before and during the first 28 days of M conception), the Food and Nutrition Board recommends that women who are capable of becoming pregnant should consume 400 <g/day of synthetic folic acid, derived from dietary supplements or fortified food.

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Vitamins


Recommendations for intake of folate are dependent on variation in bioavailability. Supplemental folate is nearly 100 percent absorbed, while absorption of folate found in foods is only about 50 percent. Fortified foods approach the level of bioavailability of folate found in supplements. No adverse effects have been associated with the consumption of normal folate-fortified foods.

Folates are found in nearly all natural foods. Protracted cooking or processing may destroy folate. Foods with the highest folate content include yeast, liver, other organ meats, fresh green vegetables, and some fruits.

Vitamin B12. Vitamin B12 can be called as cyanocobalamin. This is the only vitamin B12 preparation used in supplements. An adequate supply of vitamin B12 is essential for normal blood formation and neurological function. The absorption of vitamin B12 is dependent on several physiological steps. In the stomach, foodbound vitamin B12 is dissociated from proteins in the presence of stomach acid. Vitamin B12 then binds with protein and in the intestine the vitamin B12 binds with intrinsic factor for absorption. If there is a lack of sufficient acid in the stomach or intrinsic factor in the intestine, malabsorption occurs and the resulting condition caused is pernicious anemia.

The anemia of vitamin B12 deficiency (completely reversed by addition of B12) is indistinguishable from that seen with folate deficiency. Because up to 30 percent of people older than fifty are estimated to have atrophic gastritis with low stomach acid secretion, older adults may have decreased absorption of B12 from foods. Thus, it is recommended that most of the vitamin B12 consumed by adults greater than fifty-one years of age be obtained from fortified foods or supplements.

Vitamin B12 is present in all forms of animal tissues. It is not present in plants and thus does not occur in fruits or vegetables. Because a generous intake of animal foods is customary in the United States, B12 intake from foods is usually adequate. People who avoid eating animal products may obtain most of their requirement through fortified foods.

Vitamin C. Ascorbic acid (the chemical name for vitamin C) is a potent antioxidant in animals and plants. Vitamin C is important in the synthesis of collagen. Some evidence indicates that vitamin C reduces virus activity by inhibiting viral replication. Many anecdotal reports support a role for vitamin C supplementation to reduce the severity of cold symptoms.

Some epidemiological evidence indicates that supplemental vitamin C protects against risk for myocardial infarction. However, large-scale epidemiological studies do not suggest a benefit of vitamin C supplementation on cardiovascular health risks.

Non-heme iron absorption from food is enhanced two-to threefold in the presence of 25 to 75 mg of vitamin C, presumably because of the ascorbate-induced reduction of ferric iron to ferrous iron, which is less likely to form insoluble complexes in the intestine. However, vitamin C has no effect on increasing iron absorption from heme iron. Unlike most animal species, humans lack the ability to synthesize ascorbic acid; thus, the diet is the sole source for this vitamin.

The current requirement of vitamin C is 90 mg/day for adult men and 75 mg/day for adult women. During pregnancy the RDA is 85 mg/day, and 120 mg/day during lactation.

NOTES



This level was set as a guideline for people using dietary supplements and was based on reports of gastrointestinal symptoms reported when too much vitamin C was taken.

Almost 90 percent of vitamin C in the diet comes from fruits and vegetables, with citrus fruits, tomatoes, tomato juice, and potatoes being the major contributors. It is also added to some processed foods as an antioxidant.

Pantothenic AcidPantothenic acid was named after the Greek, meaning "from everywhere," because it is so widespread in foods. Pantothenic acid is essential in the diet because of the inability of animals and humans to synthesize the pantoic acid moiety of the vitamin. Pantothenic acid plays a primary role in many metabolic processes, such as oxidative metabolism, cell membrane formation, Mcholesterol and bile salt production, energy storage, and activation of some hormones.

Pantothenic acid deficiency in humans is rare because of its ubiquitous distribution in foods. Many health claims are made regarding the role of pantothenic acid in ameliorating rheumatoid arthritis, lowering cholesterol, enhancing athletic performance, and preventing graying of hair. However, sufficient information is lacking at this time and so firm recommendations may not be made. No reports of adverse effects of oral pantothenic acid in humans have been reported.

As mentioned above, pantothenic acid is found in a wide variety of both plant and animal foods. Because of its thermal lability and susceptibility to oxidation, significant amounts are lost during processing. Rich food sources include chicken, beef, liver, and other organ meats, whole grains, potatoes, and tomato products. Biotin. In mammals, biotin serves as a coenzyme for reactions that control such important functions as fatty acid metabolism and gluconeogenesis. Biotin is recycled upon degradation of enzymes to which it is bound. Biotin from pharmaceutical sources is 100 percent bioavailable. Deficiency is rare but has been seen in patients on parenteral nutrition without biotin supplementation. Lipoic acid and biotin have structural similarities, thus competition potentially exists for intestinal or cellular uptake. This may be of concern in settings where large doses of lipoic acid are administered or taken as supplements. Biotin is distributed widely in natural foods. Those rich in biotin include egg yolk, liver, and some vegetables.

CholineCholine has been considered a nonessential nutrient because humans can synthesize sufficient quantities. However, when hepatic function is compromised, hepatic choline synthesis is decreased and thus choline is now considered "conditionally" essential. Nutrition Board study suggested that graded doses of choline intake be studied regarding their effects on organ function, plasma cholesterol, and homocysteine levels.

Choline functions as a precursor for phospholipids and acetylcholine, and betaine. Choline and choline-containing lipids, mainly phosphatidylcholine, are abundant in foods of both plant and animal origin. Rich sources include muscle and organ meats and eggs. To date there are no nationally representative estimates of choline intake from food or supplements.

NOTES

Vitamins


Vitamin C:

Propertieszz White crystals, soluble in water, heat lavishzz Good reducing agentszz Early oxidized at 1000C in presence of oxygenzz Cannot stand cooking or canning

Sources: Guava, amla, green chilli, amaranth leaves, citrus fruits, green vegetables, potatoes, tomatoes, cheese, milk etc.Functions :Acts as antioxidant.

zz Essential for formation of collagen present between cells.zz Necessary for the formation of osteoblasts and red blood cells.zz Helps to reduce the ferric iron (Fe3+) to ferrous iron (Fe2+) and is absorbed only

in this form.zz Essential for the utilization of folic acidzz Takes part in oxidation and reduction reactions in the tissues.zz Helps in bone formation.zz Helps in wound healing.zz Prevents formation of free radical in the body.

Deficiency Diseaseszz Scurvy, a disease characterized by sore, spongy gums, loose teeth, fragile blood

vessels, swollen joints, and anemia.zz Delay in wound healing.zz Pain in bones.zz Skin becomes rough and dry.zz Pyrexia, rapid pulse and susceptibility to infection.

Vitamin B1 (Thiamine):Properties

zz White, crystalline substancezz Water-solublezz Heat labilezz Unstable at high temperature and in alkaline mediumzz Stable in acid mediumzz On oxidation it gives a yellowish dye called thiochrome.

Sources :Rice polishing, dried yeast and wheat germ are rich sources of vit. B1. Whole cereals like wheat, oats, legumes, oil seeds and nuts are good sources. Milled cereals, vegetables, fruits, meat and fish are poor sources. On milling, vit. B1 is lost from cereals.

NOTES



Functions

zz Acts as a co-enzyme in carbohydrate metabolism

zz Require for the synthesis of glycine

zz It has a specific action on nerve tissue

zz Requires for the maintenance of normal gastro-intestinal tone and motility

zz Maintains normal appetite.

Deficiency Diseases

zz Beriberi - nervous, system affected, muscles become weak and painful paralysis can occur.

zz Heart failure, wet beriberi, dry beriberi, infantile beriberi, oedemia, children's growth is impaired, keto acids accumulate in the blood, wernicke’s-korsakoff’s syndrome etc.

zz Loss of appetite, fatigue, irritability, depression and constipation occur.

Vitamin B2 (Riboflavin)

Properties

zz Yellow crystals

zz Soluble in water

zz Heat soluble in neutral and acid media

zz Destroy by light.

Sources

Milk, liver, kidney, muscle, butter, chicken, fish, yeast, cheese, raw egg, white grains, green vegetable such as spinach, peanuts, fruits such as apple, orange etc.

Functions

zz Essential for growth, essential for tissue oxidation related to carbohydrate, fat and protein metabolism.

zz Maintain mucosal, epithelial and ocular tissues.

zz Essential for normal vision.

Deficiency Diseases

Symptomszz Tongue sore at the corner of the mouth.zz Loss of hair, skin becomes dry and scaly.zz Arrest of growth.zz Dermatitis around nose and lips, inflammation of tongue, angular stomatitis and

cheilosis, photophobia, cataract etc.zz Scrotal or vulval dermatitis, intense itching etc.zz Disturb carbohydrate metabolism.

NOTES

Vitamins


3.6 SUMMARYSuch organic substances which are important for the proper functioning of our body or an organic chemical compound is called a vitamin when the organism cannot synthesize the compound in sufficient quantities, and must be supplied through the diet. For example, ascorbic acid (vitamin C) is a vitamin for humans, but not for most other animal organisms. Vitamins are important nutrients which are necessary for life. Unlike other classes of nutrients, vitamins serve no structural function nor provide significant energy. Mostly vitamins needs metabolic activation into a functional form. Some vitamins function as coenzymes, antioxidants, although some vitamins (A and D), act as hormones. Vitamins are classified in to two categories on the basis of their soluble properties: (i) Fat soluble vitamins: Vitamins-A, D, E and K are considered as fat soluble vitamins. (ii) Water soluble vitamins: Vitamins B and C are considered as water soluble vitamins.

3.7 REVIEW QUESTIONS 1. What are vitamins? Give the details of general properties of the vitamins. 2. Give the basis of classification of vitamins and their classification in two categories. 3. Explain the following points in detail: (i) Fat soluble vitamins (ii) Water soluble vitamins 4. Give the details of following vitamins and also discuss the deficiency diseases: Vitamins: A, D, E. 5. Explain the following points in detail: (a) Vitamin C, its source and deficiency diseases (b) Vitamin B, its source and deficiency diseases

3.8 FURTHER READINGSzz Organic chemistry, Volume 2 : Stereochemistry and the chemistry of natural


zz Janos Zempleni, John W. Suttie, Jesse F. Gregory III - 2013 - Medical

zz Simic, M.G., and Karel, M. (1980)Autoxidation in Food and Biological Systems, Plenum Press, New York

zz Machlin, L.J. (1984) Vitamin E, inHandbook of Vitamins: Nutritional Biochemical & Clinical Aspects (Machlin, L.J., ed.), 99–145, Marcel Dekker, New York and Basel

zz IUPAC-IUB Joint Commission on Biochemical Nomenclature (1982) Nomenclature of Tocopherols and Related Compounds: Recommendations 1981,Eur. J. Biochem. 123, 473–475.

NOTES



CHAPTER – 4

STEROIDS AND HORMONESSTRUCTURE

4.1 Learning Objectives 4.2 Introduction 4.3 Occurrence of Steroids 4.4 Steroid Nomenclature and Numbering System 4.5 Basic Skeleton of Steroids 4.6 Isolation of Steroids 4.7 Stereochemistry of Steroids 4.8 Structure Determination and Synthesis of some Steroid (cholesterol) and

Sex Hormones (testosterone and oestrone) 4.9 Occurrence 4.10 Nomenclature 4.11 Classification 4.12 Biogenesis and Physiological Effects 4.13 The Synthesis of Natural E-Series Prostaglandins 4.14 Summary 4.15 Review Questions 4.16 Further readings


zz Definition (steroids and sex hormones)zz Physiological effects / importancezz Structure of cholesterolzz Stereochemical informationzz This chapter is based on the synthesis of types of prostaglandins and their different

analogues, this short review will concentrate only on the synthesis of the E-series prostaglandins and some of their analogues.

zz Specifically, this chapter will present an up to date review of the synthetic methods involving the conjugate addition approach that is used to synthesize the E-series prostaglandins and their analogues with modified side chains only.

zz The E-series PGs are the most widely studied.

NOTES

Steroids and Hormones


4.2 INTRODUCTIONSteroid are any class of natural or synthetic organic compounds which comprise a group of cyclical organic compounds whose basis is a characteristic arrangement of seventeen carbon atoms in a four-ring structure linked together from three 6- carbon rings followed by a 5-carbon ring and an eight-carbon side chain on carbon 17. Steroids are important in biology, chemistry, and medicine. Among the synthetic steroids of therapeutic value are considered as large number of antiinflammatory agents and anabolic (growth-stimulating) agents.

The steroids basically introduces the sterols, bile acid, vitamin D and sex hormones (androsterone, testosterone, progestrone, estrogen and estrone). All the steroids have basic skeleton i.e., 1,2-cyclopentophenanthrene nucleus which is also basic skeleton of Diel's hydrocarbon.

26

Physiological effects / importance

Structure of cholesterol

Stereochemical information

Importance of steroids

2.2 Introduction

Steroid are any class of natural or synthetic organic compounds which comprise a group of cyclical organic compounds whose basis is a characteristic arrangement of seventeen carbon atoms in a four-ring structure linked together from three 6-carbon rings followed by a 5-carbon ring and an eight-carbon side chain on carbon 17. Steroids are important in biology, chemistry, and medicine. Among the synthetic steroids of therapeutic value are considered as large number of anti-inflammatory agents and anabolic (growth-stimulating) agents.


43

23'

2'

1'1

10

98

7

6

5

Molecular formula (C18H16)

So, we can say that any compound which produce Diel's hydrocarbon on distillation with selenium consider as steroids. If the distillation carried out at 420°C, then chrysene and picene both are obtained.

Chrysene + PiceneSe

420°CAll steroids

Se

360°CDiel's hydrocarbon

2.3 Occurrence of steroids

All the steroids possess sterol. Sterol is mainly found in plants and animals. The sterols are crystalline organic compound which possess an alcoholic group. Due to


26

Physiological effects / importance

Structure of cholesterol

Stereochemical information

Importance of steroids

2.2 Introduction

Steroid are any class of natural or synthetic organic compounds which comprise a group of cyclical organic compounds whose basis is a characteristic arrangement of seventeen carbon atoms in a four-ring structure linked together from three 6-carbon rings followed by a 5-carbon ring and an eight-carbon side chain on carbon 17. Steroids are important in biology, chemistry, and medicine. Among the synthetic steroids of therapeutic value are considered as large number of anti-inflammatory agents and anabolic (growth-stimulating) agents.


43

23'

2'

1'1

10

98

7

6

5

Molecular formula (C18H16)


Chrysene + PiceneSe

420°CAll steroids

Se

360°CDiel's hydrocarbon

2.3 Occurrence of steroids

All the steroids possess sterol. Sterol is mainly found in plants and animals. The sterols are crystalline organic compound which possess an alcoholic group. Due to

Prostaglandins (PGs) were discovered by Swedish physiologist (Nobel laureate), Ulf von Euler in 1935 and other investigators were given the term “Prostaglandin” anticipating the active material could be the origin from the prostate gland. PGs were first isolated and characterized by K. Bergstrom from Karolinska Institute in 1957. 2 In 1971, it was determined that aspirin-like drugs could inhibit the synthesis of PGs. PGs are classified among the family of eicosanoids along with leukotrienes (LT), thromboxanes (TX) and Lipoxins (LX). The PGs and TXs are collectively identified as prostanoids. PGs exist and are synthesized in virtually every tissues and organs of the living body. These are like hormones in that they act as chemical messengers, but they are not transported from one place to another in the body rather they are synthesized within the cells when required. They play important regulatory roles in many normal cellular functions, especially in relation to inflammatory responses, regulating fat metabolism, hormones, pain, fever as well as the cardiovascular, immune, and central nervous systems.

NOTES



prostAGlAndin structure

Prostaglandins are unsaturated carboxylic acids, consisting of a 20 carbon skeleton that also contains a five member ring and are based upon the fatty acid, arachidonic acid. There are a variety of structures one, two, or three double bonds. On the five member ring there may also be double bonds, a ketone, or alcohol groups.

51

OR1

R2

OR1

R2

OR1

R2

HOR1

R2O

R1

R2

O

HO

R1

R2HO

HO

PGA PGB PGC PGD PGE PGF

O

O

R1

R2

OR1

HOR2

OR2

R1

OR2

R1

O

OR1

R2O

R1

R2OHO

OH

PGGPGH

PGI PGJ PGK TXA TXB

COOH357

89

10

11 1115 17 19

prostanoic acid 3.2 Occurrence

Prostaglandin any of a group of naturally occurring, chemically related fatty acids that stimulate contractility of the uterine and other smooth muscleand have the ability to lower blood pressure, regulate acid secretion of the stomach, regulate body temperature and platelet aggregation, and control inflammation and vascular permeability; they also affect the action of certain hormones. Nine primary types are labeled A through I, the degree of saturation of the side chain of each being designated by subscripts 1, 2, and 3. The types of prostaglandins are abbreviated

PGE2, PGF2, and so on.

3.3 Nomenclature

In the approved nomenclature, each prostaglandin is named using the prefix 'PG' followed by a letter A to K depending on the nature and position of the substituents on the ring. Thus PGA to PGE and PGJ have a keto group in various positions on the ring, and are further distinguished by the presence or absence of double bonds or hydroxyl groups in various positions in the ring. PGF has two hydroxyl groups while PGK has two keto substituents on the ring. PGG and PGH are bicyclic endoperoxides. An oxygen bridge between carbons 6 and 9 distinguishes prostacyclin (PGI). Thromboxane A (TXA) contains an unstable bicyclic oxygenated ring structure, while thromboxane B (TXB) has a stable oxane

4.3 OCCURRENCE OF STEROIDSAll the steroids possess sterol. Sterol is mainly found in plants and animals. The sterols are crystalline organic compound which possess an alcoholic group. Due to presence of alcoholic group sterols are obtained in esterified state with fatty acids.

27

presence of alcoholic group sterols are obtained in esterified state with fatty acids.

Sterols

Obtained from Obtained from Obtained from

Plants Fungi Animals

Phytosterols Mycosterols zoosterols

eg. cholesterol

eg.Ergosterol cholestanol

stigmasterol coprostanol

Some important sterols and their sources are listed below:

common name systematic name Occurrence

Cholesterol 5-cholesten-3-ol principal sterol of most animals and all vertebrate tissues

Coprostanol 5 -cholestan-3 -ol feces of vertebrates

Cholestanol 5 -cholestan-3-ol minor vertebrate sterol: guinea pig and rabbit adrenal

dehydrocholesterol 5,7-cholestadien-3-ol mammalian skin, intestine

zymosterol 5-cholesta-8,24-dien-3-ol minor sterol of yeasts

ergosterol 5,7,22-ergostatrien-3-ol principal sterol of yeasts, ergot (Claviceps purpurea), and other fungi

stigmasterol 5,22-stigmastadien-3-ol most green plants, soybeans

sitosterol 5-stigmasten-3-ol most green plants, wheat germ

NOTES



27

presence of alcoholic group sterols are obtained in esterified state with fatty acids.

Sterols

Obtained from Obtained from Obtained from

Plants Fungi Animals

Phytosterols Mycosterols zoosterols

eg. cholesterol

eg.Ergosterol cholestanol

stigmasterol coprostanol

Some important sterols and their sources are listed below:

common name systematic name Occurrence

Cholesterol 5-cholesten-3-ol principal sterol of most animals and all vertebrate tissues

Coprostanol 5 -cholestan-3 -ol feces of vertebrates

Cholestanol 5 -cholestan-3-ol minor vertebrate sterol: guinea pig and rabbit adrenal

dehydrocholesterol 5,7-cholestadien-3-ol mammalian skin, intestine

zymosterol 5-cholesta-8,24-dien-3-ol minor sterol of yeasts

ergosterol 5,7,22-ergostatrien-3-ol principal sterol of yeasts, ergot (Claviceps purpurea), and other fungi

stigmasterol 5,22-stigmastadien-3-ol most green plants, soybeans

sitosterol 5-stigmasten-3-ol most green plants, wheat germ

28

fucosterol 5,24(28)-stigmastadien-3-ol principal sterol of marine brown algae (Fucus species)

lanosterol 8,24-lanostadien-3-ol skin, sheep wool, fat, yeasts

2.4 Steroid nomenclature and numbering system

All steroids are related to a characteristic molecular structure composed of 17 carbon atoms which are arranged in four rings conventionally denoted by the letters A, B, C, and D having 28 hydrogen atoms in four rings.

Names of fundamental structures most often used in steroid nomenclature

carbon atoms present (as

numbered in structure 6)

naturally occurring general classes

examples shown in text

gonane 1–17 None Gonane

androstane 1–19 androgens Androstane, testosterone androstanedione

pregnane 1–21 gestogens and adrenal steroids

Progesterone, cortisol aldosterone

cholane 1–24 bile acids cholic acid, sodium sodium glycocholate

cholestane 1–27 Sterols Cholesterol, scymnol

ergostane 1–28 Sterols Ergosterol, cyasterone

stigmastane 1–29 Sterols Stigmasterol, antheridiol

*Gonane and androstane themselves do not occur in nature.

4.4 STEROID NOMENCLATURE AND NUMBERING SYSTEMAll steroids are related to a characteristic molecular structure composed of 17 carbon atoms which are arranged in four rings conventionally denoted by the letters A, B, C, and D having 28 hydrogen atoms in four rings.


This parent structure (1), named gonane (also known as the steroid nucleus), may be odified in a practically unlimited number of ways by removal, replacement, or addition of a few atoms at a time; hundreds of steroids have been isolated from plants and animals.

The steroid nucleus is a three-dimensional structure, and atoms or groups are attached to it by spatially directed bonds. Although many stereoisomers of this nucleus are possible (and may be synthesized), the saturated nuclear structures of most classes of natural steroids are alike, except at the junction of rings A and B. Simplified three-dimensional diagrams may be used to illustrate stereochemical details. For example, androstane, common to a number

NOTES



of natural and synthetic steroids, exists in two forms (2 and 3), in which the A/B ring fusions are called cis and trans, respectively.

28

fucosterol 5,24(28)-stigmastadien-3-ol principal sterol of marine brown algae (Fucus species)

lanosterol 8,24-lanostadien-3-ol skin, sheep wool, fat, yeasts

2.4 Steroid nomenclature and numbering system

All steroids are related to a characteristic molecular structure composed of 17 carbon atoms which are arranged in four rings conventionally denoted by the letters A, B, C, and D having 28 hydrogen atoms in four rings.


carbon atoms present (as

numbered in structure 6)

naturally occurring general classes

examples shown in text

gonane 1–17 None Gonane

androstane 1–19 androgens Androstane, testosterone androstanedione

pregnane 1–21 gestogens and adrenal steroids

Progesterone, cortisol aldosterone

cholane 1–24 bile acids cholic acid, sodium sodium glycocholate

cholestane 1–27 Sterols Cholesterol, scymnol

ergostane 1–28 Sterols Ergosterol, cyasterone

stigmastane 1–29 Sterols Stigmasterol, antheridiol

*Gonane and androstane themselves do not occur in nature.

29

This parent structure (1), named gonane (also known as the steroid nucleus), may be modified in a practically unlimited number of ways by removal, replacement, or addition of a few atoms at a time; hundreds of steroids have been isolated from plants and animals.

The steroid nucleus is a three-dimensional structure, and atoms or groups are attached to it by spatially directed bonds. Although many stereoisomers of this nucleus are possible (and may be synthesized), the saturated nuclear structures of most classes of natural steroids are alike, except at the junction of rings A and B. Simplified three-dimensional diagrams may be used to illustrate stereochemical details. For example, androstane, common to a number of natural and synthetic steroids, exists in two forms (2 and 3), in which the A/B ring fusions are called cis and trans, respectively.

In the cis isomer, bonds to the methyl group (CH3) and to the hydrogen atom (H) both project upward from the general plane defined by the rest of the molecule, whereas in the trans isomer, the methyl group projects up and the hydrogen projects down. Usually, however, steroid structures are represented as plane projection diagrams such as 4 and 5, which correspond to 2 and 3, respectively.

29

This parent structure (1), named gonane (also known as the steroid nucleus), may be modified in a practically unlimited number of ways by removal, replacement, or addition of a few atoms at a time; hundreds of steroids have been isolated from plants and animals.

The steroid nucleus is a three-dimensional structure, and atoms or groups are attached to it by spatially directed bonds. Although many stereoisomers of this nucleus are possible (and may be synthesized), the saturated nuclear structures of most classes of natural steroids are alike, except at the junction of rings A and B. Simplified three-dimensional diagrams may be used to illustrate stereochemical details. For example, androstane, common to a number of natural and synthetic steroids, exists in two forms (2 and 3), in which the A/B ring fusions are called cis and trans, respectively.

In the cis isomer, bonds to the methyl group (CH3) and to the hydrogen atom (H) both project upward from the general plane defined by the rest of the molecule, whereas in the trans isomer, the methyl group projects up and the hydrogen projects down. Usually, however, steroid structures are represented as plane projection diagrams such as 4 and 5, which correspond to 2 and 3, respectively.

In the cis isomer, bonds to the methyl group (CH3) and to the hydrogen atom (H) both project upward from the general plane defined by the rest of the molecule, whereas in

NOTES



the trans isomer, the methyl group projects up and the hydrogen projects down. Usually, however, steroid structures are represented as plane projection diagrams such as 4 and 5, which correspond to 2 and 3, respectively.

30

The stereochemistry of rings A and B must be specified by showing the orientation of the hydrogen atom attached at C5 (that is, carbon atom number 5; steroid numbering is explained below) as either above the plane of the diagram

(designated β) or below it (α). The α-, β- symbolism is used in a similar manner to indicate the orientation of any substituent group that is attached to a saturated

(fully substituted) carbon within the steroid ring system. Bonding of β-attached

substituents is by a full line, while of α-substituents by a broken line and if not known then shown by a wavy line.

Each carbon atom of a steroid molecule is numbered, and the number is reserved to a particular position in the hypothetical parent skeletal structure (6) whether this position is occupied by a carbon atom or not.

Systematic rules agreed by the International Union of Pure and Applied Chemistry for the nomenclature. By attaching prefixes and suffixes to the name of the appropriate root structure, the character of substituent groups or other structural modification is indicated. The prefixes and suffixes include numbers, called locants, indicative of the position in the carbon skeleton at which the modification

occurs, and, where necessary, the orientation of a substituent is shown as α or β. The carbon atom at position 3, for example, is referred to as C3; a hydroxyl group

attached to C3 is referred to as a 3-OH group or, more specifically, as a 3α-OH or

The stereochemistry of rings A and B must be specified by showing the orientation of the hydrogen atom attached at C5 (that is, carbon atom number 5; steroid numbering is explained below) as either above the plane of the diagram (designated β) or below it (α). The α-, β- symbolism is used in a similar manner to indicate the orientation of any substituent group that is attached to a saturated (fully substituted) carbon within the steroid ring system. Bonding of β-attached substituents is by a full line, while of α-substituents by a broken line and if not known then shown by a wavy line.


30

The stereochemistry of rings A and B must be specified by showing the orientation of the hydrogen atom attached at C5 (that is, carbon atom number 5; steroid numbering is explained below) as either above the plane of the diagram

(designated β) or below it (α). The α-, β- symbolism is used in a similar manner to indicate the orientation of any substituent group that is attached to a saturated

(fully substituted) carbon within the steroid ring system. Bonding of β-attached

substituents is by a full line, while of α-substituents by a broken line and if not known then shown by a wavy line.


Systematic rules agreed by the International Union of Pure and Applied Chemistry for the nomenclature. By attaching prefixes and suffixes to the name of the appropriate root structure, the character of substituent groups or other structural modification is indicated. The prefixes and suffixes include numbers, called locants, indicative of the position in the carbon skeleton at which the modification

occurs, and, where necessary, the orientation of a substituent is shown as α or β. The carbon atom at position 3, for example, is referred to as C3; a hydroxyl group

attached to C3 is referred to as a 3-OH group or, more specifically, as a 3α-OH or

Systematic rules agreed by the International Union of Pure and Applied Chemistry for the nomenclature. By attaching prefixes and suffixes to the name of the appropriate root structure, the character of substituent groups or other structural modification is indicated. The prefixes and suffixes include numbers, called locants, indicative of the position in the carbon skeleton at which the modification occurs, and, where necessary, the orientation of a substituent is shown as α or β. The carbon atom at position 3, for example, is referred to as C3; a hydroxyl group attached to C3 is referred to as a 3-OH group or, more specifically, as a 3α-OH or 3β-OH group. In addition to differences in details of the steroid nucleus, the various classes of steroids are distinguished by variations in the size and structure of an atomic group (the side chain) attached at position 17.

NOTES



In addition to the usual chemical notations for substituent groups replacing hydrogen atoms (e.g., methyl-, chloro-, hydroxy-, oxo-), the following prefixes are commonly used: dehydro- (lacking two hydrogen atoms from adjacent positions); dihydro- (possessing two additional hydrogen atoms in adjacent positions); deoxy- (hydroxyl group replaced by a hydrogen atom); epi- (differing in configuration of a carbon atom bonded to two other carbon atoms); iso- (differing in configuration of a carbon atom bonded to three other carbon atoms); nor- (lacking one carbon atom); homo- (possessing one additional carbon atom); cyclo- (with a bond between two carbons that are normally not united); and seco- (with a carboncarbon bond of the nucleus broken).

31

3β-OH group. In addition to differences in details of the steroid nucleus, the various classes of steroids are distinguished by variations in the size and structure of an atomic group (the side chain) attached at position 17.

In addition to the usual chemical notations for substituent groups replacing hydrogen atoms (e.g., methyl-, chloro-, hydroxy-, oxo-), the following prefixes are commonly used: dehydro- (lacking two hydrogen atoms from adjacent positions); dihydro- (possessing two additional hydrogen atoms in adjacent positions); deoxy- (hydroxyl group replaced by a hydrogen atom); epi- (differing in configuration of a carbon atom bonded to two other carbon atoms); iso- (differing in configuration of a carbon atom bonded to three other carbon atoms); nor- (lacking one carbon atom); homo- (possessing one additional carbon atom); cyclo- (with a bond between two carbons that are normally not united); and seco- (with a carbon-carbon bond of the nucleus broken).

H

A B

C D

H

A B

C D

23-nor-5B-cholaneA-nor-5-androstane

32

H

A B

CD

C2H5

HOOC

HOOC

H

3, 5-Cyclocholestane

B-Homo-5-pregnane 2,3-Seco-5-androstane-2,3-dioic acid

2.5 Basic skeleton of steroids

The basic skeleton of all steroids posses 1,2-cyclopentophenanthrene nucleus i.e. known as Diel's hydrocarbon.

Structure of Diel's hydrocarbon:

2 3' 2'

1'1

CH3

3'-Methyl-1,2-cyclopentophenanthrene

Molecular Formula: C18H16

2.6 Isolation of steroids

The isolation of the considerable amount of pure steroids required for structure elucidation,degradation, biological testing, and other research needs (generally

4.5 BASIC SKELETON OF STEROIDSThe basic skeleton of all steroids posses 1,2-cyclopentophenanthrene nucleus i.e. known as Diel's hydrocarbon.

NOTES




32

H

A B

CD

C2H5

HOOC

HOOC

H

3, 5-Cyclocholestane

B-Homo-5-pregnane 2,3-Seco-5-androstane-2,3-dioic acid

2.5 Basic skeleton of steroids

The basic skeleton of all steroids posses 1,2-cyclopentophenanthrene nucleus i.e. known as Diel's hydrocarbon.


2 3' 2'

1'1

CH3

3'-Methyl-1,2-cyclopentophenanthrene

Molecular Formula: C18H16

2.6 Isolation of steroids

The isolation of the considerable amount of pure steroids required for structure elucidation,degradation, biological testing, and other research needs (generally

4.6 ISOLATION OF STEROIDSThe isolation of the considerable amount of pure steroids required for structure elucidation,degradation, biological testing, and other research needs (generally milligrams to grams). The methods of isolation generally involve extraction, precipitation, adsorptions, chromatography, and sometimes crystallizations. The isolated matter is purified to chemical homogeneity. The structure determination methods that are applied to determine the chemical structure of an isolated steroid, a process that involves an array of chemical and physical methods that included NMR and small molecule crystallography. Analytical methods involve in determining of structure if a steroid is present in an analytical mixture.

Procedures for isolation of steroids differ according to the chemical nature of the steroids and the scale and purpose of the isolation. Steroids are isolated from natural sources by extraction with organic solvents, in which they usually dissolve more readily than in the aqueous fluids of tissues. The source material often is treated initially with an alcoholic solvent, which dehydrates it, denatures , proteins associated with the steroids, and dissolves many steroids. Saponification either of whole tissues or of substances extracted from them by alcohol splits the molecules of sterol esters, triglycerides, and other fatty esters and permits the extraction of the sterols by means of water-immiscible solvents, such as hexane or ether, with considerable purification. Intact sterol esters or hormonal steroids and their metabolites (compounds produced by biological transformation) that are sensitive to strong acids or alkalies, however, require essentially neutral conditions for isolation, although some procedures for analysis of urinary steroids employ acid treatment, milder hydrolysis, as by enzymes, is preferred. The acidity of some steroids allows them to be held in alkaline solution, while nonacidic impurities are extracted with organic solvents.

Commercially, abundant steroids usually are purified by repeated crystallization from solvents. Small-scale laboratory isolations for investigative or assay purposes usually exploit differing polarities of the steroid and of its impurities, which may be separated by partitioning between solvents differing in polarity or by chromatography. Occasionally, special reagents may selectively precipitate or otherwise sequester the desired steroid. A classical example is

NOTES



the precipitation of 3β-hydroxy sterols such as cholesterol by the natural steroid derivative digitonin. New steroids of great physiological interest often are isolated from tissue only with extreme difficulty, because they are usually trace constituents.

The percentage recovery of known steroid hormones during their assay in small biological samples usually is assessed by adding a trace of the same steroid in radioactivity) after purification is complete.

4.7 STEREOCHEMISTRY OF STEROIDSFor the study of steroids stereochemistry we will take the e.g. of sterol. Sterol has 8 disimilar chiral centre at carbon, C-3,5,8,9,10,13,14 and 17. So the total number of optical isomer can be calculated as follow:

Formula = 2n

Where n = number of chiral centre

34

radioactive form to the initial sample, followed by radioassay (analysis based on radioactivity) after purification is complete.

2.7 Stereochemistry of steroids

For the study of steroids stereochemistry we will take the e.g. of sterol. Sterol has 8 disimilar chiral centre at carbon, C-3,5,8,9,10,13,14 and 17. So the total number of optical isomer can be calculated as follow:

Formula = 2n

Where n = number of chiral centre

1112

13 16

1514

7

643

2

1

17

20

8

5HO

189

19

10

DC

BA

If we consider the chiral centre of main nucleus then number of optical isomer will be

n = 8, So 28 = 256

If we consider the chiral centre that is outside of nucleus (C-20) then the number of optical isomer will be –

N = 9, So, 29 = 512

In sterol such chiral centre which are produced due to ring junction. If we count these chiral centre (5,8,9,10,13 and 14) then number of optical isomer are 26 = 64, however many of these are not possible because of some steric hindrance.

2.8 Structure determination and synthesis of some steroid (cholesterol) and sex hormones (testosterone and oestrone)

2.8.1 Cholesterol:

Molecular formula: C27H46O

Molecular Structure:

If we consider the chiral centre of main nucleus then number of optical isomer will be

n = 8, So 28 = 256

If we consider the chiral centre that is outside of nucleus (C-20) then the number of optical isomer will be –

N = 9, So, 29 = 512

In sterol such chiral centre which are produced due to ring junction. If we count these chiral centre (5,8,9,10,13 and 14) then number of optical isomer are 26 = 64, however many of these are not possible because of some steric hindrance.

4.8 STRUCTURE DETERMINATION AND SYNTHESIS OFSOME STEROID (CHOLESTEROL) AND SEX HORMONES(TESTOSTERONE AND OESTRONE)

cHolesterol

Molecular formula: C27H46O

Molecular StructureThe three cyclohexane rings are designated as rings A, B and C in the figure to the right and the one cyclopentane ring as ring D. Individual steroids vary, first and primarily, by

NOTES



the oxidation state of the carbon atoms in the rings and by the chains and functional groups attached to this four-ring system; second, steroids can vary more markedly via changes to the ring structure. Sterols are a particularly important form of steroids, with sterols having a cholestane-derived framework and an hydroxyl group at the C-3 ring position. Study of cholesterol is done under following points:

35

The three cyclohexane rings are designated as rings A, B and C in the figure to the right and the one cyclopentane ring as ring D. Individual steroids vary, first and primarily, by the oxidation state of the carbon atoms in the rings and by the chains and functional groups attached to this four-ring system; second, steroids can vary more markedly via changes to the ring structure. Sterols are a particularly important form of steroids, with sterols having a cholestane-derived framework and an hydroxyl group at the C-3 ring position.

Study of cholesterol is done under following points:

(I) Structure of nucleus (pressence and size of four ring)

(II) Presence and position of hydroxyl group and double bond

(III) Nature and position of side chain

(IV) Position of two angular methyl group

(I) Nature of nucleus (presence of four rings A,B,C,D):

(a) Size of ring A: When the cholesterol and cholic acid decomposed into dicarboxylic acid, then it converted into cyclopentanone via ring contraction. This suggest that ring (A) is six membered.

HOOC

HOOC

R

B

C D

or Cholic acid

Cholesterol

OC OCH3 2

Blanc's RuleA B

C D

R

O

(b) Size of ring B: When cholesterol undergoes decomposition then ring (B) is decomposed to give tricarboxylic acid which further undergoes cyclisation to give cyclopentanone. This reaction suggested that ring (B) is six membered.







35

The three cyclohexane rings are designated as rings A, B and C in the figure to the right and the one cyclopentane ring as ring D. Individual steroids vary, first and primarily, by the oxidation state of the carbon atoms in the rings and by the chains and functional groups attached to this four-ring system; second, steroids can vary more markedly via changes to the ring structure. Sterols are a particularly important form of steroids, with sterols having a cholestane-derived framework and an hydroxyl group at the C-3 ring position.

Study of cholesterol is done under following points:







HOOC

HOOC

R

B

C D

or Cholic acid

Cholesterol

OC OCH3 2

Blanc's RuleA B

C D

R

O



36

Cholesterol

COOHCOOH

R

C D

HOOC

OC OCH3 2

Blanc's Rule

C D

R

HOOC

O

B

Tricarboxylic acid (c) Size of ring C: When the deoxycholic acid is converted into dicarboxylic

acid, which yields seven membered cyclic anhydride. This reaction also suggested that ring (C) is also six membered.

Deoxycholic acid

HOOCCOOH

R

A B

DOC OCH3 2

Blanc's Rule A B

OO

O

CD

R

(d) Size of ring D: When the 5-cholestane is converted into dicarboxylic acid i.e. etiobilianic acid then it yields six membered cyclic anhydride. This reaction suggest that ring (D) is five membered.

A B5-Cholestane

OC OCH3 2

Blanc's Rule

COOH

COOH

C

Etiobilianic acid

A B

CO

O

O

D

(II) Presence and position of hydroxyl group and double bond:

(a) Presence of hydroxyl group: When the cholesterol undergoes esterification with acetic acid and benzoic acid then it yields monoacetate and monobenzoate. Presence of hydroxyl can also be confirmed that cholesterol gives cholestanone on oxidation.

(b) Position of hydroxyl group: When the cholesterol undergoes reduction then it yields dihydroderivative (i.e. cholestanol). The later compound react with CrO3 to give a saturated ketone (i.e. cholestenone). The cholestanone (a ketone) react with methyl magnesium bromide followed by hydrolysis and aromatization(dehydrogenation)to give 3',7-dimethylcyclopentophenanthrene. The above reaction confirm that hydroxyl group present in ring (A) at C-3 position.

(c) Size of ring C: When the deoxycholic acid is converted into dicarboxylic acid, which yields seven membered cyclic anhydride. This reaction also suggested that ring (C) is also six membered.

NOTES



36

Cholesterol

COOHCOOH

R

C D

HOOC

OC OCH3 2

Blanc's Rule

C D

R

HOOC

O

B



Deoxycholic acid

HOOCCOOH

R

A B

DOC OCH3 2

Blanc's Rule A B

OO

O

CD

R


A B5-Cholestane

OC OCH3 2

Blanc's Rule

COOH

COOH

C

Etiobilianic acid

A B

CO

O

O

D




(d) Size of ring D: When the 5b-cholestane is converted into dicarboxylic acid i.e. etiobilianic acid then it yields six membered cyclic anhydride. This reaction suggest that ring (D) is five membered.

36

Cholesterol

COOHCOOH

R

C D

HOOC

OC OCH3 2

Blanc's Rule

C D

R

HOOC

O

B



Deoxycholic acid

HOOCCOOH

R

A B

DOC OCH3 2

Blanc's Rule A B

OO

O

CD

R


A B5-Cholestane

OC OCH3 2

Blanc's Rule

COOH

COOH

C

Etiobilianic acid

A B

CO

O

O

D




(II) Presence and position of hydroxyl group and double bond:(a) Presence of hydroxyl group: When the cholesterol undergoes esterification

with acetic acid and benzoic acid then it yields monoacetate and monobenzoate. Presence of hydroxyl can also be confirmed that cholesterol gives cholestanone on oxidation.

(b) Position of hydroxyl group: When the cholesterol undergoes reduction then it yields dihydroderivative (i.e. cholestanol). The later compound react with CrO3 to give a saturated ketone (i.e. cholestenone). The cholestanone (a ketone) react with methyl magnesium bromide followed by hydrolysis and aromatization(dehydrogenation)to give 3',7- dimethylcyclopentophenanthrene. The above reaction confirm that hydroxyl group present in ring (A) at C-3 position.

37

Cholesterol

HO

Cholestanol

HO

H2, Pt CrO3

Cholestanone

O

HO

H3C

Se

350°C

43

23'

2'

1'1

10

98

7

6

5

H3C

CH3

3',7-dimethylcyclopentophenanthrene

(i) CH3MgBr(ii) H2O

(c) Presence of double bond: Cholesterol react with one equivalent of

hydrogen (H2) to give its dihydroderivative, also react with (Br2) to give dibromoderivative.

C27H46.OC27H46Br2OBr2 H2, Pt

C27H48O

Cholesterol Dihydrocholesterol The above reaction suggested that cholesterol contains one double bond.

(d) Position of double bond: Position of double bond in cholesterol can be confirmed by these given set of reactions. When the cholesterol undergoes reaction with H2O2 / CH3COOH then it undergoes dihydroxylation to give cholestane triol. Which further react with CrO3 to give hydroxyl cholestane dione (diketone). The later compound undergoes reaction with Zn/CH3COOH to produce cholestanedione which further undergoes oxidation to produce a tetracarboxylic acid. The above whole set of reaction suggested that position of double bond in cholesterol is in between (C5-C6).

NOTES



(c) Presence of double bond: Cholesterol react with one equivalent of hydrogen (H2) to give its dihydroderivative, also react with (Br2) to give dibromoderivative.

37

Cholesterol

HO

Cholestanol

HO

H2, Pt CrO3

Cholestanone

O

HO

H3C

Se

350°C

43

23'

2'

1'1

10

98

7

6

5

H3C

CH3

3',7-dimethylcyclopentophenanthrene

(i) CH3MgBr(ii) H2O

(c) Presence of double bond: Cholesterol react with one equivalent of

hydrogen (H2) to give its dihydroderivative, also react with (Br2) to give dibromoderivative.

C27H46.OC27H46Br2OBr2 H2, Pt

C27H48O

Cholesterol Dihydrocholesterol The above reaction suggested that cholesterol contains one double bond.


The above reaction suggested that cholesterol contains one double bond.


38

Cholesterol

HO

cholestanetriol

HO

H2O2 CrO3

Hydroxycholestanedione

O

O

CrO3

1

2

34

56

CH3COOH

OH OH OH O

OH

HOOC

HOOCCOOH

COOH

Tetracarboxylic acid

Zn/CH3COOH

(III) Nature and position of side chain:

The nature and position of side chain in cholesterol can be explained by these set of chemical reactions. The cholesterol has a hydroxyl group when it undergoes acetylation then cholesteryl acetate will be formed.The later compound is undergoes the oxidation at the side chain to give a ketone of cholesteryl acetate and isohexyl methyl ketone. The formation of isohexyl methyl ketone reveals that the point of attachment of side chain will be at ring (D) with 3' carbon.

HO

OC OCH3 2

OC

O

H3C

Cholesterol Cholesteryl acetate

CrO3

O

HO

C

O

H3C

CrO3

O

OC

O

H3C

+

O (IV) Position of two angular methyl group:

(III) Nature and position of side chain: The nature and position of side chain in cholesterol can be explained by these set of chemical reactions. The cholesterol has a hydroxyl group when it undergoes acetylation then cholesteryl acetate will be formed. The later compound is undergoes the oxidation at the side chain to give a ketone of cholesteryl acetate and isohexyl methyl ketone. The formation of isohexyl methyl ketone reveals that the point of attachment of side chain will be at ring (D) with 3' carbon.

(IV) Position of two angular methyl group: The molecular structure and molecular formula of cholesterol reveals that it

contains 27 carbon atoms. The main nucleus (1,2-cyclopentophenanthrene) contains only 17 carbon atoms, the side at ring (D) contains 8 carbon atoms, so these are (17+8 = 25) 25 carbon atoms. So the remaining two carbon atoms are present as angular methyl group. These can be explained as follow:

NOTES



38

Cholesterol

HO

cholestanetriol

HO

H2O2 CrO3

Hydroxycholestanedione

O

O

CrO3

1

2

34

56

CH3COOH

OH OH OH O

OH

HOOC

HOOCCOOH

COOH

Tetracarboxylic acid

Zn/CH3COOH

(III) Nature and position of side chain:

The nature and position of side chain in cholesterol can be explained by these set of chemical reactions. The cholesterol has a hydroxyl group when it undergoes acetylation then cholesteryl acetate will be formed.The later compound is undergoes the oxidation at the side chain to give a ketone of cholesteryl acetate and isohexyl methyl ketone. The formation of isohexyl methyl ketone reveals that the point of attachment of side chain will be at ring (D) with 3' carbon.

HO

OC OCH3 2

OC

O

H3C

Cholesterol Cholesteryl acetate

CrO3

O

HO

C

O

H3C

CrO3

O

OC

O

H3C

+

O (IV) Position of two angular methyl group: (a) When the keto-carboxylic acid is undergoes Clemenson reduction which

further subjected to the successive Barbier-Wieland degradation (Involve loss of one C-atom). So the keto-carboxylic acid undergoes only two times Barbier-Wieland degradation (B.W.D.). In the second last structure the carboxylic acid present at tertiary carbon atom, so it does not undergoes B.W.D. It confirm that one angular methyl group is present at that carbon.

39

The molecular structure and molecular formula of cholesterol reveals that it contains 27 carbon atoms. The main nucleus (1,2-cyclopentophenanthrene) contains only 17 carbon atoms, the side at ring (D) contains 8 carbon atoms, so these are (17+8 = 25) 25 carbon atoms. So the remaining two carbon atoms are present as angular methyl group. These can be explained as follow:

(a) When the keto-carboxylic acid is undergoes Clemenson reduction which further subjected to the successive Barbier-Wieland degradation (Involve loss of one C-atom). So the keto-carboxylic acid undergoes only two times Barbier-Wieland degradation (B.W.D.). In the second last structure the carboxylic acid present at tertiary carbon atom, so it does not undergoes B.W.D. It confirm that one angular methyl group is present at that carbon.

HOOCO

Ketocarboxyic acid

HOOC

Zn-HgB.W.D.

HOOC

HOOC

HCl

B.W.D.

(b) The position of another angular methyl group can be confirm as: When the

cholesterol skeleton is undergoes distillation with (Se) then it yields, Diel's hydrocarbon and chrysene. The chrysene have four six membered ring. The conversion of ring (D) from five membered to six membered suggested that the angular methyl group involve in cyclization.

HO

DistillationSe +

Diel's hydrocarbon Chrysene

(b) The position of another angular methyl group can be confirm as: When the cholesterol skeleton is undergoes distillation with (Se) then it yields, Diel's hydrocarbon and chrysene. The chrysene have four six membered ring. The conversion of ring (D) from five membered to six membered suggested that the angular methyl group involve in cyclization.

NOTES



39

The molecular structure and molecular formula of cholesterol reveals that it contains 27 carbon atoms. The main nucleus (1,2-cyclopentophenanthrene) contains only 17 carbon atoms, the side at ring (D) contains 8 carbon atoms, so these are (17+8 = 25) 25 carbon atoms. So the remaining two carbon atoms are present as angular methyl group. These can be explained as follow:

(a) When the keto-carboxylic acid is undergoes Clemenson reduction which further subjected to the successive Barbier-Wieland degradation (Involve loss of one C-atom). So the keto-carboxylic acid undergoes only two times Barbier-Wieland degradation (B.W.D.). In the second last structure the carboxylic acid present at tertiary carbon atom, so it does not undergoes B.W.D. It confirm that one angular methyl group is present at that carbon.

HOOCO

Ketocarboxyic acid

HOOC

Zn-HgB.W.D.

HOOC

HOOC

HCl

B.W.D.

(b) The position of another angular methyl group can be confirm as: When the

cholesterol skeleton is undergoes distillation with (Se) then it yields, Diel's hydrocarbon and chrysene. The chrysene have four six membered ring. The conversion of ring (D) from five membered to six membered suggested that the angular methyl group involve in cyclization.

HO

DistillationSe +

Diel's hydrocarbon Chrysene Synthesis of Cholesterol

In plants and animals, steroids is biosynthesized by similar reactions, beginning with acetic acid, assisted by a type of enzyme. The isoprenoid hydrocarbon called squalene, which occurs widely in nature, is thought to be the starting material from which all steroids are made. Enzymatic transformation of squalene produces lanosterol in animals and cycloartenol in plants, which yield cholesterol in both animals and plants. Cholesterol is then converted to bile acids and steroid hormones in animals and to steroids such as alkaloids in plants. Cholesterol and other steroids are biosynthesized by extension of the enzyme pathway by which terpenoids are synthesized. Acetate fragments derived from common nutrient materials are converted into mevalonic acid, from which the terpenoid hydrocarbon squalene is formed. One end of the squalene molecule is then oxidized, giving squalene 2,3-oxide, which, by an intramolecular cyclization reaction and structural rearrangement, yields the steroid lanosterol. This enzyme-controlled reaction may be initiated by introduction of a positive charge into the oxide ring, because it is remarkably similar to the nonenzymic, acid-catalyzed cyclizations of certain] unsaturated hydrocarbons similar in structure to squalene. Cholesterol is formed from lanosterol by further structural changes.

Male sex hormones (Androgens): Testosterone and androstenedione are the major testicular androgens. Several other less-active androgens occur naturally. Major metabolites of testosterone are androsterone and etiocholanolone.

testosterone

Testosterone is the principal male sex hormone of androgen group and found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is secreted by the testicles of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. Androgens promote male sexual behaviour and aggressiveness, muscular development, and, in humans, the growth of facial and body hair and deepening of the voice. In men, testosterone plays a key role in the development of male reproductive tissues such as the testis and prostate as well as promoting secondary sexual characteristics such as increased muscle, bone mass, and the growth of body hair. In addition, testosterone is essential for health and well-being as well as the prevention of osteoporosis. Testosterone has several major actions. It provides negative feedback inhibition on the secretion of gonadotropin-releasing hormone from the hypothalamus and the secretion of luteinizing hormone from the pituitary gland. It also directs the development of the embryonic Wolffian ducts into the vas deferens (ductus deferens) and seminal vesicles and stimulates the formation of muscle and bone. Dihydrotestosterone is responsible for sperm maturation during spermatogenesis, for the formation of the prostate gland and external genitalia, and for sexual maturation at

NOTES



puberty. Testosterone can be manufactured by chemical and microbiological modification of inexpensive steroids, such as diosgenin. It is used clinically to treat testicular insufficiency, to suppress lactation (milk production), and to treat certain types of breast cancer.

41

Sex hormones: Sex hormones can be classified in two categories

Male sex hormones (Androgens): Testosterone and androstenedione are the major testicular androgens. Several other less-active androgens occur naturally. Major metabolites of testosterone are androsterone and etiocholanolone.

2.8.2 Testosterone:

Testosterone is the principal male sex hormone of androgen group and found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is secreted by the testicles of males and the ovaries of females, although small

(I) Molecular Formula: C19H28O2

(II) Molecular Structure:

42

amounts are also secreted by the adrenal glands. Androgens promote male sexual behaviour and aggressiveness, muscular development, and, in humans, the growth of facial and body hair and deepening of the voice. In men, testosterone plays a key role in the development of male reproductive tissues such as the testis and prostate as well as promoting secondary sexual characteristics such as increased muscle, bone mass, and the growth of body hair. In addition, testosterone is essential for health and well-being as well as the prevention of osteoporosis. Testosterone has several major actions. It provides negative feedback inhibition on the secretion of gonadotropin-releasing hormone from the hypothalamus and the secretion of luteinizing hormone from the pituitary gland. It also directs the development of the embryonic Wolffian ducts into the vas deferens (ductus deferens) and seminal vesicles and stimulates the formation of muscle and bone. Dihydrotestosterone is responsible for sperm maturation during spermatogenesis, for the formation of the prostate gland and external genitalia, and for sexual maturation at puberty.

Testosterone can be manufactured by chemical and microbiological modification of inexpensive steroids, such as diosgenin. It is used clinically to treat testicular insufficiency, to suppress lactation (milk production), and to treat certain types of breast cancer.


(II) Molecular Structure: OH

O

(III) Presence of secondary alcoholic group: When testosterone react with the carboxylic acid then forms monoester (monoacetate and monobenzoate). Testosterone also undergoes to oxidation with CrO3 to give androst-4-ene-3,17-dione (i.e. diketone).

NOTES



(III) Presence of secondary alcoholic group: When testosterone react with the carboxylic acid then forms monoester (monoacetate and monobenzoate). Testosterone also undergoes to oxidation with CrO3 to give androst-4-ene- 3,17-dione (i.e. diketone).

43

OH

O

CrO3

O

O

Testosterone Androst-4-ene-3,17-dione

(IV) Presence of α,β-unsaturated ketonic group: Testosterone undergoes to conjugate addition (Michael addition), and it is sensitive to alkali. Presence

of the α,β-unsaturated ketonic group is also confirmed by the UV spectroscopy that it absorb at the 240 nm.

(V) Presence of tetracyclic ring system: Testosterone possess the tetracyclic ring system as we have already discussed in the case of the cholesterol (i.e. 1,2-cyclopentophenanthrene nucleus).

(VI) Synthesis of testosterone: From cholesterol.

HO

(i) Ac2O(ii) Br2

Cholesterol

AcOBr

Br

Cholesterol acetate dibromide

CrO3 / AcOH

AcO

O

Br Br

(i) Zn/AcOH(ii) Hydrolysis

HO

O

Dehydroepiandrosterone

(IV) Presence of α,β-unsaturated ketonic group: Testosterone undergoes to conjugate addition (Michael addition), and it is sensitive to alkali. Presence of the α,β-unsaturated ketonic group is also confirmed by the UV spectroscopy that it absorb at the 240 nm.



43

OH

O

CrO3

O

O

Testosterone Androst-4-ene-3,17-dione

(IV) Presence of α,β-unsaturated ketonic group: Testosterone undergoes to conjugate addition (Michael addition), and it is sensitive to alkali. Presence

of the α,β-unsaturated ketonic group is also confirmed by the UV spectroscopy that it absorb at the 240 nm.



HO

(i) Ac2O(ii) Br2

Cholesterol

AcOBr

Br

Cholesterol acetate dibromide

CrO3 / AcOH

AcO

O

Br Br

(i) Zn/AcOH(ii) Hydrolysis

HO

O

Dehydroepiandrosterone

NOTES



(i) Ac2O(ii) Na–C3H7OH

O

OH

Ph

(ii) Hydrolysis

CO

Cl(i)

HO

OCOPh

Oppenaur oxidation

O

OCOPh

Hydrolysis

O

OH

Testosterone Female sex hormones:

2.8.3 Oestrone (E1 or estrone):

Estrone is an estrogenic hormone secreted by the ovaries as well as adipose tissue. Estrone is an odorless, solid crystalline powder, white in color with a melting point of 254.5 °C. Estrone is one of several natural estrogens, which also include estriol and estradiol. Estrone is the least abundant of the three hormones. Estradiol is present almost always in the reproductive female body, and estriol is abundant primarily during pregnancy.

Estrone is known to be a carcinogen for human females as well as cause breast tenderness or pain, nausea, headache, hypertension, and leg cramps. In men, estrone has been known to cause anorexia, nausea, vomiting, and erectile dysfunction. Estrone is relevant to health and disease states because of its conversion to estrone sulfate, a long-lived derivative. Estrone sulfate acts as a reservoir that can be converted as needed to the more active estradiol. It is the predominant estrogen in postmenopausal women. These are the estrogens, of which estradiol is the most importent. They maintain the female reproductive tissues in a fully functional condition, promote the estrous state of preparedness for mating, and stimulate development of the mammary glands and of other feminine characteristics. Estrogenic steroids have been isolated from urines of pregnant female mammals of many species, including humans, from placental and adrenal tissues, and, unexpectedly, from the testes and urines of stallions.



oestrone (e1 or estrone)Estrone is an estrogenic hormone secreted by the ovaries as well as adipose tissue. Estrone is an odorless, solid crystalline powder, white in color with a melting point of 254.5 °C. Estrone is one of several natural estrogens, which also include estriol and estradiol. Estrone is the least abundant of the three hormones. Estradiol is present almost always in the reproductive female body, and estriol is abundant primarily during pregnancy.

Estrone is known to be a carcinogen for human females as well as cause breast tenderness or pain, nausea, headache, hypertension, and leg cramps. In men, estrone has been known to cause anorexia, nausea, vomiting, and erectile dysfunction. Estrone is relevant to health and disease states because of its conversion to estrone sulfate, a long-lived derivative. Estrone sulfate acts as a reservoir that can be converted as needed to the more active estradiol. It is the predominant estrogen in postmenopausal women. These are the estrogens, of which estradiol is the most importent. They maintain the female reproductive tissues in a fully functional condition, promote the estrous state of preparedness for mating, and stimulate development of the mammary glands and of other feminine characteristics. Estrogenic steroids have been isolated from urines of pregnant female mammals of many species, including humans, from placental and adrenal tissues, and, unexpectedly, from the testes and urines of stallions.



45

3-hydroxyestra-1,3,5(10)-triene-17-one

(III) Presence of keto group: When the estrone react with the one equivalent of hydroxyl amine and semicarbazide then yields monooxime and monosemicarbazone, it reveals that estrone have only one ketonic group.

(IV) Presence and position of the phenolic group: (a) Presence of phenolic group: Estrone forms moester derivative with the acid. Estrone also soluble in alkali to form phenoxide ion, give colouration with FeCl3, it suggest that hydroxyl group is phenolic in nature.

(b) Position of phenolic group: When the monomethyl ether derivative of Grignard reagent with 2-methyl cyclopentanone which further undergoes to distillation then results 7-methoxy-1,2-cyclopentophenenthrene, its formation refers that phenolic group present at C-3 position in benzene ring.

CH3O

H2C

CH2MgBr

+

H3C

O

CH3O

H2C

CH2

OH

H3C–H2O

CH3O

H2C

CH2

H3C AlCl3

CH3O

CH3

Se 320°

CH3O

7-methoxy-1,2-cyclopentophenenthrene

(V) Presence of the steroid nucleus: The X-ray study of the oestrone suggest

NOTES






45

3-hydroxyestra-1,3,5(10)-triene-17-one




CH3O

H2C

CH2MgBr

+

H3C

O

CH3O

H2C

CH2

OH

H3C–H2O

CH3O

H2C

CH2

H3C AlCl3

CH3O

CH3

Se 320°

CH3O

7-methoxy-1,2-cyclopentophenenthrene

(V) Presence of the steroid nucleus: The X-ray study of the oestrone suggest (V) Presence of the steroid nucleus: The X-ray study of the oestrone suggest that oestrone possess steroid nucleus. When steroids distilled with the Zinc then yields chrysene (i.e. hydrocarbon).

NOTES



(VI) Synthesis of Oestrone:

46

that oestrone possess steroid nucleus. When steroids distilled with the Zinc then yields chrysene (i.e. hydrocarbon).

(VI) Synthesis of Oestrone:

H3CO

BrHC CNa

DMF H3CO

C

N

C2H5OH

H CO

H

H3CO

NC2H5

C2H5

H2SO4

H3CO

O

O

O

OHH3cO

O O

O

TSOH

H3CO

O

Ni/H2

(i) CrO3

(ii) HBr/AcOH

H3CO

O

K/NH3NH4Cl

H3CO

OH

H3CO

O

(±) Oestrone

2.9 Summary

Steroids are obtained from animals and human beings. The isolation of steroids from animals and humas beings involve many chemical process, so it is a time consuming process. When the steroids extract is obtained then it will be purified by many chemical process. The steroids may affect many body activities like growth, male and female characteristics and reproductive process. This chapter covers the

4.9 OCCURRENCEProstaglandin any of a group of naturally occurring, chemically related fatty acids that stimulate contractility of the uterine and other smooth muscleand have the ability to lower blood pressure, regulate acid secretion of the stomach, regulate body temperature and platelet aggregation, and control inflammation and vascular permeability; they also affect the action of certain hormones. Nine primary types are labeled A through I, the degree of saturation of the side chain of each being designated by subscripts 1, 2, and 3. The types of prostaglandins are abbreviated PGE2, PGF2a, and so on.

NOTES



4.10 NOMENCLATUREIn the approved nomenclature, each prostaglandin is named using the prefix 'PG' followed by a letter A to K depending on the nature and position of the substituents on the ring. Thus PGA to PGE and PGJ have a keto group in various positions on the ring, and are further distinguished by the presence or absence of double bonds or hydroxyl groups in various positions in the ring. PGF has two hydroxyl groups while PGK has two keto substituents on the ring. PGG and PGH are bicyclic endoperoxides. An oxygen bridge between carbons 6 and 9 distinguishes prostacyclin (PGI). Thromboxane A (TXA) contains an unstable bicyclic oxygenated ring structure, while thromboxane B (TXB) has a stable oxane ring. In addition, all prostaglandins have a hydroxyl group on carbon 15 and a trans-double bond at carbon 13 of the alkyl substituent (R2).

52

ring. In addition, all prostaglandins have a hydroxyl group on carbon 15 and a trans-double bond at carbon 13 of the alkyl substituent (R2).

Further, a numerical subscript (1 to 3) is used to denote the total number of double

bonds in the alkyl substituents, and a Greek subscript (α or β) is used with prostaglandins of the PGF series to describe the stereochemistry of the hydroxyl

group on carbon 9. This is illustrated for prostaglandins PGE and PGFα of the 1, 2 and 3 series below, as examples.

The number of double bonds depends on the nature of the fatty acid precursor. Thus, the prostaglandins PGE1, PGE2 and PGE3 are derived from 8c,11c,14c-

eicosatrienoic (dihomo-γ-linolenic), 5c,8c,11c,14c-eicosatetraenoic (arachidonic) and 5c,8c,11c,14c,17c-eicosapentaenoic acids, respectively. Of these, PGE2 is the most common and is involved in many physiological processes. Dihomo-prostaglandins derived from adrenic acid (22:4(n-6) have also been detected in cell preparations, but no such compounds are produced from docosahexaenoic acid .

COOH

OHHO

O

COOH

OHHO

HO

HO

O

OH

COOH

HO OH

COOHHO

HO

O

COOH

OH

HO

HOCOOH

OH

PGF3

PGE3

PGF2

PGE2

PGF1

PGE1

COOH

8c, 11c, 14c-eicosatrienoic acid(dihomo- -linolenic acid)

COOH

5c, 8c, 11c, 14c-eicosatetraenoic acid(arachidonic acid)

5c, 8c, 11c, 14c, 17c-eicosapentaenoic acid

COOH

Numbering always starts at the carboxylic acid (1-position) and continues around Further, a numerical subscript (1 to 3) is used to denote the total number of double

bonds in the alkyl substituents, and a Greek subscript (α or β) is used with prostaglandins of the PGF series to describe the stereochemistry of the hydroxyl group on carbon 9. This is illustrated for prostaglandins PGE and PGFα of the 1, 2 and 3 series below, as examples.

The number of double bonds depends on the nature of the fatty acid precursor.

Thus, the prostaglandins PGE1, PGE2 and PGE3 are derived from 8c,11c,14ceicosatrienoic (dihomo-γ-linolenic), 5c,8c,11c,14c-eicosatetraenoic (arachidonic) and 5c,8c,11c,14c,17c-

NOTES



eicosapentaenoic acids, respectively. Of these, PGE2 is the most common and is involved in many physiological processes. Dihomoprostaglandins derived from adrenic acid (22:4(n-6) have also been detected in cell preparations, but no such compounds are produced from docosahexaenoic acid.

Numbering always starts at the carboxylic acid (1-position) and continues around the molecule for the entire 20-carbon atoms. They are named as derivatives of prostanoic acid. Thus the PG designates the compound as a derivative of prostanoic acid.

53

the molecule for the entire 20-carbon atoms. They are named as derivatives of prostanoic acid. Thus the PG designates the compound as a derivative of prostanoic acid.

CO H2

12

34

56

78

1213 15

14 16

17

18

19

2010

11

Prostanoic Acid

9

The next letter refers to the type of functionality present at the 8-12 and 15-positions. The origin of these letters refers back to the methods used in the original isolation of these compounds. Thus PGE was partitioned into an Ether layer, while PGF partitioned into phosphate buffer. This prostaglandin was isolated in Sweden where phosphate is (Fosfat). PGA and PGB were so named because of their stability to Acids and Bases. After that letters of the alphabet were just filled in - there is no significance to the letters.

O O O

PGCPGBPGA

HO

O

O

HO

PGD PGE PGF

HO

PGF

HO

HO

HO

The subscripted number refers to the number of double bonds in the prostaglandin. Thus PGE2 has 2 double bonds, one originating from the 5-position and the second

The next letter refers to the type of functionality present at the 8-12 and 15- positions. The origin of these letters refers back to the methods used in the original isolation of these compounds. Thus PGE was partitioned into an Ether layer, while PGF partitioned into phosphate buffer. This prostaglandin was isolated in Sweden where phosphate is (Fosfat). PGA and PGB were so named because of their stability to Acids and Bases. After that letters of the alphabet were just filled in - there is no significance to the letters.

53

the molecule for the entire 20-carbon atoms. They are named as derivatives of prostanoic acid. Thus the PG designates the compound as a derivative of prostanoic acid.

CO H2

12

34

56

78

1213 15

14 16

17

18

19

2010

11

Prostanoic Acid

9

The next letter refers to the type of functionality present at the 8-12 and 15-positions. The origin of these letters refers back to the methods used in the original isolation of these compounds. Thus PGE was partitioned into an Ether layer, while PGF partitioned into phosphate buffer. This prostaglandin was isolated in Sweden where phosphate is (Fosfat). PGA and PGB were so named because of their stability to Acids and Bases. After that letters of the alphabet were just filled in - there is no significance to the letters.

O O O

PGCPGBPGA

HO

O

O

HO

PGD PGE PGF

HO

PGF

HO

HO

HO

The subscripted number refers to the number of double bonds in the prostaglandin. Thus PGE2 has 2 double bonds, one originating from the 5-position and the second

The subscripted number refers to the number of double bonds in the prostaglandin.

NOTES



Thus PGE2 has 2 double bonds, one originating from the 5-position and the second from the 13-position, wherease PGE1 has only one double bond (originating from the 13-position) and PGE3 has 3 double bonds (originating at the 5,13, and 17- positions).

54

from the 13-position, wherease PGE1 has only one double bond (originating from the 13-position) and PGE3 has 3 double bonds (originating at the 5,13, and 17-positions).

O

HO

CO H2O

HO

CO H2

O

HO

CO H2

PGE3

PGE2PGE1

The subscripted Greek letter, as in PGF2α refers to the orientation of the extra hydroxyl group on the ring. Note that in most prostaglandins there is a single hydroxyl group in the ring and another on the side chain at the 15-position and that

they are always oriented a (behind the plane). PGF2α has an extra hydroxyl on the ring and so its orientation must be specified.

3.4 Classification

The structure of PGs comprises of an oxygenated cyclopentane ring with a

heptanoic acid side chain (α-side chain) and an octene side chain (ω-side chain) on adjacent positions of cyclopentane and such a basic structural unit is referred to as a prostanoic acid. PGs differ from the other eicosanoids in the substitution model on the cyclopentane ring and the side-chains, and these differences are accountable for the various biological activities of the members of the prostaglandin family. PGs are generally classified as PGA, PGB, PGC, PGD, PGE, PGF, PGG, and PGH referring to the different oxygen functionalities in the cyclopentane ring substitution patterns. For each general PG class is sub-classified based on the degree of unsaturation (i.e., PGE1, PGE2, and PGF2). The letters and numbers that follow the initial PG abbreviation indicate the nature of the unsaturation and substitution. For example, the subscript 1 in PGE1 indicates one double bond in the side chains, while the 2 in PGE2 indicates two double bonds in the side chains.

The subscripted Greek letter, as in PGF2α refers to the orientation of the extra hydroxyl group on the ring. Note that in most prostaglandins there is a single hydroxyl group in the ring and another on the side chain at the 15-position and that they are always oriented a (behind the plane). PGF2α has an extra hydroxyl on the ring and so its orientation must be specified.

4.11 CLASSIFICATION

55

CO H2

CH3

Prostanoic acid

Classification of Prostaglandins

OR1

R2

PGA

OR1

R2

PGB

R1

R2

O

PGC

R1

R2

HO

O

OR1

R2

PGE

HO

R1

R2

HO

HO

PGD

PGF , PGF

R1

R2

O

OPGG, PGH

R2

HO

R '1

O

PGI

3.5 Biogenesis and Physiological Effects

The prostaglandins are lipid mediators with physiological effects, such as regulation of the contraction and relaxation of smooth muscle tissue. They are synthesized in the cell from the essential fatty acids (EFAs).they have been implicated in inflammation, pain, pyrexia, cardiovascular disease, cancer, glaucoma, allergic rhinitis, asthma preterm labor, male sexual dysfunction and osteoporosis. Physiological action of the prostaglandins 1. Once a call responds to a prostaglandin by changing the intracellular concentrations of some key substance, those changes then trigger a sequence of reactions that produce a "physiological response" (i.e. a contraction, secretion, excitation, etc.). 2. Since the number or prostaglandins and the target cells is large, the effects of these hormones is very variable. 3. An interesting example of opposing action of a pair of prostaglandins is seen in blood coagulation. PGI2, produced by endothelial cells, acts on the local muscle to relax them, decreasing blood pressure and also acts with

NOTES



The structure of PGs comprises of an oxygenated cyclopentane ring with a heptanoic acid side chain (α-side chain) and an octene side chain (ω-side chain) on adjacent positions of cyclopentane and such a basic structural unit is referred to as a prostanoic acid. PGs differ from the other eicosanoids in the substitution model on the cyclopentane ring and the side-chains, and these differences are accountable for the various biological activities of the members of the prostaglandin family. PGs are generally classified as PGA, PGB, PGC, PGD, PGE, PGF, PGG, and PGH referring to the different oxygen functionalities in the cyclopentane ring substitution patterns. For each general PG class is sub-classified based on the degree of unsaturation (i.e., PGE1, PGE2, and PGF2). The letters and numbers that follow the initial PG abbreviation indicate the nature of the unsaturation and substitution. For example, the subscript 1 in PGE1 indicates one double bond in the side chains, while the 2 in PGE2 indicates two double bonds in the side chains.

4.12 BIOGENESIS AND PHYSIOLOGICAL EFFECTSThe prostaglandins are lipid mediators with physiological effects, such as regulation of the contraction and relaxation of smooth muscle tissue. They are synthesized in the cell from the essential fatty acids (EFAs).they have been implicated in inflammation, pain, pyrexia, cardiovascular disease, cancer, glaucoma, allergic rhinitis, asthma preterm labor, male sexual dysfunction and osteoporosis. Physiological action of the prostaglandins 1. Once a call responds to a prostaglandin by changing the intracellular concentrations of some key substance, those changes then trigger a sequence of reactions that produce a "physiological response" (i.e. a contraction, secretion, excitation, etc.). 2. Since the number or prostaglandins and the target cells is large, the effects of these hormones is very variable. 3. An interesting example of opposing action of a pair of prostaglandins is seen in blood coagulation. PGI2, produced by endothelial cells, acts on the local muscle to relax them, decreasing blood pressure and also acts with a receptor on the platelet to inhibit aggregation. It has been suggested PGI2 synthesis prevent platelets from aggregating and/or sticking to vessel walls. When a vessel is injured, the platelets produce TXA2 which blocks PGI2 binding, thus permitting platelet aggregation. Furthermore, TXA2 either inhibits PGI2 synthesis, or it has a direct action on the smooth muscle cell opposite to that of PGI2, thus increasing the muscle contraction (and the low blood pressure). Inhibition of prostaglandin synthesis. 1.

A major mechanism to explain the anti-inflammatory action of certain steroids has been traced to their inhibition of phospholipase A2.This inhibition caused a decrease of arachidonic acid (also the other polyenoic acids which are the precursors of the PG1 and PG3 series), and hence an inhibition of prostaglandin synthesis. 2. Cyclo-oxygenase is inhibited by non-steroidal antiinflamma ory agents such as aspirin, indomethacin and phenylbutazone. In fact the only known biochemical affect of aspirin is prostaglandin synthesis suppression via cyclooxygenase inhibition. Leukotrienes 1. Blood cells (Polymorphonuclear leukocytes, mast cells, etc.) appears to synthesize leukotrienes rather than prostaglandins from arachidonic acid. Again the story is complex. Each cell type specializes in which leukotrienes they synthesize or will responded to. 2. The regulatory first step involves a lipoxygenase enzyme.

NOTES



Lipoxygenase is not inhibited by aspirin, thus leukotriene production is reduced only by the anti-inflmmatory steroids. 3. Leukotriene C is synthesized by Mast cells and has been extremely potent muscle constractant that severely constricts the small airways of the lungs during an asthma attack. 4. Neutrophils synthesize another leukotriene, which alters cell motility and chemotaxis in the immune reponse. Cox is one of two isoenzymes. There are two primary cyclooxygenase enzymes: Cox1 and Cox2. Cox1 help maintain platelet and kidney function and are much needed to maintain homeostasis. Cox2 lead to the production of substances that cause acute or chronic discomfort in joints. Cox2 inhibitors stop the creation of cox2. Go to the Cox2 link on this site to read more about cox2.

Aspirin and other traditional nonsteroidal antiinflammatory drugs (NSAIDs) inhibit the enzyme cyclooxygenase (COX), which is involved in the production of prostaglandins. Prostaglandins are intercellular messengers that are found in high concentrations at sites of chronic inflammation. They are capable of causing vasodilatation, increasing vascular permeability and sensitizing pain receptors. Although many NSAIDs were developed that block the action of COX, all produced gastritis in many patients especially the elderly, those patients with a prior history of peptic ulcer disease and patients on corticosteroids. It is now known that there are two COX enzymes cyclooxygenase 1 (COX-1) and cyclooxygenase 2 (COX- 2). The traditional NSAIDs bind to the active sites of both COX-1 and COX-2. Gastritis is caused by the inhibition of COX-1, which is a gastric COX that regulates mucosal cell production of mucous. (The mucous acts as a barrier to the acid and pepsin present in gastric secretions.) Cox2 inhibitors Cox is an abbreviation for "cyclooxygenase." There are two primary cyclooxygenase enzymes: Cox-1 and cox-2. Cox1 helps maintain platelet and kidney function and are integral to maintaining homeostasis. Cox2 is one of several enzymes that lead to the formation of substances that can cause joint and connective tissue problems. Researchers discovered that cox2 enzyme is involved in several major diseases including: · Alzheimers · Rheumatic and Osteo-Arthritis · Cancer · Kidney disease · Osteoporosis A new class of cox2 inhibiting pain relief medications are beginning to enter the market. These new medications are reportedly safer alternatives to the current NSAIDs (Non-steroidal Anti-Inflammatory Drugs). The older medications can be problematic because of serious side effects: diarrhea, nausea, vomiting, kidney problems, liver problems, bleeding ulcers Therefore, drug companies are marketing safer pain medications, primarily for use on arthritis and related aches and pains.

Some diuretics, such as furosemide, may act in part by releasing prostaglandins in the kidney. Prostaglandins inhibit the action of vasepressin on the kidney tubules, resulting in enhanced urinary excretion of water. The resultant tendency to dehydration from this enhanced excretion of water leads to local secretion of another kidney prostaglandin that stimulates the secretion of renin. Renin stimulates the production of aldosterone, which has the affect of conserving sodium and water, thus combating the dehydration and elevating the depressed blood pressure. Although prostaglandins were first detected in semen, no biologic role for them has been defined in the male reproductive system. This is not true, however, for females. It has been shown that prostaglandins mediate the control of GnRH over LH secretion, modulate ovulation, and stimulate uterine muscle contraction. Discovery of this

NOTES



last property has led to the successful treatment of menstrual cramps (dysmenorrhea) through the use of NSAIDs as inhibitors of prostaglandin synthesis. Prostaglandins also play a role in inducing labor in pregnant women at term or in inducing therapeutic abortions.

57

patients on corticosteroids. It is now known that there are two COX enzymes cyclooxygenase 1 (COX-1) and cyclooxygenase 2 (COX- 2). The traditional NSAIDs bind to the active sites of both COX-1 and COX-2. Gastritis is caused by the inhibition of COX-1, which is a gastric COX that regulates mucosal cell production of mucous. (The mucous acts as a barrier to the acid and pepsin present in gastric secretions.) Cox2 inhibitors Cox is an abbreviation for "cyclooxygenase." There are two primary cyclooxygenase enzymes: Cox-1 and cox-2. Cox1 helps maintain platelet and kidney function and are integral to maintaining homeostasis. Cox2 is one of several enzymes that lead to the formation of substances that can cause joint and connective tissue problems. Researchers discovered that cox2 enzyme is involved in several major diseases including: · Alzheimers · Rheumatic and Osteo-Arthritis · Cancer · Kidney disease · Osteoporosis A new class of cox2 inhibiting pain relief medications are beginning to enter the market. These new medications are reportedly safer alternatives to the current NSAIDs (Non-steroidal Anti-Inflammatory Drugs). The older medications can be problematic because of serious side effects: diarrhea, nausea, vomiting, kidney problems, liver problems, bleeding ulcers Therefore, drug companies are marketing safer pain medications, primarily for use on arthritis and related aches and pains.

PG Receptor Endogenous Ligand

Signaling Pathway

EP1 PGE2

Increased Ca++ via PLC stimulation

EP2

PGE2

Increased cAMP via AC stimulation

EP3

PGE2

Decreased cAMP via AC inhibition

EP4

PGE2

Increased cAMP via AC stimulation

FP PGF2


58

DP

PGD2


IP

PGI2


TP

TxA2


Some diuretics, such as furosemide, may act in part by releasing prostaglandins in the kidney. Prostaglandins inhibit the action of vasepressin on the kidney tubules, resulting in enhanced urinary excretion of water. The resultant tendency to dehydration from this enhanced excretion of water leads to local secretion of another kidney prostaglandin that stimulates the secretion of renin. Renin stimulates the production of aldosterone, which has the affect of conserving sodium and water, thus combating the dehydration and elevating the depressed blood pressure. Although prostaglandins were first detected in semen, no biologic role for them has been defined in the male reproductive system. This is not true, however, for females. It has been shown that prostaglandins mediate the control of GnRH over LH secretion, modulate ovulation, and stimulate uterine muscle contraction. Discovery of this last property has led to the successful treatment of menstrual cramps (dysmenorrhea) through the use of NSAIDs as inhibitors of prostaglandin synthesis. Prostaglandins also play a role in inducing labor in pregnant women at term or in inducing therapeutic abortions.

Summary of the Physiologic Actions of the Eicosanoids

Eicosanoid Biochemical and Physiologic Action

PGD2 • Weak inhibitor of platelet aggregation

PGE1 • Bronchial Vasodilation

• Inhibitor of lipolysis

• Inhibitor of platelet aggregation

• Contraction of GI smooth muscle


58

DP

PGD2


IP

PGI2


TP

TxA2


Some diuretics, such as furosemide, may act in part by releasing prostaglandins in the kidney. Prostaglandins inhibit the action of vasepressin on the kidney tubules, resulting in enhanced urinary excretion of water. The resultant tendency to dehydration from this enhanced excretion of water leads to local secretion of another kidney prostaglandin that stimulates the secretion of renin. Renin stimulates the production of aldosterone, which has the affect of conserving sodium and water, thus combating the dehydration and elevating the depressed blood pressure. Although prostaglandins were first detected in semen, no biologic role for them has been defined in the male reproductive system. This is not true, however, for females. It has been shown that prostaglandins mediate the control of GnRH over LH secretion, modulate ovulation, and stimulate uterine muscle contraction. Discovery of this last property has led to the successful treatment of menstrual cramps (dysmenorrhea) through the use of NSAIDs as inhibitors of prostaglandin synthesis. Prostaglandins also play a role in inducing labor in pregnant women at term or in inducing therapeutic abortions.


Eicosanoid Biochemical and Physiologic Action

PGD2 • Weak inhibitor of platelet aggregation

PGE1 • Bronchial Vasodilation

• Inhibitor of lipolysis


• Contraction of GI smooth muscle

NOTES



• Stimulates hyperalgesic response (sensitize to pain)

59

PGE2

• Stimulates hyperalgesic response (sensitize to pain)

• Renal and bronchial vasodilation


• Stimulates uterine smooth muscle relaxation

• Cytoprotection: Protects GI epithelial cells from acid degradation

• Reduces gastric acid secretion

• Elevates thermoregulatory set-point in anterior hypothalamus (fever)

• Promotes inflammation

PGF2 • Stimulates breakdown on corpus luteum (luteolysis): Animals

• Stimulates uterine smooth muscle contraction

• Bronchial constrictor

PGI2

• Potent inhibitor of platelet aggregation

• Potent transient CV vasodilator, then vasodilator

• Bronchial dilator

• Uterine relaxant

• Sensitize/amplify nerve pain response

TXA2

• Potent inducer of platelet aggregation

• Potent vasconstrictor (bronchioles, renal)

• Decreases cAMP levels in platelets

• Stimulates the release of ADP and 5-HT from platelets

LTB4

• Increases leukocyte chemotaxis and aggregation

LTC/D4

• Slow-reacting substance of anaphylaxis

• Potent and prolonged contraction of ileal smooth muscle (Animals)

NOTES



60

• Contraction of lung parenchymal strips (Animals)

• Bronchoconstriction in humans

• Increased vascular permeability in skin (Animals)

5- or 12-HPETE

• Vasodilation of gastric cirulation (Animals)

5- or 12-PETE

• Aggregates human leukoctyes

• Promotes leukocyte chemotaxis

3.6 THE SYNTHESIS OF NATURAL E-SERIES PROSTAGLANDINS.

Because of the increasing demand for prostaglandins, biosynthesis which used to be the only source of these compounds became inadequate to meet this demand. Therefore, the development of an efficient PG chemical synthesis has been necessary as the only way to provide sufficient quantities of these compounds. A

simplified retrosynthetic analysis of PGF2α, PGE1, and PGE2 reveals the synthons shown in scheme 1. The Corey synthesis which consists of a two-fold Wittig type chain extension of the chiral dialdehyde synthon shown in scheme 1 will not be discussed here since it is developed through the use of cyclic systems. The two other methods suggested by the retrosynthetic analysis both involve the development of the synthesis via conjugate addition type reactions and hence they are the subject of this review.

3.6.1(I) Synthesis of PGE1, PGE2, PGE3

Organometallic reagents have played an important role in developing many synthetic methodologies to prostaglandins via the conjugate addition approach. The construction of PGs via the 1,4 addition of organocuprate reagents to a, f3-unsaturated ketones was initially developed by Sih and Fried in 1972. They found that the chiral organocuprate reagent 8 underwent conjugate addition to 2- cyclopenten-1-one to give exclusively the 1,4-addition adduct. Sih and his group later 4b developed this approach for the synthesis of natural PGE1. They reported that addition of the chiral cuprate complex 8, prepared from the organolithium derivative of the co side-chain, to the optically active cyclopentenone derivative 7 afforded, after deprotection, natural PGE1 in good yield (scheme 2). This type of conjugate addition has been modified and improved extensively in the past ten

4.13 THE SYNTHESIS OF NATURAL E-SERIESPROSTAGLANDINSBecause of the increasing demand for prostaglandins, biosynthesis which used to be the only source of these compounds became inadequate to meet this demand. Therefore, the development of an efficient PG chemical synthesis has been necessary as the only way to provide sufficient quantities of these compounds. A simplified retrosynthetic analysis of PGF2α, PGE1, and PGE2 reveals the synthons shown in scheme 1. The Corey synthesis which consists of a two-fold Wittig type chain extension of the chiral dialdehyde synthon shown in scheme 1 will not be discussed here since it is developed through the use of cyclic systems. The two other methods suggested by the retrosynthetic analysis both involve the development of the synthesis via conjugate addition type reactions and hence they are the subject of this review.

(i) syntHesis oF pGe1, pGe2, pGe3Organometallic reagents have played an important role in developing many synthetic methodologies to prostaglandins via the conjugate addition approach. The construction of PGs via the 1,4 addition of organocuprate reagents to a, f3- unsaturated ketones was initially developed by Sih and Fried in 1972. They found that the chiral organocuprate reagent 8 underwent conjugate addition to 2- cyclopenten-1-one to give exclusively the 1,4-addition adduct. Sih and his group later 4b developed this approach for the synthesis of natural PGE1. They reported that addition of the chiral cuprate complex 8, prepared from the organolithium derivative of the co side-chain, to the optically active cyclopentenone derivative 7 afforded, after deprotection, natural PGE1 in good yield (scheme 2). This type of conjugate addition has been modified and improved extensively in the past ten years as the search for more selective reagents and catalysts continued. Weiss and co-workers also developed a novel method for effecting the 1,4-conjugate additions to cyclopentenones using trialkyl-trans-1-alkenylalanates reagents. They reported the synthesis of PGE1 and congeners

NOTES



via conjugate addition of alanate reagents, carrying the co side-chain of prostaglandin, to the cyclopentenone derivative (±)-1.

61

years as the search for more selective reagents and catalysts continued. Weiss and co-workers also developed a novel method for effecting the 1,4-conjugate additions to cyclopentenones using trialkyl-trans-1-alkenylalanates reagents. They reported the synthesis of PGE1 and congeners via conjugate addition of alanate reagents, carrying the co side-chain of prostaglandin, to the cyclopentenone derivative (±)-1.

O

C H5 11Cl

3

Nal/acetone

O2

C H5 11I

I

OH

C H5 11

( )-(1)

optical

resolution

I

OH1

C H5 11

Penicilliumdecumbens

2

2 THF, - 100 Co

4

Li +

O

O

H

OC H2 5

Al

Scheme 1 : Synthetic routes to optically active trans-1-(S)-hydroxy-1-iodo-octene, 1 Scheme 1 : Synthetic routes to optically active trans-1-(S)-hydroxy-1-iodo-octene, 1

II. The three-component conjugate addition method.

For directness and high flexibility, the three-component coupling process has been developed recently as the shortest and most convenient synthetic route for the preparation of PGs. As the name suggests, the three-component coupling process is a one-pot combination of the cyclopentenone ring and the two side chains to construct the PG skeleton. The synthesis is initiated by a nucleophilic transfer of the m side-chain unit to a protected 4-(R)-hydroxy-2-cyclopentenone 9 followed by an electrophilic trapping of the enolate intermediate 10 with a side-chain equivalent (RαX) leading to the required prostaglandin skeleton 11 (scheme 3).

NOTES



62

O

OR

(CH ) - CO CH2 5 2 3

5, R = THP6, R = Si (C H )2 5 3

7, R = Si(CH ) - Bu3 2

1

+8

LiCu

O

O

R1

2

- (n-C H ) P4 9 3

R = n-C H1 5 11

0 Co

dry ether

O

OTHP O

O

CO CH2 3

2. Rhizopus oryzae1. H O3

+

O

OH OH

COOH

Scheme 2 : Synthesis of PGE1 by conjugate cuprate aaddition

II. The three-component conjugate addition method.

For directness and high flexibility, the three-component coupling process has been developed recently as the shortest and most convenient synthetic route for the preparation of PGs. As the

name suggests, the three-component coupling process is a one-pot combination of the cyclopentenone ring and the two side chains to construct the PG skeleton. The synthesis is initiated by a nucleophilic transfer of the m side-chain unit to a protected 4-(R)-hydroxy-2-cyclopentenone 9 followed by an electrophilic trapping

of the enolate intermediate 10 with a side-chain equivalent (RαX) leading to the required prostaglandin skeleton 11 (scheme 3).

Scheme 2 : Synthesis of PGE1 by conjugate cuprate aaddition

Since organocuprate reagents have been used to deliver organic groups to the position of a, unsaturated ketones, one might expect that conjugate addition of the w side-chain unit to the 4-hydroxy-2-cyclopentenone followed by alkylation of the resulting enolate with alkyl halides carrying the side-chain could lead directly to PGs derivatives. However, in reality, such a process is not easy to achieve (see below).

63

Since organocuprate reagents have been used to deliver organic groups to the position of a, unsaturated ketones, one might expect that conjugate addition of the side-chain unit to the 4-hydroxy-2-cyclopentenone followed by alkylation of the resulting enolate with alkyl halides carrying the side-chain could lead directly to PGs derivatives. However, in reality, such a process is not easy to achieve (see below).

O

OR

9

R M

O M +

R

OR

10

R X

O

R

OR

R

11 R is a suitable protecting group ,

RM is the organometallic complex carrying the lower side chain of PG,

RM = Li , Cu, etc.

Rx is the halide carrying the upper side chain of PG, Rex. X = I or Br.

Scheme 3 : Basic strategy of the three-component coupling process.

The second objective was to develop a method for the conjugate addition that allows clean enolate trapping which has been the troublesome step in the three-component coupling process. Organometallic reagents have been employed extensively to facilitate such coupling; however, they have to be used in excess in order to ensure the conjugate addition. When Noyori and his co-workers reinvestigated the use of excess of the side-chain equivalent, they found that the excess organometallic compound only made the reaction system more complicated. This was rationalized by the fact that if the nucleophile carying the m side-chain is not used in one equivalent, the resulting enolate would not be the only strong nucleophile present in the reaction system. Thus, this would disturb the reaction between the enolate species and the . side-chain electrophile. In this context, they prepared the organocopper reagent from equimolar amounts of copper(l) iodide and the organo lithium compound (carrying the side-chain) and 2-3 equivalents of tri-nbutylphosphine. When this reagent was allowed to react with 2-cyclopentenone, the conjugate addition product was obtained in high yield.

R is a suitable protecting group ,

NOTES



RwM is the organometallic complex carrying the lower side chain of PG,RwM = Li , Cu, etc.

Rax is the halide carrying the upper side chain of PG, Rex. X = I or Br.

Scheme 3 : Basic strategy of the three-component coupling process.

64

More importantly, the enolate was trapped efficiently with one equivalent of an aldehyde to give the aldol adduct On these bases, they synthesized PGE1 in five steps 7, 9 via the three-component coupling process which recently10 came to be known as the Aldol route.

O

OTHP

(a) R Li-CuI-2.6 (n-C H ) P4 9 3

(b) R CHO

O

OTHP OTHP

CO CH2 3

CO CH2 3

17

O

OTHP OTHP 18

CO CH2 3

19

O

OTHP OTHP

O

HO OHPGE1

COOH

12, R = THP13, R = Si(C H )14, R = Si(CH ) - Bu

2 5 3

3 2

1

CH SO Cl3 2

4-dimethylaminopyridine

zinc dustCH COOH - (CH ) CH OH3 3 2

(5 : 95)

CH COOH - H O - THF2 2

porcine liver esterase

R CHO - OHC-(CH ) - COOCH 2 5 3

ORR 1 - I

15, R = THP16, R = Si(CH ) - Bu3 2

1

OH

Scheme 4 . Convergent synthesis of PGE1 ( aldol route).

The second objective was to develop a method for the conjugate addition that allows clean enolate trapping which has been the troublesome step in the threecomponent coupling process. Organometallic reagents have been employed extensively to facilitate such coupling; however, they have to be used in excess in order to ensure the conjugate addition. When Noyori and his co-workers reinvestigated the use of excess of the w side-chain equivalent, they found that the excess organometallic compound only made the reaction system more complicated. This was rationalized by the fact that if the nucleophile carying the m side-chain is not used in one equivalent, the resulting enolate would not be the only strong nucleophile

NOTES



present in the reaction system. Thus, this would disturb the reaction between the enolate species and the w . side-chain electrophile. In this context, they prepared the organocopper reagent from equimolar amounts of copper(l) iodide and the organo lithium compound (carrying the w side-chain) and 2-3 equivalents of tri-nbutylphosphine. When this reagent was allowed to react with 2-cyclopentenone, the conjugate addition product was obtained in high yield.

More importantly, the enolate was trapped efficiently with one equivalent of an aldehyde to give the aldol adduct On these bases, they synthesized PGE1 in five steps 7, 9 via the three-component coupling process which recently10 came to be known as the Aldol route.

The synthesis is illustrated in scheme 4. When the organocopper reagent formed from iodide 15 was coupled with the enone 12, and the resulting enolate was trapped with methyl 6 formylhexanoate, the desired aldol 17 was obtained in 83% yield. Removal of the C-7 hydroxyl group by methanesulfonyl chloride and 4- (dimethylamino) pyridine then gave 18 in 92 % yield. Exposure of 18 to zinc dust in 2-propanol / acetic acid (95 : 5) gave 19 in 84% yield. The yield of 19 was improved to 90% when tributyl tin hydride and di-t-butylperoxide were used. Removal of the tetrahydropyranyl protective groups and subsequent enzymatic hydrolysis of the ester functional group afforded natural PGE1 in 56% overall yield.

Another, even shorter, synthesis of natural PGE1 using this three-component coupling process has been reported. The method involved the tandem conjugate addition of the lower side chain equivalent to enone 14 followed by Michael addition of the generated enolate across a nitroolefin carrying the w--side chain to give the expected conjugate addition adduct which was easily transformed to PGE1. As illustrated in Scheme 4 the four chiral centres of natural PGE1 are constructed in an efficient way. The absolute configuration at C-11 and C-15 is established at the stage of preparation of starting enone and the w- side-chain components. The trans relationship of the three ring-substituents is effected by the conjugate addition of the organo copper reagent and subsequent operations. The trans relationship is favoured kinetically because of steric interactions with the C- 11 functionality. At this stage, it is important to emphasize the point that the synthesis of (-)-PGE1 presented above is still considered to be an indirect route for construction of the PGE1 skeleton. That is to say that although the synthesis employs the three-component coupling process mentioned above, subsequent operations had to be done in order to construct the required PGE1 derivative 19.

Recently, a three-step route to PGE2 was established through the tandem conjugate addition/alkylation sequence (scheme5). The treatment of the enone 14 with the organocopper reagent, prepared from 16 in the presence of hexamethyl phosphoric triamide (HMPA) and triphenyltin chloride, followed by addition of Zallylic iodide 20 afforded the PGE2 derivative 21 in 78% yield. This was a single-pot preparation. Natural PGE2 is then obtained by removal of the protecting groups. When this three-component coupling is done with the proparyglic iodide 22, the 5,6-dehydro-PGE2 derivative 23 is produced. Compound 23 serves as a common intermediate for the synthesis of some naturally occuring PGs. For example, partial hydrogenation of the 5,6 triple bond could be done over 5% Pd/BaSO4 catalyst to

NOTES



give PGE2. Unfortunately the above direct approaches do not work for the construction of PGE1 skeleton because a reactive a side-chain equivalents such as 20 or 22 would be needed to achieve efficient enolate trapping . Recently, the discovery that dimethyl zinc was found to enhance the alkylation of lithium enolates improved this triply convergent synthesis.

66

preparation. Natural PGE2 is then obtained by removal of the protecting groups. When this three-component coupling is done with the proparyglic iodide 22, the 5,6-dehydro-PGE2 derivative 23 is produced. Compound 23 serves as a common intermediate for the synthesis of some naturally occuring PGs. For example, partial hydrogenation of the 5,6 triple bond could be done over 5% Pd/BaSO4 catalyst to give PGE2. Unfortunately the above direct approaches do not work for the construction of PGE1 skeleton because a reactive a side-chain equivalents such as 20 or 22 would be needed to achieve efficient enolate trapping . Recently, the discovery that dimethyl zinc was found to enhance the alkylation of lithium enolates improved this triply convergent synthesis.

O

OSiR3

14

(a) R I

(a) R I

(b) R1

O

OSiR3

COOCH3

21OSiR3

O

OSiR3

14

(b) R2

SiR - Si (CH ) - t-C H3 3 2 4 9

I

16 OSiR3

R I =

R = 1 I

20CO CH2 3

CO CH2 3I

22

R = 2

variety of PG’s

23

O

OSiR3

COOCH3

OSiR3

Scheme 5 : Direct three component coupling process for the synthesis of the PGE2

derivative 21 and the 5, 6-dihydro-PGE2 derivative 23.

66

preparation. Natural PGE2 is then obtained by removal of the protecting groups. When this three-component coupling is done with the proparyglic iodide 22, the 5,6-dehydro-PGE2 derivative 23 is produced. Compound 23 serves as a common intermediate for the synthesis of some naturally occuring PGs. For example, partial hydrogenation of the 5,6 triple bond could be done over 5% Pd/BaSO4 catalyst to give PGE2. Unfortunately the above direct approaches do not work for the construction of PGE1 skeleton because a reactive a side-chain equivalents such as 20 or 22 would be needed to achieve efficient enolate trapping . Recently, the discovery that dimethyl zinc was found to enhance the alkylation of lithium enolates improved this triply convergent synthesis.

O

OSiR3

14

(a) R I

(a) R I

(b) R1

O

OSiR3

COOCH3

21OSiR3

O

OSiR3

14

(b) R2

SiR - Si (CH ) - t-C H3 3 2 4 9

I

16 OSiR3

R I =

R = 1 I

20CO CH2 3

CO CH2 3I

22

R = 2

variety of PG’s

23

O

OSiR3

COOCH3

OSiR3

Scheme 5 : Direct three component coupling process for the synthesis of the PGE2

derivative 21 and the 5, 6-dihydro-PGE2 derivative 23.

Scheme 5 : Direct three component coupling process for the synthesis of the PGE2 derivative 21 and the 5, 6-dihydro-PGE2 derivative 23.

Johnson and Penning succeeded in eliminating the equilibration of the enolate resulting from the conjugate addition of the w- side-chain to the enone in the three component coupling process. They postulated that enolate equilibration would be suppressed in the presence of a-oxygen functionality constrained in the five membered ring. This requirement was satisfied by the enone 24 which was prepared in six steps in 40% yield from cyclopentadiene. When the conjugate addition was carried out, the trapping of enolate and subsequent alkylation went cleanly to afford the expected conjugate addition/ enolate trapping adduct in excellent yield.

67

Johnson and Penning succeeded in eliminating the equilibration of the enolate resulting from the conjugate addition of the side-chain to the enone in the three component coupling process. They postulated that enolate equilibration would be suppressed in the presence of a-oxygen functionality constrained in the five membered ring. This requirement was satisfied by the enone 24 which was prepared in six steps in 40% yield from cyclopentadiene. When the conjugate addition was carried out, the trapping of enolate and subsequent alkylation went cleanly to afford the expected conjugate addition/ enolate trapping adduct in excellent yield.

O

O

O

All the discussion above has dealt with PGE1 and PGE2 and nothing has been said concerning PGE3· This was done intentionally because most of the literature work has been done on PGE1 and PGE2. PGE3 is actually a PGE2 analogue in which there is additional

double bond between C-17 and C-18. Therefore most of the chemistry presented above should apply for PGE3. The difficulty in synthesizing PGE3 lies in the synthesis of the lower side-chain with the extra double bond. Recently, Okamoto and coworkers developed a highly efficient synthesis of natural PGE3.

PGE3 :

Prostaglandin E3 (PGE3) is formed via the cyclooxygenase (COX) metabolism of eicosapentaenoic acid.1 In human ocular tissue, it comprises 2.4% of the COX products formed.1 When applied to the eyes of a rabbit, a 1 µg dose of PGE3 decreases intraocular pressure from 21 mmHg to about 17 mmHg.2

NOTES



All the discussion above has dealt with PGE1 and PGE2 and nothing has been said concerning PGE3· This was done intentionally because most of the literature work has been done on PGE1 and PGE2. PGE3 is actually a PGE2 analogue in which there is additional double bond between C-17 and C-18. Therefore most of the chemistry presented above should apply for PGE3. The difficulty in synthesizing PGE3 lies in the synthesis of the lower w side-chain with the extra double bond. Recently, Okamoto and coworkers developed a highly efficient synthesis of natural PGE3.

PGE3 :Prostaglandin E3 (PGE3) is formed via the cyclooxygenase (COX) metabolism of eicosapentaenoic acid.In human ocular tissue, it comprises 2.4% of the COX products formed. When applied to the eyes of a rabbit, a 1 μg dose of PGE3 decreases intraocular pressure from 21 mmHg to about 17 mmHg.

68

O

HO

OH

COOH

3.7 Summary

The many actions of the prostaglandins in reproductive physiology are truly remarkable, as is our rapidly expanding knowledge of these effects. Our knowledge remains far from complete, however, and much more research is needed to fully elucidate the role(s) of prostaglandins in many physiologic processes, particularly in areas such as luteolysis, where the hope is that these biologically active lipids may provide a method for regulating menstruation and fertility. Other important fields of clinical significance that deserve further attention are parturition and ductus arteriosus function. Further developments in prostaglandin research are likely to include the clinical application of more selective inhibitors, antagonists, and long-acting superpotent agonist analogues of prostaglandins.

3.8 Review Question

1. What molecule is the source of arachidonic acid and where is it found?

2. What role does arachidonic acid play in prostaglandin production.

3. What is the action of prostaglandins synthesis on diuretics ?

4. Outline the physiological roles of prostaglandins in the body.

5. Identify the prostaglandins that belong to the omega-3 and omega-6 prostaglandin.

6. Give the brief account of the synthesis of prostanoids highlight the clinical significance if any of this pathway.

7. How are prostaglandins catabolized in the body?

8. Discus the synthesis of PGE1, PGE2.

3.9 Reference and Suggested reading

4.14 SUMMARYThe many actions of the prostaglandins in reproductive physiology are truly remarkable, as is our rapidly expanding knowledge of these effects. Our knowledge remains far from complete, however, and much more research is needed to fully elucidate the role(s) of prostaglandins in many physiologic processes, particularly in areas such as luteolysis, where the hope is that these biologically active lipids may provide a method for regulating menstruation and fertility. Other important fields of clinical significance that deserve further attention are parturition and ductus arteriosus function. Further developments in prostaglandin research are likely to include the clinical application of more selective inhibitors, antagonists, and long-acting superpotent agonist analogues of prostaglandins.

Steroids are obtained from animals and human beings. The isolation of steroids from animals and humas beings involve many chemical process, so it is a time consuming process. When the steroids extract is obtained then it will be purified by many chemical process. The steroids may affect many body activities like growth, male and female characteristics and reproductive process. This chapter covers the study of steroids and sex hormones e.g. cholesterol, testosterone and oestrone under molecular formula, structure, separation, chemical reaction and synthesis.

4.15 REVIEW QUESTIONS1. What molecule is the source of arachidonic acid and where is it found?

2. What role does arachidonic acid play in prostaglandin production.

NOTES



3. What is the action of prostaglandins synthesis on diuretics?

4. Outline the physiological roles of prostaglandins in the body.

5. Identify the prostaglandins that belong to the omega-3 and omega-6 prostaglandin.

6. Give the brief account of the synthesis of prostanoids highlight the clinical significance if any of this pathway.

7. Give the introduction of steroids including their occurrence and isolation ?

8. Write short note on following points

(i) Nomenclature of steroids

(ii) Stereochemistry of steroids

9. Give the details about the structure of cholesterol.

10. Explain the following in cholesterol:

(a) Presence and nature of hydroxyl group

(b) Presence and position of double bond

(c) Presence of cyclopentophenanthrene nucleus

(d) Synthesis of cholesterol

11. Explain the detailed structure of the testosterone.

12. Give the evidences of the presence of following in oestrone –

(a) Presence and position of hydroxyl (phenolic) group

(b) Presence of steroid nucleus

(c) Synthesis of oestrone

13. How are prostaglandins catabolized in the body?

14. Discuss the synthesis of PGE1, PGE2.

4.16 FURTHER READINGSzz Textbook of organic chemistry, Vol II by I L Finar

zz Chemistry of natural products, Vol 1- 12, by Atta-Ur-Rahman

zz introduction to the chemistry of terpenoids and steroids, by William Templeton

zz Systematic identification of flavonoid compounds by Mabry & Markham

zz Organic chemistry, Volume 2 : Stereochemistry and the chemistry of natural products, fifth edition – I.L. Finar.


zz Biogenesis of natural products, Pergamon (1967, 2nd edn.)

zz Steroids reaction mechanism, Elsevier (1968).

NOTES



CHAPTER – 5

ROTENOIDS AND PORPHYRINS

STRUCTURE 5.1 Learning Objectives

5.2 Introduction: Rotionoids

5.3 Introduction: Porphyrine

5.4 Summary




zz To structure and synthesis of rotenoids

zz To understand the synthesis of hemoglobin

zz To discus the synthesis of chlorophyll

5.2 INTRODUCTION: ROTIONOIDSThe organic compounds which are having Rotexen ring system are called rotenoids. Rotenoids are present in several tropical and subtropical plants. These are derived from isoflavone and found in same plants containing isoflavone. Rotanoids are isoflavnons that have been modified with one extra carbon atom.

Rotenone is an odourless, colourless, crystalline ketonic chemical compound used as an insecticide, piscicide, and pesticide. It is classified by world health organization as moderately hazardous and mildly toxic to humans and other mammels but it is extremely toxic to insects, pest and fishes. It occurs naturally in the seeds and stems of several plants, such as the jicama vine plant; and the roots of several members of Fabaceae. However, rotenone has a serious limitation to its widespread usage due to its susceptibility towards ultraviolet rays or solar irradiation. Owing to rotenone’s high photolability, either breaking down or isomerizing in the presence of sunlight will decrease its bioactivities under field conditions, resulting in poor persistence in the environment and inadequate field performance.

NOTES

Rotenoids and Porphyrins


structure: rotenone

The molecular formula of rotenone is C23H22O6. It is a colourless crystalline compound having melting point 165-166ºC. Rotenone (fig.1) was first isolated by Geoffroy in 1892 from Lonchocarpus nicou. Several other related compounds are also known and all of them possess the same fused tetracyclic skeletal structure named rotexen (fig.2).

71

hydrolytic cleavage to yield derrisic acid (fig.5). Derrisic acid, on oxidation with alkaline hydrogen peroxide, breaks down to derric acid (fig.6) which on further oxidation gives rissic acid (fig.7). When rotenone is subjected to a vigorous treatment with hot alkali, it undergoes extensive degradation to yield tubaic acid (fig.8).

Fig.1

Fig. 2

Fig. 1

71

hydrolytic cleavage to yield derrisic acid (fig.5). Derrisic acid, on oxidation with alkaline hydrogen peroxide, breaks down to derric acid (fig.6) which on further oxidation gives rissic acid (fig.7). When rotenone is subjected to a vigorous treatment with hot alkali, it undergoes extensive degradation to yield tubaic acid (fig.8).

Fig.1

Fig. 2

Fig. 2

Rotenone is 6a,12a,4’, 5’-tetrahydro-2,3-dimethoxy-5’-isopropenylfurano-(3’,2’,8,9) 6H-rotexen-12-one. Rotenone can be readily dehydrogenated using oxidizing agents such as potassium permanganate in acetone, potassium ferricyanide in methanol, perbenzoic acid in chloroform, manganese dioxide in acetone or iodine-sodium acetate in ethanol to obtain 6a, 12a-dehydrorotenone (fig.3). On treatment with alcoholic potash dehydrorotenone gives an unstable intermediate (fig.4) which undergoes further hydrolytic cleavage to yield derrisic acid (fig.5). Derrisic acid, on oxidation with alkaline hydrogen peroxide, breaks down to derric acid (fig.6) which on further oxidation gives rissic acid (fig.7).

When rotenone is subjected to a vigorous treatment with hot alkali, it undergoes extensive degradation to yield tubaic acid (fig.8).

NOTES



72

Fig.3

Fig.4

Fig. 3

72

Fig.3

Fig.4

Fig. 4

72

Fig.3

Fig.4

Fig. 5

NOTES



72

Fig.3

Fig.4

Fig. 6

73

Fig.5 Fig. 6

Fig.7 Fig.8

This salicylic acid derivative (it gives a violet colour with alcoholic ferric chloride) gives a dihydro derivative (Fig.9) on catalytic hydrogenation.It indicate presence of phenolic –OH group. Both (Fig.8) and (Fig.9) on prolonged hydrogenation yield the optically inactive tatrahydrotubaic aciod (Fig.10). The latter on decarboxylation, by heating to its melting point, gives 2-isoamylresorcinol (tetrahydrotubanol) (Fig.11).

Fig. 7

73

Fig.5 Fig. 6

Fig.7 Fig.8


Fig. 8

This salicylic acid derivative (it gives a violet colour with alcoholic ferric chloride) gives a dihydro derivative (Fig.9) on catalytic hydrogenation. It indicate presence of phenolic –OH group. Both (Fig.8) and (Fig.9) on prolonged hydrogenation yield the optically inactive tatrahydrotubaic aciod (Fig.10). The latter on decarboxylation, by heating to its melting point, gives 2-isoamylresorcinol (tetrahydrotubanol) (Fig. 11).

NOTES



73

Fig.5 Fig. 6

Fig.7 Fig.8


Fig. 9

73

Fig.5 Fig. 6

Fig.7 Fig.8


Fig. 10

74

Fig.9 Fig.10

Fig.11

The reactions described above thus revealed the two parts of rotenone structure. Combining these data, the complete pentacyclic structure of rotenone was deduced.

Besides this derrisic acid forms a mono oxime derivative with NH2OH. It gives negative test of aldehyde which indicate the presence of ketonic group. Zeisel method indicate the presence of two –OCH3 group. On estrification with diazomethane derrisic acid produces a mono methyl ester, which indicate the presence of one –COOH group. The presence of carbonyl group in hydrogenation products points out that Tubanol is attached to derrisic acid through –CO- group in derrisic acid.

4.2.2 Synthesis of rotenone

The conversion of derrisic acid (Fig.5) to dehydrorotenone (Fig.3) was readily achieved but the selective hydrogenation of the latter to rotenone proved difficult.

Fig. 11

The reactions described above thus revealed the two parts of rotenone structure. Combining these data, the complete pentacyclic structure of rotenone was deduced. Besides this derrisic acid forms a mono oxime derivative with NH2OH. It gives negative test of aldehyde which indicate the presence of ketonic group. Zeisel method indicate the presence of two –OCH3 group. On estrification with diazomethane derrisic acid produces a mono methyl ester, which indicate the presence of one –COOH group. The presence of carbonyl group in hydrogenation products points out that Tubanol is attached to derrisic acid through –CO- group in derrisic acid.

syntHesis oF rotenone

The conversion of derrisic acid (Fig.5) to dehydrorotenone (Fig.3) was readily achieved but the selective hydrogenation of the latter to rotenone proved difficult. Ultimately, in 1958, Miyano andMatsui succeeded in synthesising rotenone from dehydrorotenone by a two-step process involving reduction with sodium borohydride followed by Oppenauer oxidation of

NOTES



the resulting secondary carbinol. The first step is a Hoesch condensation between tubanol hydrate (Fig.12) and methyl derric acid nitrile (Fig.13). The resulting compound (Fig.14) on treatment with phosphorus tribromide in pyridine underwent dehydration to yield racemic methyl derrisate (Fig.15). On reaction with sodium acetate in acetic anhydride, racemic methyl derrisate give racemic dehydrorotenone (Fig.3). The meta rotenone obtained from (Fig.3) by the two-step reduction/oxidation mentioned above could be converted into (-)-rotenone by refluxing with carbon tetrachloride when the natural rotenone-CCl4 separated out.

75

Ultimately, in 1958, Miyano andMatsui succeeded in synthesising rotenone from dehydrorotenone by a two-step process involving reduction with sodium borohydride followed by Oppenauer oxidation of the resulting secondary carbinol. The first step is a Hoesch condensation between tubanol hydrate (Fig.12) and methyl derric acid nitrile (Fig.13). The resulting compound (Fig.14) on treatment with phosphorus tribromide in pyridine underwent dehydration to yield racemic methyl derrisate (Fig.15). On reaction with sodium acetate in acetic anhydride, racemic methyl derrisate give racemic dehydrorotenone (Fig.3). The meta rotenone obtained from (Fig.3) by the two-step reduction/oxidation mentioned above could be converted into (-)-rotenone by refluxing with carbon tetrachloride when the natural rotenone-CCl4 separated out.

Fig.12 Fig.13 Fig.14

Fig. 12 Fig. 13

75



Fig. 14

75



75



Fig. 15

NOTES



Rotenone is difficult to handle in the pure state, as it is sensitive to light and oxygen, quickly decomposing to less toxic products.

Crombie and co-workers reported a novel and elegant synthesis of racemic isorotenone (Fig16 & 17).

76

Fi

Fig.15



Fig.16

Fig.17

He used isoflavone (Fig.18) as the starting material. This compound, on treatment with dimethylsulphoxonium methylide (Fig.19) gave the vinyl coumaranone. On

14

Fig. 16

76

Fi

Fig.15



Fig.16

Fig.17

He used isoflavone (Fig.18) as the starting material. This compound, on treatment with dimethylsulphoxonium methylide (Fig.19) gave the vinyl coumaranone. On

14

Fig. 17

77

treatment with pyridine, (35) underwent a rearrangement to yield the target compound. Using this method, other rotenoids, including rotenone,have been synthesized.

Fig.18

Fig.19

Fig. 18

NOTES



He used isoflavone (Fig.18) as the starting material. This compound, on treatment with dimethylsulphoxonium methylide (Fig.19) gave the vinyl coumaranone. On treatment with pyridine, (35) underwent a rearrangement to yield the target compound. Using this method, other rotenoids, including rotenone,have been synthesized.

77


Fig.18

Fig.19

Fig. 19

77


Fig.18

Fig.19

5.3 INTRODUCTION: PORPHYRINEThe natural porphyrine pigment contains a complex cyclic structure composed of four pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). Four pyrrole rings combine through methane bridges and resultant structure is known as porphins. Substituted porphines are called porphyrins.The porphyrin macrocycle has 26 (delocalized) pi electrons. Porphyrin macrocycles are highly conjugated systems and consequently they typically have very intense absorption bands in the visible region.

Two most important derivatives of porphyrrine are chlorophyll and hemoprotein(porphyrins combined with metals and protein).These compounds play vital role in biological activities.

NOTES



78

4.3 Introduction: Porphyrine

The natural porphyrine pigment contains a complex cyclic structure composed of

four pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). Four pyrrole rings combine through methane bridges and resultant structure is known as porphins. Substituted porphines are called porphyrins.The porphyrin macrocycle has 26 (delocalized) pi electrons. Porphyrin macrocycles are highly conjugated systems and consequently they typically have very intense absorption bands in the visible region.

Two most important derivatives of porphyrrine are chlorophyll and hemoprotein(porphyrins combined with metals and protein).These compounds play vital role in biological activities.

Structure of porphine(Parent porphyrine)

4.3.1 Hemoglobin

Approximately one third of the mass of a mammalian red blodd cell is hemoglobin. Its major function is to carry oxygen from lunges through the arteries to the tissues and help to carry carbon dioxide through the veins back to lungs. The ability of hemoglobin to bind oxygen depends on the presence of a bound prosthetic group called heme. The heme group gives blood to its distinctive red color.

4.3.1.1 Structure of hemoglobin

Structure of porphine(Parent porphyrine)

HeMoGlobin

Approximately one third of the mass of a mammalian red blodd cell is hemoglobin. Its major function is to carry oxygen from lunges through the arteries to the tissues and help to carry carbon dioxide through the veins back to lungs. The ability of hemoglobin to bind oxygen depends on the presence of a bound prosthetic group called heme. The heme group gives blood to its distinctive red color.

Structure of HemoglobinHemoglobin is mainly divided into two parts viz heme part and globin part.

(I) Heme structure: Heme consists of an organic component and a central iron atom. The organic component called protoporphyrin, is made up of four pyrrol rings linked by methyl bridges to form a tetrapyrrole ring. Four methyl groups, two venyl groups, and two propionate side chains are attached.

79

Hemoglobin is mainly divided into two parts viz heme part and globin part.

(I) Heme structure:

Heme consists of an organic component and a central iron atom. The organic component called protoporphyrin, is made up of four pyrrol rings linked by methyl bridges to form a tetrapyrrole ring. Four methyl groups, two venyl groups, and two propionate side chains are attached.

The iron atom lies in the center of the protoporphyrin, bonded to the four pyrrol nitrogen atom. Under normal conditions, the iron is in the ferrous(Fe+2 ) oxidation state. The iron ion can form two additional bonds, one on each side of the heme plane.These binding sites are called the 5th and 6th co-ordination sites. The iron ion lies approxmetly 0.4 Å outside the porphyrin plane because an iron ion, in this form, is slightly too large to fit into the well-defined hole within the porphyrin ring.The binding of O2 molecule at 6th co-ordination site of the iron ion substantially rearranges the electrons within the iron so that iron becomes effectively smaller, allowing it to move into plane of porphyrin.

Structure of heme

(II) Protein Structure:

The hemoglobin molecule is made up of four polypeptide chains: two alpha chains of 141 amino acid residues each and two beta chains of 146 amino acid residues each. The alpha and beta chains have different sequences of amino acids,

Structure of heme

The iron atom lies in the center of the protoporphyrin, bonded to the four pyrrol nitrogen atom. Under normal conditions, the iron is in the ferrous(Fe+2 ) oxidation state. The iron ion can form two additional bonds, one on each side of the heme plane.These binding sites are called the 5th and 6th co-ordination sites. The iron ion lies approxmetly 0.4 Å outside the porphyrin plane because an iron ion, in this form, is slightly too large to fit into the well-defined hole within the porphyrin ring.

NOTES



The binding of O2 molecule at 6th co-ordination site of the iron ion substantially rearranges the electrons within the iron so that iron becomes effectively smaller, allowing it to move into plane of porphyrin.

(II) Protein Structure: The hemoglobin molecule is made up of four polypeptide chains: two alpha chains of 141 amino acid residues each and two beta chains of 146 amino acid residues each. The alpha and beta chains have different sequences of amino acids, but fold up to form similar three-dimensional structures. The four chains are held together by noncovalent interactions. There are four binding sites for oxygen on the hemoglobin molecule, because each chain contains one heme group. In the alpha chain, the 87th residue is histidine F8 and in the beta chain the 92nd residue is histidine F8. A heme group is attached to each of the four histidines. The hemoglobin molecule is nearly spherical, with a diameter of 55 Å. The four chains are packed together to form a tetramer. The heme groups are located in crevices near the exterior of the molecule, one in each subunit. Each alpha chain is in contact with both beta chains. However, there are few interactions between the two alpha chains or between the two beta chains.

80

but fold up to form similar three-dimensional structures. The four chains are held together by noncovalent interactions. There are four binding sites for oxygen on the hemoglobin molecule, because each chain contains one heme group. In the alpha chain, the 87th residue is histidine F8 and in the beta chain the 92nd residue is histidine F8. A heme group is attached to each of the four histidines. The hemoglobin molecule is nearly spherical, with a diameter of 55 Å. The four chains are packed together to form a tetramer. The heme groups are located in crevices near the exterior of the molecule, one in each subunit. Each alpha chain is in contact with both beta chains. However, there are few interactions between the two alpha chains or between the two beta chains.

Hemoglobin molecule

Each polypeptide chain is made up of eight or nine alpha-helical segments and an equal number of nonhelical ones placed at the corners between them and at the ends of the chain. The helices are named A-H, starting from the amino acid terminus, and the nonhelical segments that lie between the helices are named AB, BC, CD, etc. The nonhelical segments at the ends of the chain are called NA at the amino terminus and HC at the carboxyl terminus.

To form the tetramer, each of the subunits is joined to its partner around a twofold symmetry axis, so that a rotation of 180 degrees brings one subunit into congruence with its partner. One pair of chains is then inverted and placed on top

Hemoglobin molecule

Each polypeptide chain is made up of eight or nine alpha-helical segments and an equal number of nonhelical ones placed at the corners between them and at the ends of the chain. The helices are named A-H, starting from the amino acid terminus, and the nonhelical segments that lie between the helices are named AB, BC, CD, etc. The nonhelical segments at the ends of the chain are called NA at the amino terminus and HC at the carboxyl terminus.

To form the tetramer, each of the subunits is joined to its partner around a twofold symmetry axis, so that a rotation of 180 degrees brings one subunit into congruence with its partner. One pair of chains is then inverted and placed on top of the other pair so that the four chains lie at the corners of a tetrahedron. The four subunits are held together mainly by nonpolar interactions and hydrogen bonds. There are no covalent bonds between subunits.

NOTES



The twofold symmetry axis that relates the pairs of alpha and beta chains runs through a water-filled cavity at the center of the molecule. This cavity widens upon transition form the R structure to the T structure to form a receptor site for the allosteric effector DPG (2,3 diphosphoglycerate) between the two beta chains. The heme group is wedged into a pocket of the globin with its hydrocarbon side chains interior and its polar propionate side chains exterior.

syntHesis oF HeMoGlobin

Hemoglobin synthesis requires the coordinated production of heme and globin. Heme is the prosthetic group that mediates reversible binding of oxygen by hemoglobin. Globin is the protein that surrounds and protects the heme molecule. Hemoblobin is synthesized inside developing red blood cells(In immature RBC’s Cytosol) during Intermediate normoblast stages- It begins in the proerythroblasts and continues slightly even into the reticulocyte stage. Chemical steps in the formation of hemoglobin are as:

1. 2 alpha ketoglutonic acid(from Creb's cycle) + 2 glycine → pyrrole

2. 4 pyrrole → protoporphyrine

3. porphyrine + Fe+ → heme

4. 4 heme + 4 polypeptide chain(2 alpha + 2 beta) → 1 hemoglobin molecules

(i) Heme Synthesis: Heme is synthesized in a complex series of steps involving enzymes in the mitochondrion and in the cytosol of the cell. The first step in heme synthesis takes place in the mitochondrion, with the condensation of succinyl CoA and glycine by aminolevulic acid synthase to form 5-aminolevulic acid (ALA). This molecule is transported to the cytosol where a series of reactions produce a ring structure called coproporphyrinogen III. This molecule returns to the mitochondrion where an addition reaction produces protoporhyrin IX.

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(i) Heme Synthesis:

Heme is synthesized in a complex series of steps involving enzymes in the mitochondrion and in the cytosol of the cell. The first step in heme synthesis takes place in the mitochondrion, with the condensation of succinyl CoA and glycine by aminolevulic acid synthase to form 5-aminolevulic acid (ALA). This molecule is transported to the cytosol where a series of reactions produce a ring structure called coproporphyrinogen III. This molecule returns to the mitochondrion where an addition reaction produces protoporhyrin IX.

The enzyme ferrochelatase inserts iron into the ring structure of protoporphyrin IX to produce heme. Deranged production of heme produces a variety of anemias. Iron deficiency, the world's most common cause of anemia, impairs heme synthesis thereby producing anemia. A number of drugs and toxins directly inhibit heme production by interfering with enzymes involved in heme biosynthesis.

Heme synthesis

(ii) Globin Synthesis:

Two distinct globin chains (each with its individual heme molecule) combine to form hemoglobin. One of the chains is designated alpha. The second chain is called "non-alpha". With the exception of the very first weeks of embryogenesis, one of the globin chains is always alpha. A number of variables influence the nature of the non-alpha chain in the hemoglobin molecule. The fetus has a distinct non-alpha

Heme synthesis

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The enzyme ferrochelatase inserts iron into the ring structure of protoporphyrin IX to produce heme. Deranged production of heme produces a variety of anemias. Iron deficiency, the world's most common cause of anemia, impairs heme synthesis thereby producing anemia. A number of drugs and toxins directly inhibit heme production by interfering with enzymes involved in heme biosynthesis.

(ii) Globin Synthesis: Two distinct globin chains (each with its individual heme molecule) combine to form hemoglobin. One of the chains is designated alpha. The second chain is called "non-alpha". With the exception of the very first weeks of embryogenesis, one of the globin chains is always alpha. A number of variables influence the nature of the non-alpha chain in the hemoglobin molecule. The fetus has a distinct non-alpha chain called gamma. After birth, a different non-alpha globin chain, called beta, pairs with the alpha chain. The combination of two alpha chains and two non-alpha chains produces a complete hemoglobin molecule (a total of four chains per molecule).

The combination of two alpha chains and two gamma chains form "fetal" hemoglobin, termed "hemoglobin F". With the exception of the first 10 to 12 weeks after conception, fetal hemoglobin is the primary hemoglobin in the developing fetus. The combination of two alpha chains and two beta chains form "adult" hemoglobin, also called "hemoglobin A". Although hemoglobin A is called "adult", it becomes the predominate hemoglobin within about 18 to 24 weeks of birth.

The pairing of one alpha chain and one non-alpha chain produces a hemoglobin dimer (two chains). Two dimers combine to form a hemoglobin tetramer, which is the functional form of hemoglobin.

cHloropHyll

The trapping of light energy is the key to photosynthesis. The first event is the absorption of light by a photoreceptor molecule. The principal photoreceptor in the chloroplasts of most green plant is chlorophyll-a, a substituted tetrapyrrole. The four nitrogen atoms of the pyrroles are coordinated to a magnesium ion. There are actually 2 main types of chlorophyll, named a and b. They differ only slightly, in the composition of a side chain (in a it is -CH3, in b it is CHO). Both chlorophylls absorb light most strongly in the red and violet parts of the spectrum. Green light is absorbed poorly. Thus when white light shines on chlorophyllcontaining structures like leaves, green light is transmitted and reflected and the structures appear green.

Structure of ChlorophyllIn chlorophyll nitrogen atoms of four pyrrole rings coordinated to magnesium ion in a square planar arrangement. Thus in chlorophyll the central ion is magnesium, and the large organic molecule is a porphyrin. Unlike a porphyrin such as heme, chlorophyll has a reduced pyrrol ring and an additional 5-carbon ring fused to one of the pyrrol rings. Another distinctive

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feature of chlorophyll is the presence of phytol, a highly hydrophobic 20-carbon alcohol, estrified to an acid side chain.

There are several forms of chlorophyll. The structure of one form, chlorophyll a, is shown below-

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There are several forms of chlorophyll. The structure of one form, chlorophyll a, is shown below-

Note the system of alternating single and double bonds that run around the porphyrin ring. Although single and double bonds are drawn in fixed positions, but actually the "extra" electrons responsible for the double bonds are not fixed between any particular pair of carbon atoms but instead are free to migrate around the ring. This property enables these molecules to absorb light and make them effective photoreceptor. Chlorophyll a’s peak molar extinction coefficient (a measure of a compound’s ability to absorb light) is higher than 105 M-1cm-1, among the highest observed for organic compound.

4.5.2.2 Synthesis of chlorophyll

In higher plants, Chlorophyll is synthesized in chloroplasts through the cooperative activity of many enzymes. In the first phase of chlorophyll biosynthesis, the glutamic acid is converted to 5-aminolevulinic acid (ALA). This reaction is unusual in that it involves a covalent intermediate in which the glutamic acid is attached to a transfer RNA molecule. This is one of a very small number of examples in biochemistry in which a tRNA is utilized in a process other than protein synthesis. Two molecules of ALA are then condensed to form porphobilinogen (PBG), which ultimately form the pyrrole rings in chlorophyll. The next phase is the assembly of a porphyrin structure from four molecules of

Note the system of alternating single and double bonds that run around the porphyrin ring. Although single and double bonds are drawn in fixed positions, but actually the "extra" electrons responsible for the double bonds are not fixed between any particular pair of carbon atoms but instead are free to migrate around the ring. This property enables these molecules to absorb light and make them effective photoreceptor. Chlorophyll a’s peak molar extinction coefficient (a measure of a compound’s ability to absorb light) is higher than 105 M-1cm-1, among the highest observed for organic compound.

syntHesis oF cHloropHyll

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PBG. This phase consists of six distinct enzymatic steps, ending with the product protoporphyrin.

All the biosynthesis steps up to this point are the same for the synthesis of both chlorophyll and heme. But here the pathway branches and the fate of the molecule depend on which metal is inserted into the center of the porphyrin. If magnesium is inserted by an enzyme called magnesium chelatase, then the additional steps needed to convert the molecule into chlorophyll take place; if iron is inserted, the species ultimately becomes heme.

In higher plants, Chlorophyll is synthesized in chloroplasts through the cooperative activity of many enzymes. In the first phase of chlorophyll biosynthesis, the glutamic acid is converted to 5-aminolevulinic acid (ALA). This reaction is unusual in that it involves a covalent intermediate in which the glutamic acid is attached to a transfer RNA molecule. This is one of a very small number of examples in biochemistry in which a tRNA is utilized in a process other than protein synthesis. Two molecules of ALA are then condensed to form porphobilinogen (PBG), which ultimately form the pyrrole rings in chlorophyll. The next

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phase is the assembly of a porphyrin structure from four molecules of PBG. This phase consists of six distinct enzymatic steps, ending with the product protoporphyrin.

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The next phase of the chlorophyll biosynthetic pathway is the formation of the fifth ring by cyclization of one of the propionic acid side chains to form protochlorophyllide. The pathway involves the reduction of one of the double bonds in ring, using NADPH. This process is driven by light in angiosperms and is carried out by an enzyme called protochlorophyllide oxidoreductase (POR). Seedlings of angiosperms grown in complete darkness lack chlorophyll, because the POR enzyme requires light. These etiolated plants very rapidly turn green when exposed to light. The final step in the chlorophyll biosynthetic pathway is the attachment of the phytol tail, which is catalyzed by an enzyme called chlorophyll synthetase.

4.4 Summary

The organic compounds which are having Rotexen ring system are called rotenoids. Rotenone is an odourless, colourless, crystalline ketonic chemical

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4.4 Summary


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4.4 Summary



5.4 SUMMARYThe organic compounds which are having Rotexen ring system are called rotenoids. Rotenone is an odourless, colourless, crystalline ketonic chemical compound used as an insecticide, piscicide, and pesticide. Roatnone can be synthesed from dehydrorotenone by a two-step process involving reduction with sodium borohydride followed by Oppenauer oxidation of the resulting secondary carbinol.

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The natural porphyrine pigment contains a complex cyclic structure composed of four pyrrole subunits. Porphyrin macrocycles are highly conjugated systems and consequently they typically have very intense absorption bands in the visible region. Two most important derivatives of porphyrrine are chlorophyll and hemoprotein (porphyrins combined with metals and protein).These compounds play vital role in biological activities.

Approximately one third of the mass of a mammalian red blodd cell is hemoglobin. It consists of two parts: heme part and globin part(a protein). Heme consists of an organic component and a central iron atom. The hemoglobin molecule is made up of four polypeptide chains. Hemoblobin is synthesized inside developing red blood cells during Intermediate normoblast stages.

In chlorophyll nitrogen atoms of four pyrrole rings coordinated to magnesium ion in a square planar arrangement. Thus in chlorophyll the central ion is magnesium, and the large organic molecule is a porphyrin. Chlorophyll is synthesized by glutamic acid in chloroplasts through the cooperative activity of many enzymes.

5.5 REVIEW QUESTIONS1. What is the Introduction: Rotionoids?

2. Explain the Introduction: Porphyrine.

3. Discuss the synthesis of hemoglobin.

4. What is the synthesis of chlorophyll?

5.6 FURTHER READINGSzz Textbook of organic chemistry, Vol II by I L Finar

zz Chemistry of natural products, Vol 1- 12, by Atta-Ur-Rahman

zz introduction to the chemistry of terpenoids and steroids, by William Templeton

zz Systematic identification of flavonoid compounds by Mabry & Markham

zz Organic chemistry, Volume 2 : Stereochemistry and the chemistry of natural products, fifth edition – I.L. Finar.


zz Biogenesis of natural products, Pergamon (1967, 2nd edn.)

zz Steroids reaction mechanism, Elsevier (1968).

m.sc. chemistry of natural products

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