Chapter Two

Functionalized Ketene Dithioacetals in

Organic Synthesis

2.1 Introduction

Ketene dithioacetals having functional groups at a-positions are extremely 1-3 important multifunctional intermediates in organic synthesis. Functional groups

present at the a position are usually electron withdrawing groups, due to the relative

ease in their preparation. a-Nitro ketene dithioacetals have been found to be highly

versatile synthon for the synthesis of h e t e r ~ c ~ c l e s . ~ a -0x0 ketene dithioacetals have a

carbonyl group in the a-position. Ketene dithioacetals possesssing aldehyde, keto,

ester or amide functionalities belongs to this class. Any carbonyl compound having an

acrive methylene group can be transformed to corresponding a-0x0 ketene dithioacetals

under suitable conditions. Since carbonyl group is abundant in organic molecules a

large number of a-0x0 ketene dithioacetals are also known. a - 0 x 0 ketene dithioacetals

also have been used extensively for the construction of heterocycles. Moreover they

have been recognized as valuable multifunctional intermediates suitable for selective

construction of new carbon-carbon bonds. Ketene dithioacetals with nitrile group at the

cx-position may also be considered similar to a-0x0 ketene dithioacetals. because they

also give comparable reaction with nucleophiles. Ketene dithioacetals having

pyridinium groups and sulfonyl groups at the a-position have also been synthesized and

showr? to be valuable intermediates in organic synthesis. 3-15

2.2 Synthesis of Functionalized Ketene Dithioacetals

The most popular and convenient method for the preparation of functionalized

ketene dithioacetals involve the reaction of an active methylene compound with carbon

disulfide in the presence of a suitable base followed by alkylation.

1 2

Scheme 1

Thuillier and Vialle were the first to develop a high yield synthesis of a-0x0

ketene dithioacetals starting from ketone^.'^'^' They have treated ketones with carbon

disulfide in the presence of sodium amylate followed by two equivalents of the

alkylating agent. Aliphatic. cyclic and aryl alkyl ketones undergo smooth reaction

under these conditions to give the corresponding ketene dithioacetals. Several groups

have developed alternative methods for the synthesis of ketene dithioacetals from active

~nethylene compounds mostly the difference is in the choice of the base or solvent to I X matcll the starting substrate. Lithium 2,6-di-t-butyl-4-methyl phenoxide. lithium

19-ZI 22-29 dialky! amide, sodium hydride, potassium t-butoxide 30.32and ~ ~ : / a l u r n i n a . ~ ~

Acyl ketene dithioacetals prepared from aliphatic ketones, such as acetone, ethyl

~nethyl ketone efc have been used for further functionalization. The aldol type

condensation of acyl ketene dithioacetals with aromatic aldehydes and substituted

cinnarnaldehydes afford cinnamoyl ketene dithioacetals and their higher enyl

homologues (Scheme 2). 34-36

The ketene dithioacetals derived from ethyl acetoacetate also undergo

condensation reactions with aromatic aldehydes. The reaction involve insitu hydrolysis

of the ester functiona!ity and decarboxylation to afford cinnamoy! ketene

dithioacetals.'" Condensation of acyl ketene dithioacetals with N,N-dimethyl

forrnarnide diethyl acetal gave the enaminoketones 7 (Scheme 3).38

5

n=O, 1, 2

R I , R,, R3=H, CH3

Scheme 2

These compounds would have interesting applications since the highly versatile

enarninoketone functionality and the ketene dithioacetal moiety are present in the same

niolecule. I t has been found that the enaminoketones 7 undergo chemoselective

addition of Grignard reagents to afford alkenoyl ketene dithioacetals 8 (Scheme 4).

8

R,=Me, Et, i-Pr, n-Pr etc

Scheme 4

While the polarized ketene dithioacetals having alkyl or benzyl substituents on

sulfur can be conveniently prepared by alkylation of the dithiolate anions, phenylthio

substituted ketene dithioacetals are obtained by nucleophilic displacenlent of P-dichloro

groups by aryl thiolate anions (Scheme 5).39

Scheme 5

Aroyl ketene dithioacetals have been synthesized in a similar fashion from

bromoethynyl aryl ketones.40 Another approach towards the synthesis of functionalized

ketene dithioacetals involves addition of electron rich substrates such as enamines,

enolizable carbonyl compounds, enolates phenols and naphthols to trithiocarbenium salts, 1 .41-47 Friedel-Crafts type acylation of ketene dithioacetals or trithioorthoacetates

also lead to the formation of a-0x0 ketene dithioacetals (Schemes 6 and 7).4"51

Scheme 6

Scheme 7

The reaction of trithiocarbonates with the carbanions derived from N,N-

dimethyl amino substituted acetonitriles also afford ketene dithioacetals functionalized

at the a-position with nitrile group or sulfoxide group respectively (Scheme

Scheme 8

Though the number of methods available for the synthesis of functionalized

ketene dithioacetals are not very large, they are sufficient to make these compounds

available with diverse structural features. Extensive research has been carried out 011

[lie application of these compounds as synthetic intermediates.

2.3 Functionalized Ketene Dithioacetals in Organic Synthesis

The ketene dithioacetal functionality alone has a high potential in organic

synthesis, due to its ability to behave as a donor or acceptor in the formation of a new

carbon-carbon bond^.^^,^^ The diverse reactivity pattern of ketene dithioacetals and

their application in the synthesis of complex molecules have been reviewed by ~ o l b . '

In this section, only the reactivity of functionalized ketene dithioacetals, particularly

\ t those with electronwithdrawing substituents at the a-position would be discussed.

2.3.1 a-0x0 Ketene Dithioacetals

a-0x0 ketene dithioacetals have been extensively used as three carbon synthons

in the synthesis of a variety of five membered and six membered heterocycles due to

their 1,3-bielectrophilic character. Reactions of hydrazine hydrate with a-0x0 ketene

dithioacetals lead to the formation of pyrazoles (Scheme 9). 56-66

21 22

Scheme 9

Similar reactions with hydroxyl amine afford functionalized oxa~oles .~ '

Junjappa and co-workers have recently developed conditions for the regioselective

reaction of hydroxyl amine with aroyl ketene dithioaceta~s.~' They have found that in

the pH range 5-9, nucleophilic attack of nitrogen on the carbonyl group is preferred,

while at lower pH(2.2) conjugate addition of the nitrogen is favoured. This leads to the

selective formation of 3-alkylthio or 5-alkylthio pyrazoles respectively.(Scheme 10).

Scheme 10

The reaction of guanidine and amidines with a-0x0 ketene dithioacetals lead to

the formation of substituted pyrimidines. 62,69-72 Pott's group has extended this method

for the synthesis of several pyrimidine derivatives, with diverse structural features." A

synthesis of 2.6-thienoyl pyrimidine has been shown in scheme I I .

DMF, Bennc~~e A

Scheme 11

Similar approaches towards the synthesis of heterocycles involve the reactions

of cyanoacetamide anions as well. The reaction of cyanoacetamide with a-0x0 ketene

dithioacetals in the presence of sodium isopropoxide in isopropanol afford the

pyridones 30 (Scheme 12). 74.75

NaO i-Pr * i-PrOH, NCCH2CONH2

Scheme 12

The arnidine functionality which is part of a heterocycle can also participate in the

reaction with a-0x0 ketene dithioacetal leading to the formation of fused pyrimidines.

2-Aminopyridines and 2-aminobenzothiazole have been used for the synthesis of

annulated pyrimidines 33 (Scheme 13).'~

Scheme 13

The enamine functionality of 6-aminouracils and 3-amino pyarazoles also

behave as a 1,3-binucleophile in their reaction with a-0x0 ketene dithioaceta~s.".~~

The reactions of N,N-dimethyl-6-aminouracil with a-0x0 ketene dithioacetal have been

shown in scheme 14.

29

Scheme 14

Some binucleophiles such as 1,2 or 1,3 diamines, aminoalcohols and

aminothiols undergo conjugate addition displacing both the alkylthio groups leading to 78-82

the formation of a-0x0 ketene aminals, 0, N-acetals and N, S-acetals respectively.

Scheme 15 shows the reaction of 2-mercaptoethylamine with a-0x0 ketene

dithioacetals.

Scheme 15

Displacement of one or both of the alkylthio groups of tile ketene dithioacetal

also afford a-oxoketene N,S-acetals or aminals r e ~ ~ e c t i v e l y . ~ ~ - ~ ~ There are some

reactions which transform acyl ketene dithioacetals directly to functionalized

heterocycles. For example the reaction of u-0x0 ketene dithioacetals with P,S,, offer a

convenient procedure for the synthesis of 3-thione-1.2-dithiols 38 (Scheme 16). 32.16.86-89

p4s10 * Xylene, A

Scheme 16

Several functionalized ketene dithioacetals having active methylene group

attached to atleast one of the sulfur atom undergo intramolecular aldol type

condensation to afford thiophene derivatives. Alkylation of the dithiolate dianions

obtained by the reaction of active methylene ketones with carbon disulfide in the

presence of a base with a-haloketones, esters or nitriles is shown in (Scheme 17,,27.7&."-92

Scheme 17

a-0x0 ketene dithioacetals have also been found to afford thiophene derivatives

41 on treatment with Simmons-Smith reagent (Scheme 18). The authors have proposed

that the reaction involve an intermediate ylide, resulting from the reaction of the

carbenoid methylene with the methylthio

Scheme 18

Besides the synthesis of heterocycles, several valuable transformations have

been reported on a-0x0 ketene dithioacetals. One class of reactions involve

nucleophilic addition of hydride or carbon nucleophiles to the carbonyl carbon of the a-

0x0 ketene dithioacetal followed by Lewis acid or protic acid catalyzed solvolysis. The

overall transformation involve a reductive or alkylative 1,3-carbonyl group

transposition (Scheme 19).

Scheme 19

Junjappa's group have successfully achieved a selective reduction of the

carbonyl group using an excess of sodium borohydride in refluxing absolute ethanol.

Subsequent solvolysis in the presence of boron trifluoride etherate in refluxing

methanol gave the a,P-unsaturated esters 43 in good yields with high 94 stereoselectivity. Similar 1.3-cabonyl group transpositions leading to the formation of

a,P-unsaturated thiol esters or carboxylic acids were also observed with HBF, in the

presence of mercury salt^.^^.'^ Reductive transpositions were further extended to the

preparation of conjugated pentadienoates and heptatrienoates. 97.98 The reductive

carbonyl group transposition methodology has also been used for the synthesis of y-

butyrolactone derivative^.^' The a,P-unsaturated ester 46 prepared from the ketene

dithioacetal 44 with an a-allyl substituent underwent cyclization to the a-ylidene-y-

butyrolactone 47 (Scheme 20).

Scheme 20

1,2-Addition of organometallic reagents such as ~ r i ~ r $ r d reagents or

organolithium reagents also give similar carbinols which undergo carbonyl group

transposition under solvolytic conditions. Thus the addition of methyl magnesium

iodide to a-0x0 ketene dithioacetal on subsequent solvolysis afford the a,b-unsaturated

ester 49 depending on the reaction conditions (Scheme 21). 100,101

The Grignard reagents derived from higher homologues of alkyl halides

underwent sequential 1.4 and 1,2-addition to the. a-0x0 ketene dithioacetal. The

subsequent carbonyl group transposition reaction lead to the formation of a , b -

unsaturated ketones. 101

0 SCH3 OH SCH3 CH3 0

R v S C H 3 a e d v ~ ~ ~ 3 CH3 MeOH &l\\r OCH3 rt d R'

Scheme 21

There are several reports on the reaction of functionalized carbanions to the

102 carbonyl group of the a-0x0 ketene dithioacetals. Reformatsky reagents, enolates

derived from esters and ketones 103-105 and the lithium salts prepared from hydrazone

are used as the nucleophiles. The intermediate y-hydroxy ketones are highly

functionalized and undergo further useful transformations. Scheme 22 illustrates a

synthesis of substituted pyrones 53 employing such intermediates.

Selective reduction of a-0x0 ketene dithioacetals resulting in the removal of one

of the alkylthio group leading to the formation of vinylogous thiol esters is a valuable

transformation. Junjappa's group have used a combination of nickel chloride and

sodium borohydride to achieve this objective.Io6 In a recent report they have disclosed

that zinc in the presence of zinc chloride and tetramethyl ethylene diamine (TMEDA)

in refluxing ethanol has been found to be very efficient in the selective displacement of one

of the methylthio groups of a-0x0 ketene dithioacetal (Scheme 23).1°'

OBut

THF, HBF4_ -78OC THF

Scheme 22

Scheme 23

By varying the amount of ZnCIz-TMEDA further reduction of the double bond or

complete reduction of the ketene dithioacetal hnctionality to a methyl group are also

possible. Selective reduction of the double bond in a-0x0 ketene dithioacetals could also

be achieved by treating them with magnesium in methanol. 108.109

One of the extensively studied aspects of a-0x0 ketene dithioacetal chemistry is

the addition of allylic or propargylic carbanions selectively to the carbonyl group followed

by Lewis-acid catalyzed cycloaromatization reactions. The addition of allyl or methallyl

magnesium bromide to a-0x0 ketene dithioacetals and subsequent cycloaromatization in

the presence of BF3.Et20 or HBFd lead to the formation of substituted benzenes 57

(Scheme 24). 110.111

Scheme 24

This method offer an opportunity to construct aromatic compounds starting from

aliphatic precursors. Besides the substitution pattern of the products could be designed by

choosing the appropriate starting substrates. This method has been further extended to

the synthesis of stilbenes.ll2 Similar reactions with benzyl magnesium chloride give

napthoannulation, but benzyl Grignard undergo sequential 1,4 and 1.2-addition to a-0x0

ketene dithioacetal. This results in the formation of naphthalene derivatives substituted

with a benzyl group rather than the methylthio group.'13

The propargyl magnesium bromide gave exclusive 1,2-addition products with a-

0x0 ketene dithioacetals. Subsequent cycloaromatization in the presence of

borontrifluoride etherate in methanol gave methoxy substituted benzene derivatives 60

(Scheme 25).'IJ

H

'v:I33 R + H-C-C - I

R'

tlr3.u2u MeOH

SCH3

Scheme 25

Similar reactions have also been reported with lithioacetonitrile while the treatment

of the intermediate P-hydroxy nitrile with boron trifluoride etherate gave the nitrile

substituted diene, cyclization involving intramolecular migration of methylthio group could

be achieved in the presence of phosphoric acid (Scheme 26).'15

Benzoheterocycles are conventionally prepared by starting witha suitable aromatic

substrate and constructing the desired heterocycles on that. An alternate approach would

be to start with a suitable heterocyclic precursor on which an aromatic ring could be

annulated. The reaction of 3-methyl-5-lithio-methyl isoxazole with a-0x0 ketene

dithioacetals followed by cycloaromatization gave benzisoxazoles 64 in good yields

(Scheme 27). 116.117

Scheme 26

63

Scheme 27

Similar cyclizations have been reported with other allyl anions as well. The allyl

anions derived from 2-picoline,118 P-pyrrolidino crotonate, 1,3,6-trimethyl uracil'I9 and

2,3-dimethyl-l-phenylpyrazoline-5-one'20 have been found to undergo nucleophilic

addition to a-0x0 ketene dithioacetals and subsequent cyclization leading to the formation

of quinolizinium salts, amino substituted benzene derivatives, quinazoline derivatives and

indazolone derivatives respectively.

A rather unusual reaction of Reformatsky reagent involve the addition of a second

molecule of reagent to the initial adduct followed by cyclization, leading to the formation

of substituted salicylates (Scheme 28).12'

Scheme 28

The addition of Methyl Grignard on a-0x0 ketene dithioacetals followed by

dehydration lead to the formation of sulfur substituted butadienes which are valuable

intermediates for cycloaddition reactions. Similar dienes can also be obtained by Wittig

reaction 122,123 These dienes have been shown to undergo facile (4+2) cycloaddition

reactions with electron deficient dienes (Scheme 29). 124.125

Several nucleophilic reagents have been found to undergo conjugate addition with

functionalized ketene dithioacetals. Ketene dithioacetals having two electron withdrawing

groups at the a-position are particularly susceptible to conjugate additions.

Scheme 29

The chemo and sterioselective addition of organocuprates to a-0x0 ketene

dithioacetals has been explored in much detail. 18.126-129 The high chemo and

sterioselectivity, that could be achieved in this reaction is evident in the example

highlighted in the scheme 30.

(PhSCuMc)Li THF f l S C H 3

Scheme 30

There are a number of reports on the conjugate addition of functionalized

nucleophiles to polarized ketene dithioacetals which involve subsequent cyclizations

leading to the formation of a variety of heterocycles. For example the anion generated

from a-pyridyl acetonitrile adds to the ketene dithioacetal 72 prepared from ethyl cyano

acetate followed by cyclization to quinolizones 73 (Scheme 3 1).13'

72 73

Scheme 31

A large number of similar transformations are available in the ~ i t e r a t u r e . ~ ~ " " ~

Conjugate additions of enolates to the polarized ketene dithioacetals derived from ethyl

cyanoacetate or diethyl malonate undergo further cyclization leading to the formation of

substituted 2-pyrones 76 (Scheme 32). 135-138

Scheme 32

This method have been hrther extended to the synthesis of 2-pyrone derivatives I annulated to other heterocyclic systems as well. 139,140 The 1,S-diketones formed on the

conjugate addition of enolates to a-0x0 ketene dithioacetals have been found to cyclize to

the pyrilium salts which could be then transformed into pyridine derivatives on treatment

with ammonium salts. 141-145 One example shown in scheme 33 illustrates the preparation

of'a valuable base 79.

78

Scheme 33

2.3.2 Nitro Ketene Dithioacetals

The simple nitro ketene dithioacetals is prepared in high yield by the alkylation of

the dithiolate dianion obtained by the reaction of nitromethane with carbon disulfide in the

presence of a base. The nitro ketene dithioacetals are valuable two carbon synthons

particularly useful in the synthesis of heterocycles which should have a nitro or amino

substituent. While the a-carbon acts as a donor, the P-carbon behaves as an acceptor

Functionalized nucleophiles add to the P-carbon, of the nitro ketene dithioacetals and the

adducts formed could be cyclized subsequently. Scheme 34 shows the reaction of

anthranilic ester with nitro ketene dithioacetal. The initial addition of two molecules of

anthranilic ester leads to the formation of an intermediate quinolone derivative 82. Further

cyclization to the diazepinone 83 could be accomplished in the presence of Raney

nickelihydra~ine.'~~

The reaction of amines with nitro ketene dithioacetals lead to the formation

of correspomding N,S-acetals or aminals. 147.148 Scheme 35 shows a synthesis of pyrrole

derivatives from nitro ketene N,S-acetals. The amino acetaldehyde diethylacetal adds to

the nitro ketene dithioacetal to afford an intermediate ketene N,S-acetal which on further

cyclization give the nitro substituted pyrrole 86. 147.IJ0.15li

Scheme 34

Scheme 35

Carbanions generated from active methylene compounds also add to nitro ketene-

S,S-acetals. 131.152 Scheme 36 shows the reaction of 3-cyanomethyl indole-2-carboxylate

leading to the formation of carbazole derivatives 88.lS3

Scheme 36

The reaction of pyridinium-N-ylides with nitro ketene dithioacetals is accompanied

by cyclization and denitration leading to the formation of the indolizine derivative 90

(Scheme 37).154

89 80 90

Scheme 37

2.4 Conclusions

Functionalized ketene dithioacetals are important synthetic intermediates in organic

chemistry. They can be prepared with virtually any functional group at the a-positions.

They have a wide spectrum of reactivity. They have found applications in almost every

field of organic synthesis such as selective formation of new carbon-carbon bonds,

functional group manipulations, carbonyl group migrations, aromatic annulation reactions.

synthesis of a large variety of heterocycles and synthesis of complex natural products. The

sulfur substituents present in the products are also useful for further synthetic

transformations.

2.5 References

Dieter, R.K. Tetrahedroll 1986, 42, 3029.

Junjappa, H.; Ila, H.; Asokan, C.V. Tetrahedron 1990,16, 5423.

Kolb, M. Synthesis 1990, 171.

Tominaga, Y.; Matsuda, Y. J. Heterocycl. chem. 1985, 22, 937.

Krohnke, F.; Vierlach, K. Chem. Ber. 1962, 95, 1108.

Krohnke, F.; Zecher, W. Chem. Ber. 1962, 95, 1128.

Tominaga, Y.; Miyake, Y.; Fujito, H.; Matsuda, Y.; Kobayashi, G. Yukugaku

Zasshi 1977, 97, 927.

Kakehi. A,; Ito, S.; Kinoshita, N.; Abaka. Y . Bull. Chem. Soc. Jupatl 1988.

61. 2055.

Kakehi, A ; Ito, S.; Matssumoto, S.; Morimoto, Y. Chem. Lett. 1987, 2043.

Kakehi, A ; Ito, S.;Yonezu, S.; Mamta, K.; Yuito, K. Hetercycles 1985, 23.

33.

Kakehi, A,; Ito, S.;Yonezu, S.; Maruta, K.; Shiohark, M.; Adachi, K. Hlrll. ('hem.

SOC. Juparl 1987, 60, 1867.

Tominaga, Y.; Gotou, H.; Oniyama, Y.; Nishimura, Y.; Matsuda, Y. Chem.

Pharm. Bull. 1985, 33, 3038.

Krohnke, F.; Steuernagel, H. Chem. Ber. 1964, 97, 11 18.

Gompper, R.; Wetzel, B.; Elser, W. Tetrahedron Lett. 1968, 5519.

Mizuyama, K.; Tominaga, Y.; Mastuda, Y.; Kobayashi, G. Yakzrgaktr Zasshi

1974, 94, 704.

Gommper, R.; Kutter, E. ., Schmidt, R. R. C'hem. Ber. 1965, 98, 1374

Jensen, K.; Burchardt, 0.; Lohse, C . Acta Chem. Scand. 1967, 21, 2797.

Gompper, R.; Scheafer, H.Chem. Ber. 1967, 100, 591.

Nakayama, J.; Sujie, Y.; Hoshimo, M. Chem. Lett. 1987, 939.

Bohme, H.; Arens, G. Liebigs Ann. Chem. 1982. 1022.

Stachel, H. D. Chem. Ber. 1962, 95,2166.

Ishihara, T.; Seki, T.; Yarnanaka, Y.; Ando. T. Nippon Kagaku Kaishi 1985,

2048.

Hojo, M.; Masuda, R.; Sano, H.; Saegusa, M. Syt~thesis 1986, 137.

Hojo, M.; Masuda, R. J. Org. Chem. 1975, 40, 963.

Bhattachajee, S. S., Ila, H.; Junjappa, H. Sytlthesis 1984, 249.

Yokoyama, M.; Tsuji, K.; Hayashi, M.; Imamoto, T. J. Chenr. Soc. Perkit, Trans I

1984, 85.

Kolb, M. The Chemistry ofkefenes, Allenes and related comporrt~d~, Patai,

S. (ed), Wiley, Chichester 1980, Part.2, 669.

Grobel, B.-Th.; Seebach. D. Synthesis 1977, 357. , >

Gompper,R.; Topfl, W. Chem. Ber.1962, 95, 2881./"

Chauhan, S. M.S.; Junjappa, H. Synthesis 1975,798,

Apparao, S.; Rahman. A.; Ila, H.; Junjappa, H. Synthesis 1982. 792.

Rudorf, W.D.; Augustin, M. J. Prakt. Chem. 1978,320, 585.

Rudorf, W.D. Tetrahedron 1978, 34, 725.

Stachel, H.D. Arch. Pharm. 1962, 295,224.

Saquet, M.; Thullier, A. C. R. Acad. Sci. Baris, S'er. C.1968, 266.

Kashima, K.; Hidaki, S.; Tominaga, Y.; Matsuda, Y.; Kobyashi. G., Sakemi,

K. Yakt~gaku Zasshi 1979, 99.38.

Singh, G.; Deb, B.; Ila, H.; Junjappa, H. Sytithcsis 1987, 286

Singh, G.; Ila, H.; Junjappa,H. Sytlthesis 1985, 165.

Singh, G.; Ila, H.; Junjappa, H. J. Chem. Soc. Perkin Trans.1 1987, 1945.

Gompper, R.; Topfl, W. Chem. Ber. 1962, 95, 2881.

Purkayastha, M. L.; Ila. H.; Junjappa, H . Syr~the.si.s 1989, 20.

Gompper, R.; Topfl, W. <,'hem. Ber. 1962, 95, 2871

Singh, L. W.; Gupta, A. K.; Ila, H.; Junjappa, H. Synthesis 1984, 5 15.

Chauhan, S. M. S.; Junjappa, H. Tetrahedron 1976, 32, 1779.

Chauhan, S. M. S.; Junjappa, H. Tetrahedron 1976, 32, 191 1

Potts, K. T. ; Cipullo, M. J.; Ralli, P.; Theodoridis, G. J. Org. Chem. 1983, -18,

484 1 .

Rastoyi, R. R.; Kumar, A,; Ila, H.; Junjappa, H. J. Chem. Soc. I'erkin lrntr.s 1.

1978, 549.

Rastogi, R. R.; Ila, H.; Junjappa, H. J. Chern. Soc. (,'hem. C7)mnirnr. 1975. 645

Gompper, R.; Topfl, W. (.'hem. Ber. 1962, 95, 287.

Tominaga, Y.; Kohra, S.; Okuda, H.;Ushirogocha, A ; Matsuda,-Y.;

Kobayashi, G. Chem. Phnrm. BUN. 1984, 32, 122.

Augustin, M.; Groth,. Ch. J. Prakt. Chem. 1979, 321, 2 15.

Rudorf, W. D.; Augustin, M. J. Prakf. Chem. 1977, 319, 545.

Huang, Z.; Zhang, P. Chin. Chem. Len. 1990, 1, 167.

Huang, Z.; Shi, X. Synthesis 1990, 162.

Huang, Z.; Shi, X. Chem. Ber. 1990, 123, 541

Rudorf, W.D.; Augustin, M. J. Prakt. Chem. 1977,545, 3 19.

Kumar.A.; Aggarwal, V.;Ila, H.; Junjappa, H. Synthesis 1980 748.

Augustin, M.: Groth. Kusten, H.; Peseke,K, Wiechmann Ch. ./. Prakt. (,'hem

1979, 321, 205.

Rioult, P.;Vialle, J .; Htrll . Soc. ('him. Fr. 1965 ,33 15.

Saquet,M.; Thuillier, A. Blrll. Chem. Soc. C'him. I+. 1966, 1582.

Rioult, P.;Vialle, J. Htrll. Soc. Chim. Fr. 1968, 4483.

Maijnan, J.; Vialle, J. Ihid, 1973, 2388.

Gompper, R.; Schafer, H. Chem. Ber. 1967, 591.

Dalgaard, L.; Jensen, L.; Lawesson, S. 0. Tetrahedron 1974, 30, 93.

Mansour, N. B.; Rudorf, W. D.; Augustin, M. Z. Chem. 1981,21, 69.

Thomas, A,; Ila, H.; Junjappa, H. 7ktrahedrorr Lett. 1989, 30, 3093.

Myrboh, B.; Ila, H.; Junjappa, H. .I. Org. ( 'hen. 1983, 48, 5327.

Reddy, K.R.; Roy, A.; Ila, H.; Junjappa, H. Tetrahedron 1995, 51, 10941

Datta, A.; Ila, H.; Junjappa, H. Tetrahedron Letr. 1988, 29, 497.

Masson. S.; Thuillier, A. Tetrahedron Lett. 1980, 21,4085.

Bauman, J. G.; Barber, R. B.; Gless, R. D.; Rapoport, R. Tetrahedron Lett.

1980, 21,4777.

Gupta, A. K.; Ila, H.; Junjappa, H. Tetrahedron 1989, 45, 1509.

Kumar, B.; Suryawanshi, S. N.; Bhakuni, D. S. Tetrahedron Lett. 1995, 36,

4625.

Johansen, 0. H.; Undheim, K. Acta. Chem. Scurrd. (B) 1979,33, 460.

Dieter, R. K.; Fishpaugh, J . R.; Silks, L. A. Tetrahedron Let1 1982, 3751

Dieter, R. K.; Silks, L. A ; Fishpaugh, J. R.; Kastner, M. F. .I. Am. ('her11 Soc.

1985, 107, 4679.

Mehta, 9. K.; Ila, H.; Junjappa, H. Tetruhedrotl I,e/t. 1995, 1225.

Kobayashi. G.; Furukawa. S.; Matsuda, Y.; Matsunaga, S. Yuk71gukr1 %LI.S.S/~;

1970, Y O , 127.

Kobayashi. G.; Furukawa, S.; Matsuda, Y.; Matsunaga, S. Yukugcku Zusshi

1969, 89, 203.

Kobayashi, G.; Matsuda, Y.;Natsuki, R.; Tominaga, Y.; Maseda, C.; Awaya,

H. Yakugaku Zasshi 1974, 94, 50.

Kobayashi, G.; Furukawa, S.; Matsuda, Y.; Natsuki, R.; Matsunaga, S ('hem.

Pharm. BUN. 1970, 18, 124.

Kobayashi, G.; Matsuda, Y.; Natsuki, R.; Sone, M. Chem. Pharm. BUN. 1972,

20, 657.

Tominaga, Y.; Ushirogochi, A,; Matsuda, Y. J. Heterocycl. Chem. 1987, 24 ,

1557.

Tominaga, Y.; Ushirogochi, A.; Matsuda, Y.; Kobayashi, G. (.'hem. Phartn.

Rfrll. 1984, 32, 3384.

Tominaga, Y.; Ushirogochi, A,; Matsuda, Y.; Kobayashi, G. Heterocycles 1977.

8 , 193.

Tominaga, Y.; Matsuda, Y.; Kobayashi, G. Heterocycles 1976, -1. 1493.

Kobayashi, G.; Matsuda, Y. Japan Palerlt 703 1 284, 1971. CA 1971, 74, 76326i

Kobayashi, G.; Matsuda, Y. Japan Patent 7 104 741, 1971. CA 1971, 74,

141848.

Potts, K. T.; Cipullo, M. J.; Ralli. P.; Theodoridis, G. J. Ani. ('hem. Soc. 1981.

103, 3584.

Potts, K. T.; Ralli, P.; Theodoridis, G.; Winslow, P. A. 0rg. Syt~rhesis 1985,

64, 189.

Potts, K . T.; Winslow, P. A. Syr~thesis 1987, 839.

Potts, K. T.; Cipullo, M. I . ; Ralli, P.; Theodoridis, G. J. Org. Chem. 1982, 47,

3028.

Potts, K. T.; Winslow, P. A. J. Org. Chem. 1985, 50, 5405.

Schafer, ii.; Gewald, K.; Scifert, M. J. Prakt. Chem. 1976, 318, 39.

Borfier, A.; Kristen, H.; Peseke, K.; Michalik, M. J. Prakt. Chem. 1986, 328, 21

Barzen, R.; Schunack, W. Arch. Pharm. 1982, 315, 680.

Mertens, H . ; Troschutz, R. Arch. Pharm. 1986, 319, 161

Kumar, A ; Ila, H.; Junjappa, H. J. Chem. Soc. Chem. Commirtr. 1976, 593.

Tominaga, Y.; Kurata, K.; Awaga, H.; Matsuda, Y.; Kobayashi, G. Heterocycles

1978, 9 , 399.

Peseke, K . ; Quincoces-Suarez, J. 2. Chem. 1981, 21, 405.

Kobayashi, G.; Matsuda, Y.; Natsuki, R.; Tominaga, Y . Yahrgak~r Zasshi 1972.

92, 713.

Tominaga, Y.; Hidaki, S.; Matsuda, Y.; Kobayashi, G. Yakirpirrrkir Zasshi 1984,

104, 440.