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CHAPTER-I

SECTION - A

An efficient method of making (S)-3-

aminomethyl-5-methylhexanoic acid

(Pregabalin)

INTRODUCTION

Introduction

Epilepsy is a common chronic neurological disorder characterized by recurrent

unprovoked seizures1, 2

. These seizures are transient signs and symptoms of abnormal

excessive or synchronous neuronal activity in the brain.3 About 50 million people

worldwide have epilepsy with almost 90 % of these people being in developing

countries.4 Epilepsy is more likely to occur in young children or people over the age of

65 years however it can occur at any time.5 Epilepsy is usually controlled but cannot be

cured with medication, although surgery may be considered in difficult cases. However

over 30 % of people with epilepsy do not have seizure control even with the best

available medications.6, 7

Not all epilepsy syndromes are lifelong – some forms are

confined to particular stages of childhood. Epilepsy should not be understood as a single

disorder but rather as syndromic with vastly divergent symptoms but all involving

episodic abnormal electrical activity in the brain.

Mutations in several genes have been linked to some types of epilepsy. Several

genes that code for protein subunits of voltage-gated and ligand-gated ion channels have

been associated with forms of generalized epilepsy and infantile seizure syndromes.8

Several ligand-gated ion channels have been linked to some types of frontal and

generalized epilepsies.

Epilepsy is usually treated with medication prescribed by a physician; primary

caregivers, neurologists and neurosurgeons all frequently care for people with epilepsy.

However it has been stressed that accurate differentiation between generalized and partial

seizures is especially important in determining the appropriate treatment. In some cases

the implantation of a stimulator of the vagus nerve or a special diet can be helpful.

Neurosurgical operations for epilepsy can be palliative, reducing the frequency or

severity of seizures or in some patients an operation can be curative.

The mainstay of treatment of epilepsy is anticonvulsant medications. Often

11

anticonvulsant medication treatment will be lifelong and can have major effects on

quality of life. If a person's epilepsy cannot be brought under control after adequate trials

of two or three different drugs that person's epilepsy is generally said to be medically

refractory. A study of patients with previously untreated epilepsy demonstrated that 47 %

achieved control of seizures with the use of their first single drug. 14 % became seizure

free during treatment with a second or third drug. An additional 3 % became seizure-free

with the use of two drugs simultaneously. Other treatments in addition to or instead of

anticonvulsant medications may be considered by those people with continuing seizures.

The first anticonvulsant was bromide, suggested in 1857 by Charles Locock who

used it to treat women with hysterical epilepsy. Potassium bromide was also noted to

cause impotence in men. Authorities concluded that potassium bromide would dampen

sexual excitement and thought to cause the seizures. In fact bromides were effective

against epilepsy and also caused impotence, it is now known that impotence is a side

effect of bromide treatment which is not related to its anti-epileptic effects. It also

suffered from the way it affected introducing the idea of the epileptic personality which

was actually a result of the medication.

Phenobarbital was first used in 1912 for both its sedative and antiepileptic

properties. By the 1930s, the development of animal models in epilepsy research led to

the development of phenytoin by Tracy Putnam and H. Houston Merritt which had the

distinct advantage of treating epileptic seizures with less sedation. By the 1970s national

institutes of health initiative the anticonvulsant screening program headed by J. Kiffin

Penry served as a mechanism for drawing the interest and abilities of pharmaceutical

companies in the development of new anticonvulsant medications.

Currently there are 20 medications approved by the Food and Drug Administration

for the use of treatment of epileptic seizures in the US. Pregabalin (Lyrica) drug used for

neuropathic pain and as an adjunct therapy for partial seizures with or without secondary

generalization in adults.9 It was designed as a more potent successor to gabapentin,

12

as gabapentin has most common side effects in adult patients which includes dizziness,

drowsiness, and peripheral edema; these mainly occur due to higher doses (1200 mg per

day) in the elderly. Also children 3–12 years of age were observed to be susceptible to

mild-to-moderate mood swings, hostility, concentration problems and hyperactivity.

Although there are several cases of hepatotoxicity reported in the literature. Recent

studies have shown that Pregabalin is effective at treating chronic pain in disorders such

as fibromyalgia and spinal cord injury.

Structure

Pregabalin is known by a variety of different names. Chemically it is known as S-

3-aminomethyl-5-methylhexanoic acid with eight carbon atoms, amino group at one end

and carboxylic group at other end (as shown below). A chiral centre is embeded at the

third position which has an isobutyl group. There are two isomers of Pregabalin (1)

namely, R and S isomer, the S isomer is pharmacologically more active and has fewer

side effects than R isomer. The first synthesis of Pregabalin (1) was evaluated by

Hoekstra et. al. 10, 11

and later reviewed by Ordonez and Cativiela.12

NH2

OHO

1

OOH

NH2

Pregabalin Gabapentin

13

Biological action of Pregabalin

Pregabalin is known by (S)-3-aminomethyl-5-methylhexanoic acid 1, it is also

called as β-isobutyl-γ-aminobutyric acid or isobutyl GABA which is related to the

endogenous inhibitory neurotransmitter γ-aminobutyric acid (GABA) which is involved

in the regulation of brain neuronal activity. Convulsions can be controlled by controlling

the metabolism of the neurotransmitter γ-aminobutyric acid. When the concentration of

GABA diminishes below a threshold level in the brain, convulsion results and when the

GABA level rises in the brain during convulsions, the seizures terminate. The term

seizure means excessive unsynchronized neuronal activity that disrupts normal function.

Because of the importance of GABA as an inhibitory neurotransmitter, and its effect on

convulsive states and other motor dysfunctions, a variety of approaches have been

reported to increase the concentration of GABA in the brain. In one approach,

compounds that activate L-glutamic acid decarboxylase (GAD) have been used to

increase concentrations of GABA, as the concentrations of GAD and GABA vary parallel

and increased GAD concentration result in increased GABA concentrations. The (RS)-3-

aminomethyl-5-methylhexanoic acid, a GAD activator has the ability to suppress seizures

while avoiding the undesirable side effect of ataxia. It has been discovered that the

anticonvulsant effect of isobutyl-GABA is stereoselective. S-enantiomer of isobutyl

GABA shows better anticonvulsant activity than R-stereoisomer which is also used for

neuropathic pain and as an adjunct therapy for partial seizures with or without secondary

generalization in adults.13-14

It has also been found effective for generalized anxiety

disorder and is as of 2007, approved for this use in the European Union. Pregabalin is

marketed by Pfizer under the trade name Lyrica. Recent studies have shown that

Pregabalin is effective at treating chronic pain in disorders such as fibromyalgia and

spinal cord injury. In June 2007, Pregabalin became the first medication approved by the

U.S. Food and Drug Administration specifically for the treatment of fibromyalgia. It is

considered to have a low potential for abuse and a limited dependence liability if

misused, and is thus classified as a Schedule V drug in the U.S.

14

Synthesis of chiral compounds

In current day scenario chiral compounds take lot more attention due to their

selective pharmacological behavior. In chiral chemistry there are two main strategies for

the preparation of enantiopure compounds. These methods involve preparing the

compounds in racemic form, and later the two enatiomers. The first strategy is

asymmetric synthesis : the use of various techniques to prepare the desired compound in

high enantiomeric excess. Techniques encompassed include the use of chiral starting

materials, the use of chiral auxiliaries, chiral catalysts and the application of asymmetric

induction. The use of enzymes may also produce the desired chiral compound. Chiral

auxillariries require for asymmetric synthesis are very costly and the additional protection

and deprotection of the functional groups which ultimately increases the cost, number of

steps, decreases yield, productivity and the pollution issue to discard chiral auxillaries.

The second strategy is followed for the synthesis of chiral compounds is the chiral

resolution. Chiral resolution in stereochemistry is a process for the separation of racemic

compounds into their enantiomers. It is an important tool in the production of optically

active drugs on commercial scale. Derivatization of racemic compounds is possible with

optically pure reagents by forming pair of diastereomers which can be separated by

conventional techniques. Derivatization is possible by salt formation between racemic

amine with chiral acid or racemic acid with chiral amine resulting in the formation of

diastereomeric salt. This diastereomeric salt can be separated by simple crystallization

technique and will afford the chiral amine or acid as the salt. Simple neutralization

process using acid or base will provide the free chiral amine or acid.

Literature Review

A thorough literature search on Pregabalin reveals that in the early stage not many

synthetic chemists were interested in the synthesis considering the size and structural

simplicity of this molecule. But once the biological activity and pharmaceutical

15

importance of Pregabalin was known, the scientific community was attracted towards

its synthesis; as a result, a number of synthetic routes for the synthesis of Pregabalin

have been documented in the literature and are as follows.

Hoekstra et al : 10

[Norephedrine–based synthetic route, 1997]

Hoekstra and co-workers utilized chiral auxiliary 17, which was prepared from

(+) Norephedrine. Acylation of 17 for the synthesis of Pregabalin with 4-

methylpentanoyl chloride (16) furnished the compound 18. Alkylation of 18 at -35 °C

using LDA and tert butyl bromoacetate in THF to afford 19. Oxidative removal of

chiral auxillary of 19 with LiOH, H2O2 in THF gave free acid 20. Reduction of 20 was

done in DMS and borane in THF to give 21 which was further treated with tosyl

chloride to give 22. Tosylated compound 22 was treated with sodium azide in DMSO to

give azido tert butyl ester 23. Hydrolysis of 23 under basic conditions afforded azido

carboxylic acid 24 which was further hydrogenated under neutral condition to give

Pregabalin (1).

C l

O

N HO

O

C H3P h

NO

O

C H3P h

O

NO

O

C H3P h

O

O R

O

N H2

C O O HC O O H

N3

C O O R

N3

C O O R

O T s

C O O R

O H

C O O R

O H

O

+

1 7 1 8 1 9

2 0 2 1 2 2

2 3 2 4

L D A B rC H 2C O 2 tB u

1 6

n -B u L i, T H F

L iO H , H 2O 2

T H F B H 3S M e 2 , T H F T o s yl ch lo r id e

N a N 3 , D M S O H yd ro lys is P d /C , T H F

1

Scheme-1

16

Hoekstra et al : 10

[L-Leucine approach, 1997]

In another approach Hoekstra and co-workers have utilized L-Leucine for synthesis

of Pregabalin. The amino group of L-Leucine (25) was first converted into a bromo group

using sodium nitrate and sodium bromide to give intermediate 26 which was

transesterified to give tert butyl ester 27 using tert butyl acetate. Displacement of bromide

with excess of sodium salt of diethyl malonate gave 28 in good yield. Hydrolysis of tert

butyl ester of 28 using formic acid provided free acid 29. Reduction of acid 29 using THF

followed by in situ cyclisation and decarboxylation afforded lactone 30. The opening of

chiral lactone 30 using trimethylsilyl iodide in ethanol was followed by nucleophilic

displacement of the iodo group by azide provided ester derivative 23. Hydrolysis of the

ester derivative 23 using potassium hydroxide in ethanol gave acid 24 which on

subsequent reduction using Pd / C yielded the desired compound Pregabalin (1).

OH

O

NH2

H OH

O

BrH OtBu

O

BrH

OtBu

O

H CH2(CO

2Et)

2

OH

O

H CH2(CO

2Et)

2

O

O

H

O

HI

OEt

O

H

OEt

N3

O

H

OH

N3

NH2

COOH

25 26 27

28 30

2423

29

31

t BuOAc

BF3.HOAc

NaCH(CO2Et)2

THFHCOOH

BH3 SMe2,

HCl

TMSI, EtOHNaN3

CH3CN KOH, EtOH

H2 , Pd/C

NaNO2, NaBr

H2SO4

1

Scheme-2

17

Hoekstra et al : 10

[Enzymatic route, 1997]

Cyanoacetamide (32) was condensed with isovaleraldehyde (2) in presence of

diisopropylamine to give intermediate 33 which on hydrolysis using dil HCl afforded 3-

isobutylglutaric acid (9). Esterification of 9 with isopropanol in acidic condition yielded

diisopopyl-isobutylglutarate 34. Diester intermediate was treated with procine liver

esterase to give enatiomerically pure monoester 35. The monoester was further treated

with lithium methoxide and formamide in a mixed solvent system gave amide

intermediate 13 which on hoffmann degradation furnished Pregabalin (1).

O

NH2

NC

O

H

NH2

O

NH2

O

CN

NC

OH O

OH

O

OiPr O

OiPr

O

OH O

OiPr

O

OH O

NH2

O

NH2

COOH

32 2 33 9

34 35 13

NaOBr H2O

HCl, H2O

IPA, H+

H2O, DMSO

PLE

Li, MeOH THF

Formamide

DIPA

MTBE

1

Scheme-3

Mark J. Burk et al : 16

[Asymmetric hydrogenation, 2003]

Isovaleraldehyde (2) was condensed with acrylonitrile (36) in presence of base

DABCO to give hydroxy nitrile intermediate 37 which on treatment with ethyl

chloroacetate in presence of base pyridine at room temperature afforded ethyl carbonate

derivative 38. Palladium catalyzed carbonylation of ethyl carbonate derivative

18

38 using palladium acetate in ethanol furnished cyano ester derivative 39 which on

hydrolysis using potassium hydroxide in water yielded potassium salt of compound 40.

The compound 40 was hydrogenated in presence of [(R, R)-(Me-DuPHOS)Rh(COD)]-

BF4 at 55 °C furnished enatiomerically pure cyano acid derivative 41 which on

hydrolysis using KOH followed by reduction in Raney nickel provided Pregabalin (1).

CHOOH

CN

CN

COOEt

COOH

NH2

CN

CN

COOK

OCO2Et

CN

COOK

CN

+

2 36 37

1

38

39 40 41

DABCO2,6 di-tert-butyl-4-methylphenol

ClCO2Et, Pyridine

CH2Cl2

Pd(OAC)2, PPh3

EtOH, CO KOH, water[(R,R)-(Me-DuPHOS)Rh(COD)]-BF4, MeOH

KOH, Raney NiEtOH, AcOH

+ +--

Scheme-4

Hamersak et al : 17

[Quinine mediated desymmetrization of cyclic anhydride, 2007]

Isovaleraldehyde (2) was condensed with cyanoacetamide (32) in presence of

piperidine followed by in situ hydrolysis with hydrochloric acid and anhydride formation

using accetic anhydride afforded 3-isobutylglutaric anhydride 10 in good yield. The 3-

isobutylglutaric anhydride 10 was further treated with quinine and cinnamyl alcohol in

toluene followed by resolution was achieved in presence (S)-(-)-α-phenylethylamine

furnished enatiomerically pure monoester 42. The monoester 42 was treated with

diphenyl phosphoryl azide and triethylamine using benzyl alcohol yielded amino acid

ester derivative 43 subsequently debenzylation of amino acid ester derivative 43 in

refluxing ethanol and palladium acetate–triphenyl phosphine afforded compound 44.

Reduction of compound 44 using Pd / C yielded desired compound Pregabalin (1).

19

CHO

NH2

O

CN

O OO

NH

OO

OO

O

OH

OO

NH

OOH

OO

NH2

COOH

+

2 32 10 42

43

44

CH2Cl2, Piperidine

HCl, Ac2O

Quinine, Toluene

Cinnamyl alcohol (S)-PEA

(PhO)2PON3, Et3N

Benzyl alcohol

Pd(OAc)2, PPh3

Morpholine, EtOHPd/C, Aq MeOH

1

Scheme-5

Sammis et al : 18

[TMSCN route, 2003]

Addition of hydrogen cyanide to α, β-unsaturated imides 45 using aluminum salen

catalyst and TMSCN yielded chirally pure cyano derivative 46 which on hydrolysis using

sodium hydroxide in THF afforded cyano acid compound 47 in good yield. Reduction of

cyano acid compound 47 using PtO2 furnished Pregabalin (1) in good yield.

20

NH

O O

NH

O O CN

OH

O CN NH2

COOH

45 46

47

Aluminum salen complexTMSCN, Isopropanol

Sodium hydroxideTHF 5% PtO2, H2

1

Scheme-6

Maymon Asher et al : 19

[Nitro ester route, 2006]

Diester compound 48 was treated with nickel chloride and sodium borohydride at

0-5 °C afforded monoester lactam 49 which on treatment with sodium hydroxide in

presence of ethanol furnished lactam compound 50 in high yield. It was further treated

with conc HCl at 125 °C followed by treatment with TEA in MDC gave Pregabalin (1).

CO2R

1CO

2R

2

NO2

NH

O

R2CO

2

NH

O

NH2

COOH

4849

50

NaOH, EtOH

HCl

NiCl2.6H2O

NaBH4, MeOH

HCl, CH2Cl2

TEA

1

Scheme-7

21

Tiwari Anand et al : 20

[Asymmetric synthesis, 2007]

Glutaric anhydride (10) was treated with (S)-PEA and 4-dimethylaminopyridine at -

10 to -15 °C afforded chirally pure compound 51 in good yield subsequently amidation

using ethyl chloroformate and triethylamine yielded compound 52. Esterification of 52

with sodium methoxide and bromine in solvent methanol at -40 to -45 °C and

crystallization in diisopropyl ether furnished the compound 53. The compound 53 was

further treated in tetrahydrofuran and water at -40 to -60 °C in presence of liquid

ammonia and sodium metal to give monoamide derivative 54 which was further treated

with HCl solution at 100 to 110 °C for 24 h followed by treatment with tributylamine and

extraction in isobutanol afforded Pregabalin (1).

NH

O

OH

O

R

ArO OO

NH

O

NH2

O

R

Ar

NH

O

NHCOOR

O

R

ArNH

2

O

NHCOOR

O

NH2

COOH

51 52

53 54

10

(S)-PEA DMAP

ECF, TEAAq ammonia

Sodium methoxideBromine, Methanol

THF, waterAmmonia, Sodium

Aq HCl, IsobutanolTributylamine

1

Scheme-8

22

Hoekstra et al : 10

[Resolution using R-(+)-α-methylbenzylamine, 1997]

3-isobutylglutaric acid (9) was treated with acetic anhydride at reflux temperature

afforded 3-isobutylglutaric anhydride (10) which on treatment with ammonia in MTBE

yielded amide derivative 11. Resolution of amide derivative 11 was achieved using R-

(+)-α-methylbenzylamine in chloroform and ethanol furnished R-Amide MBA salt

intermediate 12 which was further treated with aqueous HCl gave R-Amide 13 free from

R-(+)-α-methylbenzylamine. It was subjected under hoffmann degradation using sodium

hypobromite yielded Pregabalin 1 with high purity.

OH O

OH

O

OO O OH O

NH2

O

O O

NH2

O

NH3

OH O

NH2

ONH

2

COOH

9

12

10

NaOH, Br2

Aq HCl

13

Acetic anhydride

MTBE, NH3

Aq HCl

EtOH, CHCl3

R-(+)-MBA Aq HCl

1

11

+

-

Scheme-9

Hoekstra et al 10

[(S)-(+)-mandelic acid route, 1997]

Isovalderaldehyde (2) was condensed with diethyl malonate (3) using di-n-propyl

amine to give α, β-unsaturated diester 4 in high yield. Compound 4 was further treated

with potassium cyanide in ethanol to give β-cyano diester 5 followed by a

decarboxylation step using dimethyl sulfoxide and sodium chloride to give cyano ester

derivative 6 Hydrolysis of the ester group of 6 followed by reduction of the nitrile group

to the amine gives racemic Pregabalin (7). Racemic Pregabalin (7) is resolved using

23

(S)-(+)-mandelic acid in isopropanol and water mixture gives the chirally pure mandalate

salt 8 which on neutralization using tetrahydrofuran and water gave crude 1. Final

purification from isopropanol and water provide pure Pregabalin (1).

O

H

COOEt

COOEt

EtOOC COOEt

CN

COOEt

CNNH

2

COOH

NH3

COOH

OH

OOC

NH2

COOH

COOEt

COOEt

+

1

2

4

7

5

6

3

8

+

Di-n-propylamine KCN, EtOH

MTBEKOH, Raney Ni

EtOH

(S)-(+)-mandelic acid IPA

THF / Water

-

Scheme-10

Zhang Guisen et al 22

[3-isobutyl glutarimide route, 2006]

3-isobutylglutaric acid (9) was treated with carbamide at 160-180 °C to give 3-

isobutyl glutarimide (15) which on treatment with sodium hypochlorite at 60 °C to give

racemic Pregabalin (7). Racemic Pregabalin (7) was resolved using (S)-(+)-mandelic acid

in isopropanol and water mixture gives the chirally pure mandalate salt which on

neutralization using tetrahydrofuran and water gave crude 1. Final purification from

isopropanol and water provided pure Pregabalin (1).

24

OH

O

OHONH

OO

NH2

OHO

NH2

COOH

9

7

15

Carbamide

Sodium

hypochlorite(S)-(+)-mandelic acid IPA

1

Scheme-11

Shanghui Hu et al 23

[Enzymatic route, 2005]

Cyano ester derivative 5 was treated with enzyme lipase at pH 8.0 to give chirally

pure compound 55 which on treatment with Raney nickel at temp 25-30 °C to afford

lactam compound 56. Lactam compound 56 on treatment with water and HCl to furnish

Pregabalin (1).

CN

COOEtEtOOC

CN

COO-

COOEt

K+

NH

OHOOC

NH2

COOH

5 55

56

Lipase Buffer pH 8.0

Raney Ni, H2 H2O, HCl

1

Scheme-12

25

Veronica Rodriguez et al 24

[Pregabalin synthesis via radical mechanism, 2007]

Mesylated compound 57 was treated with S-(-)-MBA (58) in presence of

triethylamine and MDC to give compound 59 which on treatment with n-BuLi in THF at

temperature -78 °C to afford compound 60. The compound 61 was obtained by the

reaction of Bu3Sn with compound 60 at -78 °C which on treatment for Birch reduction to

give compound 50. The compound 50 on treatment with KOH and H2O to give

Pregabalin (1).

OMs

Ph3P

NH2

Ph

NH

Ph

Ph3P

NH

Ph

NO H

Ph

NH

O

NH2

COOH

O

+

57 58 59 60

61 50

TEA, MDCn-BuLi, THF

Birch reductionKOH, H2OBu3Sn

1

+ Br -

Scheme-13

26

PRESENT WORK

Present Work

The retro synthetic analysis depicted in scheme-1 suggested that Pregabalin (1)

could be prepared through mandalate salt 8 derived from compound 7, that could be

generated from cyano diester compound 6, compound 6 could be prepared from

compound 5, which could synthesized from compound 4 derived from isovaleraldehyde

(2) and diethyl malonate (3).

COOEt

COOEt

EtOOC COOEt

CN

COOEt

CN

NH2

COOH

NH3

COOH

OH

OOCNH2

COOH

-

O

H

COOEt

COOEt

1

4

7

5

6

8

+

+

2 3

Scheme-1 Retrosynthesis of Pregabalin (1)

27

Synthesis of Pregabalin 1

Synthesis of Pregabalin (1), one of the proven Anti epileptic drug started from

commercially available isovaleraldehyde (2) was condensed with diethyl malonate (3)

using base di-n-propylamine in n-hexane at reflux temperature resulted in diester

compound 4 with 88 % yield. The 1H NMR spectrum of compound 4 showed the olefinic

proton signal at δ 7.02 ppm (t, 1H, J = 7.9 Hz) and resonance specific for aldehyde group

of isovaleraldehyde was absent in proton NMR spectrum (Scheme-2).

COOEt

COOEt

O

H

COOEt

COOEt

4

+

2 3

Di-n-propyl aminen-hexane

Scheme-2

The compound 4 was subjected with KCN for 1, 4 addition in ethanol at room

temperature for 18 h followed by extraction with n-hexane and concentration gave cyano

ester compound 5. In proton NMR spectrum, signal at 3.22 (m, 1H) was observed for

proton adjacent to cyano group and signal at 3.94 (d, 1H) was observed for proton

adjacent to diester group and absence of olefinic signal at δ 7.02 ppm (t, 1H, J = 7.9 Hz)

and confirmed the formation of compound 5 (Scheme-3).

COOEt

COOEt

EtOOC COOEt

CN

4 5

KCN, EtOH

Scheme-3

Next, cyano ester compound 5 was oxidative decarboxylated using solution

containing sodium chloride, DMSO and water at 137 °C to 148 °C for 8.5 h to furnish the

cyano ester compound 6 in 85 % yield which was confirmed by proton NMR spectrum in

which proton NMR signal at 1.10-1.40 (m, 6H) in diester compound 5 was changed to

1.1-1.4 (m, 4H) and NMR signal at 4.1-4.2 (m, 4H) changed to 4.2 (q, 2H).

28

Absence of resonances specific for ethyl ester 2.53 (dd, 1H, J = 16.6 Hz) and 2.70

(dd, 1H, J = 16.5 Hz) was observed (Scheme-4).

EtOOC COOEt

CN

COOEt

CN

5 6

NaCl, DMSOWater

Scheme-4

To get the racemic compound 7, the ester group of compound 6 was hydrolysed

using KOH followed by reduction of the nitrile group to the amine group (-CH2 NH2)

using Raney nickel which was confirmed to be racemic compound 7 by 1H NMR in

which absence of proton NMR signal at 1.1-1.4 (m, 4H) and 4.2 (q, 2H) for -CH2CH3.

The mass spectrum was observed at 160 (M+

+ H) and M.P. was 166-167 °C. IR

spectrum showed band at 3304 cm-1

corresponding to NH2 group (Scheme-5).

COOEt

CNNH

2

COOH

76

KOH Raney Ni

Scheme-5

Resolution of compound 7 was successfully accomplished involving use of (S)-(+)-

mandelic acid in water and isopropanol to form compound 8 which was analysed for

percentages of both the enantiomers. It contained typically around 99.30 % of the desired

S-enantiomer of 7 along with 0.70 % of the R-enantiomer of 7. The proton NMR

spectrum specific for aromatic region of (S)-(+)-mandelic acid was appeared at 7.2 (s,

5H). Deshieldng of proton NMR signals was observed due to formation of mandalate salt

(Scheme-6).

NH2

COOH

NH3

COOH

OH

OOC

7 8

+(S)-(+)-mandelic acidIPA, Water

-

Scheme-6

29

The reported procedure for desaltification of 8 involved heating with THF and

water mixture in the range of 65-80 °C followed by filtration to obtain crude 1 which was

further recrystallized in IPA and water to obtain pure Pregabalin (1). In the proton NMR

spectrum resonance specific for aromatic region of (S)-(+)-mandelic acid appeared at 7.2

(s, 5H) were absent and mass spectrum was observed at 160 (M+

+ H). The structure

assigned was also confirmed by elemental analysis. The chiral purity by HPLC of 1 was

in excellent agreement with that reported values (Scheme-7).

NH3

COOH

OH

OOC NH2

COOH

-

8

+

THF, WaterIPA, Water

1

Scheme-7

Although the above mentioned desaltification process works, but it is far from

satisfactory as,

i) It requires temperature in the range of 65-80 °C.

ii) The process is also prone to undergo acid catalyzed decomposition of Pregabalin and

formation of lactam impurity on prolonged heating at 65-80 °C.

iii) Tetrahydrofuran is highly flammable and is liable to develop peroxides on standing,

which then become explosive, during solvent recovery.

iv) Mixing of water with tetrahydrofuran makes it unsuitable for recovery.

v) As THF is used for desaltification; which increases the cost of API and also reduces the

productivity.

To ensure the commercial viability of manufacturing process of Pregabalin we felt

there was need for developing an alternative, cost effective and safe method for

desaltification of 8. We envisaged that the most appropriate and straight forward process

would be to design a protocol in which the desaltification of 8 would go in presence of

cheaply available solvent and base at ambient temperature (Scheme-8).

30

NH3

COOH

OH

OOCNH

2

COOH

Solvent

Base25-30 °C

18

+

-

Scheme-8

In order to develop the desaltification process, we decided to find out best suitable

organic or inorganic base which could neutralize the salt and converts the compound 8

into Pregabalin (1) with high yield and purity. Accordingly a number of experiments

were set up using different bases like aqueous ammonia, diisopropylethylamine, DBU,

triethylamine and tromethamine as well as inorganic bases such as sodium bicarbonate

and sodium hydroxide in isopropanol at 25 to 30 °C. We could finally concluded that

with the use of inorganic bases lower yields were obtained however with the use of

organic bases lower yields and also purity was at lower side. In aqueous ammonia

surprisingly we could get good results and finally we could concluded that desaltification

of 8 using different bases were inadequate except aqueous ammonia with respect to yield

and quality of API. Table-1 provides the results. Pregabalin (1) obtained after

desaltification of 8 using aqueous ammonia and isopropanol was confirmed by 1H NMR,

13C NMR and elemental analysis. Chiral purity was analysed by chiral HPLC.

31

Table –1 : Study of various bases for desaltification of 8[a]

[a] Reactions were performed on 5.0 g scale using 1.0 eq base wrt 8 at 25 to 30

oC in isopropanol.

Once we arrived at a conclusion that aqueous ammonia is the base of choice for

desaltification of 8, we focused our investigation on finding out the best suitable solvent

for desaltification of 8 in presence of base aqueous ammonia which would give the

possible result with respect to yield and purity. To achieve this, a number of experiments

were conducted using solvents like n-hexane, toluene, dichloromethane, acetonitrile and

methanol from which we could finally concluded that desaltification of 8 in acetonitrile

and methanol was found satisfactory but as acetonitrile is class one solvent which would

cause residual solvent problem in API and secondly with use of methanol yield obtained

after desaltification of 8 was not feasible with respect to cost of API. However solvents

like n-hexane, toluene and dichloromethane were inadequate to carry out desaltification

of 8 which could afford impure Pregablin. However in isopropanol desaltification of

compound 8 would give the possible result with respect to yield and purity of Pregabalin

(1). Table 2 provides the results.

32

Base HPLC

purity

(%)

Chiral

purity

(%)

Yield

(%)

NaOH 99.29 100 52

NaHCO3 99.60 99.98 56

DIPEA 99.82 99.97 67

TEA 99.60 99.98 64

DBU 99.60 99.98 36

Trometha

mine

95.40 99.96 55

Aq

Ammonia

99.48 99.96 78

Table -2 : Study of different solvents for desaltification of 8[a]

[a] Reactions were performed on 5.0 g scale using 5.0 volume solvent wrt 8 at 25 to 30

oC in

aqueous ammonia.

Once we achieved at final conclusion that aqueous ammonia is base of our choice

and isopropanol is the most suitable solvent for the desaltification of 8 we focused our

investigation on finding the optimal condition i.e. quantity of aqueous ammonia and

isopropanol which would give best possible result in the desaltification of 8. To achieve

this many experiments were conducted varying solvent volume and qty of base from

which we concluded that 1.0 volume aqueous ammonia and 5.0 volume of isopropanol

wrt compound 8 is the best optimized condition under which the desaltification of 8 is

most efficient.

Once we concluded quantity of liquor ammonia and isopropanol, we focused our

investigation on finding out the reaction temperature and time which would give best

possible result in the desaltification of 8. To finalise the reaction temperature and time,

number of experiments were carried out by varying the temperature and time, from the

experiments we found that 1.0 h would be the minimal time required for complete

33

Solvent HPLC

purity

(%)

Chiral

purity

(%)

Yield

(%)

ACN 99.87 100 75

n-Hexane 96.57 100 62

Ethyl acetate 89.25 100 64

Toluene 91.11 99.98 72

MDC 85.03 99.98 75

Methanol 99.78 100 53

Acetone 99.77 100 70

THF 99.76 100 72

Isopropanol 99.48 99.96 78

desaltification of compound 8 at ambient temperature would be the most suitable and

economical viable process. From the experiments we concluded that the use of 1.0

volume of aqueous ammonia and 5.0 volume of isopropanol at 25 to 30 °C for 1.0 h

would be the best optimized condition under which desaltification of 8 is most efficient

with respect to yield and quality of API. Table 3 provides results.

Table -3 : Study of temperature for desaltification of 8[a]

[a] Reactions were performed on 50.0 g scale using 5.0 volume isopropanol and 1.0 volume

aqueous ammonia wrt 8 at different temperature.

Conclusion

A superior method of making Pregabalin (1) from the compound 8 has been

developed. This protocol has made the process for desaltification of 8 more efficient and

industrially feasible involving less number of steps.

34

Temp

°C

HPLC

purity

(%)

Chiral

purity

(%)

Yield

(%)

10-15 99.61 100 77

25-30 99.65 100 78

75-80 99.82 99.98 78

EXPERIMENTAL

SECTION

Experimental

Preparation of 2-carboxyethyl-5-methylhex-2-enoic acid ethyl ester (4).

COOEt

COOEt

Di-n-propylamine (20.0 g, 0.198 mol) and glacial acetic acid (24.0 g, 0.40 mol)

was added to the solution containing isovaleraldehyde (361.6 g, 4.2 mol), diethyl

malonate (640.8 g, 4.0 mol) and n-hexane (1000 mL). The reaction mass was heated to

reflux temperature and the generated water was removed azeotropically. The reaction

mixture was cooled to 40 °C and washed with water (2 X 800 mL). The organic layer

was concentrated under vacuum completely to afford 2-carboxyethyl-5-methylhex-2-

enoic acid ethyl ester (4) 1H NMR (CDCl3) : δ 0.91-1.02 (m, 6H), 1.23-1.37 (m, 6H),

1.81 (m, 1H), 2.16-2.23 (m, 2H), 4.19-4.36 (m, 4H), 7.02 (t, 1H, J = 7.9 Hz), B.P. 101-

104 °C.10

Preparation of 2-carboxyethyl-3-cyano-5-methylhexanoic acid ethyl ester (5).

2-carboxyethyl-5-methylhex-2-enoic acid ethyl ester (4) (692.7 g, 3.03 mol),

potassium cyanide (172.6 g, 2.65 mol) and ethanol (700 g) was stirred at 25 to 40 °C for

18 h. After completion of the reaction, n-hexane (890 mL) and solution containing glacial

acetic acid (175 g) and water (820 mL) was added slowly below 35 °C and the layers

were separated. The aqueous layer was extracted with n-hexane (890 mL). The organic

layers were combined and washed with water (420 mL) and the organic layer was

distilled out under vacuum completely to afford 2-carboxyethyl-3-cyano-5-

methylhexanoic acid ethyl ester (5) as a colorless liquid (752.6 g, 93 %). HPLC Assay :

86 %, 1H NMR (DMSO-d6) : δ 0.90 (t, 6H, J = 6.1 Hz), 1.18-1.21 (m, 6H), 1.26 (m, 1H),

1.54-1.66 (m, 2H), 3.22 (m, 1H), 3.94 (d, 1H), 4.70-4.21 (m, 4H).10

35

CN

COOEtEtOOC

Preparation of 3-cyano-5-methylhexanoic acid ethyl ester (6).

COOEt

CN

2-carboxyethyl-3-cyano-5-methylhexanoic acid ethyl ester (5) (80 g, 0.313 mol)

was combined to the solution containing sodium chloride (21 g, 0.359 mol), dimethyl

sulfoxide (238 g) and water (10.8 g). The reaction mass was heated to 137-148 °C for 8.5

h. The reaction mass was cooled to 50 °C and extracted with methyl tert butyl ether (2 X

125 g). The organic layers were combined and concentrated under vacuum to afford 3-

cyano-5-methylhexanoic acid ethyl ester (6) as a brown oil (51.3 g, 85 %). GC purity :

86.2 %, 1H NMR (CDCl3) : δ 0.88-0.99 (m, 6H), 1.19-1.40 (m, 4H), 1.63 (m, 1H), 1.78

(m, 1H), 2.53 (dd, 1H, J = 6.8, 16.6 Hz), 2.70 (dd, 1H, J = 6.1, 16.5 Hz), 3.05 (m, 1H),

4.21 (q, 2H, J = 7.1 Hz). 10

Purity by HPLC conditions :

Inertsil C8 (250 X 4.6 mm) at 210 nm and eluted with buffer (2.72 g KH2PO4 in 1000 mL

DM water and adjusted pH to 7.5 using KOH solution : acetonitrile (500:500)

Chiral purity by HPLC conditions : The chiral HPLC was recorded on chiral AD-H

(250 X 4.6 mm) at 210 nm and eluted with n-hexane : ethanol : TFA (950:50:1)

Preparation of 3-aminomethyl-5-methylhexanoic acid (7).

NH2

COOH

3-cyano-5-methylhexanoic acid ethyl ester (6) (50.1 g, 0.273 mol), methanol (53

mL), potassium hydroxide (17.8 g, 0.317 mol) and water (56 mL) was transferred to a

hydrogenerator containing Raney nickel (15.0 g) and reduced at 50 psi pressure for 15 h

after which the Raney nickel was removed by filtration and washed with methanol (35

mL) and DM Water (100 mL). Glacial acetic acid (22.8 g) was added to the filtrate and

36

heated at 70-75 °C to dissolve the solids followed by cooling of the reaction mass to 0-5

°C was done to crystallize the product. The precipitated solids were filtered, washed with

chilled isopropanol and dried under vacuum at 50-55 °C to give racemic 3-aminomethyl-

5-methylhexanoic acid (7) as a white solid (31.0 g, 74 %). M.P. 166-167 °C 10

, IR (KBr) :

648, 852, 1207, 1408, 1540, 1661, 2176, 2586, 2862, 3304 cm-1

, Mass (m/z) : 160 (M+

+

H), 182.0 (M+

+ Na), 1H NMR (D2O) : δ 0.86-0.89 (m, 6H), 1.20 (t, 2H, J = 7.07 Hz),

1.63 (m, 1H), 2.13-2.33 (m, 3H), 2.90-3.02 (m, 2H), HPLC purity : 99.04 %.

Resolution of 3-aminomethyl-5-methylhexanoic acid using (S)-(+)-mandelic Acid (8).

NH3

COOH

OH

OOC

+

-

3-aminomethyl-5-methylhexanoic acid (7) (29.7 g, 0.168 mol) and (S)-(+)-

mandelic acid (39.3 g, 0.258 mol) was charged to solution of 3 % v/v water in

isopropanol (244 mL) and heated to 65-80 °C to dissolve the solids. The reaction mass

was cooled to 30 to 32 °C and seeded with S-S salt to crystallize the diastereomeric

mandalate salt enriched with S, S isomer and stirred for 30 min. The precipitated solids

were filtered and washed with water / isopropanol solution and dried under vacuum at 50-

55 °C to furnish compound 8 as a white solid (24.7 g, 85 %). M.P 133-135 oC

10, Mass

(m/z) : 325 (M+

+ Na), 1H NMR (D20) : δ 0.68-0.72 (m, 6H), 1.05 (t, 2H, J = 7.15 Hz),

1.46 (m, 1H), 2.03-2.26 (m, 3H), 2.81 (d, 2H, J = 6.1 Hz), 4.90 (s, 1H), 7.2 (s, 5H),

Chiral purity by HPLC : S-enantiomer : 99.69 % and R- enantiomer : 0.31 %.

Preparation of (S)-3-aminomethyl-5-methylhexanoic acid (1).

(S)-(+)-mandelic acid salt of (S)-3-aminomethyl-5-methylhexanoic acid (8) (52.55

g, 0.169 mol) (containing ~0.70 % of R-enantiomer) was charged to a solution containing

isopropanol (262.5 mL) and aqueous ammonia (52.55 mL) at 25-30 °C and stirred for 1

h. The solids were filtered and washed with 13.90 L isopropanol to get crude (S)-3-

37

NH2

COOH

aminomethyl-5-methylhexanoic acid (1) which was recrystallized from mixture of

isopropanol (80.9 mL) and water (80.9 mL) to afford pure Pregabalin (1) as a white solid

(21.0 g, 78 %). M.P. 177-180 °C 10

, IR (KBr) : 701, 822, 860, 1279, 1334, 1389, 1429,

1551, 1644, 2209, 2955 cm-1

, Mass (m/z) : 160 (M+

+ H), 1H NMR (D20) : δ 0.67-0.70

(m, 6H), 1.0 (t, 2 H, J = 7.1 Hz), 1.45 (m, 1H), 1.96-2.11 (m, 3H), 2.75-2.80 (m, 2H),

HPLC purity : 99.48 %, Chiral purity : S-enantiomer : 99.96 % and R-enantiomer : 0.04

%, Elemental analysis : Anal for C8H17NO2 C, 60.29; H, 10.67; N, 8.79. Found C, 60.47;

H, 10.75; N, 8.79.

38

SPECTRA

1H NMR spectrum of Compound 5 in DMSO d6

1H NMR spectrum of Compound 7 in D2O

39

Purity by HPLC chromatogram of compound 7

Mass spectrum of compound 7

40

IR spectrum of compound 7

Chiral purity of compound 8

41



42



43



44

Chiral purity chromatogram of compound 1

HPLC purity chromatogram of compound 1

45

CHN analysis of compound 1


46

CHAPTER-I

SECTION-B

An efficient process for racemization of

enriched S-3-(carbamoylmethyl)-5-

methylhexanoic acid: A Pregabalin

intermediate

Although the previously mentioned mandalate route (Section-A) for synthesis of

Pregabalin (1) is most suitable, more efficient and industrially feasible, the major

disadvantage of the process is the unavoidable loss of 50 % of the unwanted isomer

during resolution step which goes in the mother liquor could not be racemized and in turn

increases the cost of API.

To overcome this issue we focused to find out an alternative route in which

unwanted isomer after resolution can be recovered from filtrates and could be converted

back to the racemate which in form can be subjected for resolution process resulting in

higher yield and decreasing the cost of API. After studying of number schemes, we have

chosen resolution route involving resolution of 11 using R-(+)-α-methylbenzylamine in

chloroform and ethanol and racemization of unwanted isomer 14 back to compound 11.

Having bridged the gap from the earlier racemization process of 14 to compound 11

which involves hazardous reagents and long reaction time inspired us to look into other

alternatives in which racemization of 14 can go in one step and increase the throughput of

the reaction and productivity at plant scale.

47

PRESENT WORK

Present Work

As part of our research program the retrosynthetic analysis indicates in scheme-1

suggested that Pregablin (1) could be generated by an alternative route involving

Hoffmann degradation of compound 13, derived from racemic amide 11, that could be

prepared from 3-isobutylglutaric anhydride 10, the compound 10 could be generated from

diacid compound 9, derived from isovalaraldehyde (2) and ethylcyanoacetate (62).

OO OOH O

NH2

O

O O

NH2

O

NH3

OH O

NH2

ONH

2

COOH

OH O

OH

O

O

H

O

EtOCN

O

EtOCN

12

1011

131

9

+

262

+ -

Scheme-1 Retrosynthesis of Pregabalin (1)

48

Synthesis of Pregabalin (1) using resolution of racemic amide (11)

Manufacturing process of Pregabalin was sucessfully accomplished involving

Knovengeal condensation of commercially available isovalaraldehyde (2) with

ethylcyanoacetate (62) in n-hexane and di-n-propylamine at reflux temperature and

generated water was removed azeotropically. When the reaction appears to complete the

solvent was removed to yield 2-cyano-5-methyl hex-2-enoic acid ethyl ester which was

not isolated or purified and used in next step by reaction with diethyl malonate followed

by hydrolysis and decarboxylation using aqueous HCl afforded 3-isobutylglutaric acid

(9). The proton NMR resonance specific for aldehyde group of isovaleraldehyde was

absent. Further the support of structure was provided by appearance of an IR absorption

band at 1704 cm-1

for keto group (Scheme-2).

OH O

OH

O

O

H

O

EtOCN

9

+

2

dI-n-propylaminen-hexane

Diethyl malonateAq HCl

62

Scheme-2

3-isobutylglutaric anhydride (10) was prepared by refluxing 3-isobutylglutaric

acid (9) with acetic anhydride for 5.0 h and then distilling the mixture. The spectral data

were in good agreement with the reported value. Further the support of structure was

provided by appearance of an IR absorption band at 1760 cm-1

for anhydride group. The

compound was also confirmed by elemental analysis (Scheme-3).

OH O

OH

O O OO

9 10

Acetic anhydride

Scheme-3

49

Next anhydride 10 was reacted with ammonia at room temperature to form

racemic compound 11. In this step cyclic anhydride is opened and one of the carbonyl

group was converted to amide which was confirmed by elemental analysis. Appearance

of IR absorption band at 3367 cm-1

for amide group was also supported the formation of

compound 11 (Scheme-4).

OH O

NH2

O

O OO

1110

Aqueous ammonia

Scheme-4

The R-(+)-α-methylbenzylamine salt of R-3-(carbamoylmethyl)-5-

methylhexanoic acid (12) was obtained by reacting compound 11 with R-(+)-α-

methylbenzylamine in chloroform and ethanol at 50 °C. The resolution of the desired

isomer was confirmed by chiral purity by HPLC. It was also confirmed deshielding of

proton NMR signal (Scheme-5).

OH O

NH2

O

O O

NH2

O

NH3

1211

R-MBA, CHCl3

EtOH + -

Scheme-5

R-3-(carbamoylmethyl)-5-methylhexanoic acid (13) free from R-(+)-α-

methylbenzylamine was obtained by dissolving salt (12) in water and acidifying the

solution to pH 1-2 which was confirmed by mass and proton NMR (Scheme-6).

O O

NH2

O

NH3

OH O

NH2

O

12 13

Aq HCl+ -

Scheme-6

50

The compound 13 was kept for hoffmann degradation using NaOBr and further

purified in IPA and water mixture afforded Pregabalin (1). The mass spectrum of 1 was

observed at 160 (M+

+ H). It was also confirmed by proton NMR and elemental analysis

(Scheme-7).

OH O

NH2

O

NH2

COOH

13 1

NaOBr, IPA Water

Scheme-7

The unwanted isomer 14 which goes in chloroform and ethanol mother liquor

during the resolution of 11 was isolated followed by racemization back to compound 11

as per literature procedure. The isolation process of compound 14 involves an extraction

of the chloroform filtrate with aqueous sodium hydroxide solution followed by

acidification of aqueous layer with concentrated hydrochloric acid. The acidic solution

was heated under reflux for 24 h and extracted with MTBE and concentration of the

MTBE layer afforded the corresponding 3-isobutylglutaric acid (9). The resulting

symmetrical 3-isobutylglutaric acid (9) was transformed into racemic 11 by the literature

procedure (Scheme-8).

O OO

H

OH O

NH2

O

OH O

OH

O

OH O

NH2

O

14

1110

9

Aq HCl

Acetic anhydride Aq ammonia

Scheme-8

51

Although the above mentioned racemization process works, it involves multi step

operations involving the conversion of compound 14 to diacid 9 followed by the

conversion of 9 to the corresponding anhydride 10 using acetic anhydride and finally

hydrolysis of anhydride (10) using aqueous ammonia to give racemic amide 11.10

Acetic

anhydride which is listed U. S. DEA list II precursor and restricted in many other

countries is used in the documented process and also all reactions are having longer

reaction period.

To ensure the commercial viability of the manufacturing process of Pregabalin, we

concluded that there was an immediate need for developing an alternative strategy for the

racemization of 14. We envisaged that the most appropriate and straight forward

approach would be to design a protocol in which the racemization of 14 would go

through the cyclic amide intermediate 15 which on hydrolysis should give racemic amide

(11) (Scheme-9).

NH

OO

HOH O

NH2

O

OH O

NH2

O

15 1114

Scheme-9

Symmetric glutarimide synthesis (Scheme-10) from glutaric acid monoamide has

been reported by heating the glutaric acid at 220 to 225 °C.21

NH

OO

HOOC CONH2

Heating at220-225°C

Scheme-10

To check the efficacy of the reference procedure, the compound 14 was heated at

220 to 225 °C, the reaction was monitored by TLC and after completion, the reaction

mixture was cooled and the compound was isolated by filtration and confirmed to be 3-

isobutyl glutarimide (15) by the 1H NMR and Mass spectroscopic data.

52

The 1H NMR indicated that β proton appeared at a deshilded position (δ 1.64 from

1.50 ppm). 3-isobutyl glutarimide 15 was confirmed also by IR with a peak at 1685 cm-1

.

The isolated glutarimide 15 was treated with aqueous NaOH followed by the isolation of

compound 11 after conc HCl work up. The 1H NMR β proton of compound 15 at δ 1.64

ppm shifted to δ 1.55 ppm (Scheme-11).

NH

OO

HOH O

NH2

O

OH O

NH2

O

15 1114

ToluenePiperidine

NaOHAq HCl

Scheme-11

Once, we were successful in isolating intermediate 15 on small scale, we decided

to find out an optimal condition or process for the formation of glutarimide 15 which

could be racemized and hydrolysed back to the desired racemic compound 11.

Accordingly, a number of experiments were set up with varying reaction conditions using

different solvents and bases like piperidine, DBU, diisopropylamine, triethylamine,

diisopropylethylamine in toluene and finally concluded that, piperidine is base of choice

as compared to other bases under which the racemization process is most efficient and the

results are listed in table 1.

Table 1 : Study of different bases for racemization of 14[a]

Base Chiral purity (%)

R S

Piperidine Toluene 50.01 49.99

DBU Toluene 50.01 49.98

DIPA Toluene 50.86 49.14

DIPEA Toluene 47.21 52.78

TEA Toluene 37.57 62.42

[a] Reaction were carried out on 5.0 g scale using different bases in 5.0 vol of solvent at reflux

temperature.

53

Once we were successful in finalizing piperidine as base of choice for the

formation intermediate 15, we decided to find out the most suitable solvent for the

racemization of 14 which would give the best results with respect to yield and purity. To

achieve this number of experiments were conducted varying the solvents like n-hexane,

ethyl acetate, chloroform, MTBE and toluene. We could finally concluded that the

formation of the compound 15 in above solvents was inadequate except in toluene under

which racemization of 14 was achieved with R-isomer 50.01 % and S-isomer 49.99 %

and the results are listed in table 2.

Table 2 : Study of different solvents for racemization of 14[a]

Base Solvent Chiral purity (%)

R S

Piperidine n-Hexane 27.24 72.75

Piperidine Toluene 50.01 49.99

Piperidine Ethyl acetate 11.13 88.86

Piperidine Chloroform 18.93 81.06

Piperidine MTBE 15.46 84.53

[a] Reactions were carried out on 5.0 g scale using different solvents with piperidine at reflux

temperature.

Once we arrived at a conclusion that piperidine is the base of choice and toluene is

the most suitable solvent for the racemization, we focused our investigation on finding

the optimal condition i.e. quantity of base / solvent / temperature / time which would give

best possible result in the racemization step. To achive this number of experiments were

conducted varying of reagents from which we could finally concluded that 0.04 eq of

piperidine and 5.0 volume of toluene is the best optimal condition under which the

racemization process is most efficient. To finalise the reaction time, reactions were

carried out where we enforced the racemization process by chiral HPLC, from the

experiments we found that 10 h would be the minimal time period required to get

complete racemization of compound 14. From all these experiments we concluded that

the most economical and viable process would be the use of 0.04 eq piperidine in

54

refluxing toluene for 10 h followed by pH adjustment using aqueous NaOH and

maintaining at 60 °C for 1.0 h and treatment with aqueous HCl to give racemic amide 11

with good yield and purity.

After finalizing piperidine as a base and higher boiling solvent toluene for

racemization step, our next investigation was to establish the optimal quantity of

piperidine and toluene to ensure the maximum yield with good purity. We conducted

many experiments and observed that 0.04 equivalent of piperidine and 5.0 volume of

toluene wrt compound 14 produced the best yield of racemized compound 11.

Conclusion

A superior method for racemization of (S)-3-(carbamoylmethyl)-5-methylhexanoic

acid (14) through the formation of 3-isobutyl glutarimide intermediate (15) followed by

treatment with aqueous HCl in one pot reaction sequence. This protocol has made the

process of manufacturing Pregabalin more efficient and commercially viable involving

less number of steps and complete atom economy.

OH O

NH2

OH O

NH2

O

O O

NH2

O

NH3

OH O

NH2

O

NH

O O

12 111

+

Mother Liquor

1415

+ -

Aqueous HCl

Aqueous HCl

Piperidine

Toluene

Resolution Aqueous HCl

Hoffman

55

EXPERIMENTAL

SECTION

Experimental

Chiral purity by HPLC conditions : The chiral HPLC was recorded on chiral AD-H

(250 X 4.6 mm) at 210 nm and eluted with n-hexane : ethanol : TFA (950:50:1)

Preparation of 3-isobutylgllutaric acid (9).

Ethyl cyanoacetate (62.4 g, 0.551 mol), n-hexane (70 mL), isovaleraldehyde (52.1

g, 0.60 mol) and di-n-propylamine (0.55 g, cat) were heated under reflux temperature and

generated water was removed azeotropically. The reaction mass was distilled out

completely, diethyl malonate (105.7 g, 0.66 mol) and di-n-propylamine (0.55 g, cat) was

charged to the reaction mass and maintained for 1 h to afford cyano ester derivative

which was kept hydrolysis and decarboxylation using dil HCl solution at reflux

temperature followed by extraction with toluene and concentration under vacuum to

afford 3-isobutylgllutaric acid (9) as an oil (88.7 g, 85 %). 1H NMR (DMSO d6) : δ 0.82

(d, 6H, J = 6.5 Hz), 1.13 (dd, 2H, J = 6.6, 6.5 Hz), 1.58 (m, 1H), 2.18-2.19 (m, 5H), 13

C

NMR (CDCl3) : δ 22.9, 24.91, 29.7, 38.7, 43.3, 174.1.10

Elemental analysis : Anal for

C9H16O4 : was calcd after recrystallization C, 57.44; H, 8.51. Found C, 57.24; H, 8.50.

Preparation of 3-isobutylgllutaric anhydride (10).

OO O

3-isobutylglutaric acid (156 g, 0.829 mol) in acetic anhydride (100 g, 0.975 mol)

was heated under reflux condition for 16 h. The reaction mass was distilled out under

vacuum below 70 °C to afford 3-isobutylgllutaric anhydride (10) (129 g, 91.5 %). B.P.

128 °C, 1

H NMR (DMSO d6) : δ 0.84 (d, 6H, J = 6.5 Hz), 1.12-1.16 (m, 2H), 1.65 (m,

1H), 2.21-2.79 (m, 3H), 3.30-3.58 (m, 2H), 13

C NMR (DMSO d6) : δ 22.55, 24.46, 25.97,

35.59,

56

HOOC COOH

43.34, 168.02.10

Elemental analysis : Anal for C9H14O3 : was calcd after recrystallization

C, 63.51; H, 8.29. Found C, 63.23; H, 8.11.

Preparation of 3-(carbamoylmethyl)-5-methylhexanoic acid (11).

OH O

NH2

O

3-isobutylglutaric anhydride (391 g, 2.30 mol), liquor ammonia (308 g), water

(431 g) and MTBE (200 g) were heated to 55 °C for 1 h. It was cooled to 20-25 °C and

the layers were separated after which the pH of the aqueous layer was adjusted to 1.0

using dil HCl and the solids separated were filtered and dried under vacuum to afford 11

as a white solid (408 g, 94.8 %). 1H NMR (DMSO-d6) : δ 0.84 (d, 6H, J = 6.5 Hz), 1.07-

1.17 (m, 2H), 1.50-1.72 (m, 1H), 1.98-2.25 (m, 5H), 6.75 (s, 1H), 7.30 (s, 1H), 11.6 (s,

1H), IR (KBr) : 655, 1214, 1278, 1461, 1585, 1668, 1700, 2514, 2622, 2962, 3367 cm-1

,

Anal calculated for C9H17NO3 : C, 57.73; H, 9.15; N, 7.48. Found C, 57.87; H, 9.28; N,

7.49.10

Preparation of R-(+)-α-methylbenzylamine salt of R-3-(carbamoylmethyl)-5-

methylhexanoic acid (12).

O O

NH2

O

NH3

+ -

3-(carbamoylmethyl)-5-methylhexanoic acid (11) (170.0 g, 0.909 mol) in

chloroform (1.65 L) and ethanol (40 mL) was heated at 55 °C. R-(+)-α-

methylbenzylamine (60 g, 0.495 mol) was added to the reaction mass and stirred for 1 h.

The precipitated solids were filtered and washed with chloroform (300 mL) and dried

under vacuum to get R-(+)-α-methylbenzylamine salt of (R)-(-)-3-(carbamoylmethyl)-5-

methylhexanoic acid as a white solid (12) (105 g, 68 %).M.P. 123-126 °C. 10

, IR (KBr) :

700, 1399, 1525, 1658, 2217, 2955, 3189, 3377, 3500 cm-1

, 1H NMR (DMSO-d6) : δ 0.84

(d, 6H, J = 2.59 Hz), 1.09-1.12 (m, 2H), 1.28 (d, 3H, J = 6.7 Hz), 1.60 (m, 1H), 2.02-2.1

57

(m, 5H), 4.03(q, 1H, J = 6.7 Hz), 5.02 (brs, 3H), 6.69 (s, 1 H), 7.18-7.33 (m, 6 H), Chiral

purity : R-enantiomer 100 % : S-enantiomer not detected. Anal calculated for C16H28N2O3

: C, 66.20; H, 9.15; N, 9.08. Found C, 66.00; H, 9.50; N, 9.22.

Preparation of R-3-(carbamoylmethyl)-5-methylhexanoic acid (13).

OH O

NH2

O

R-(+)-α-methylbenzylamine salt of (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic

acid (12) (100 g, 0.325 mol) was added in water (300 mL) and pH of the reaction mass

was adjusted to 1-2 using HCl at 0-5 °C. The solids were filtered, washed with water

(100 mL) and dried at 50-55 °C to give R-3-(carbamoylmethyl)-5-methylhexanoic acid

(13) as a white solid (55 g, 90 %). 1H NMR (DMSO-d6) : δ 0.84 (d, 6H, J = 6.5 Hz),

1.07-1.17 (m, 2H), 1.50-1.72 (m, 1H), 1.98-2.25 (m, 5H), 6.75 (s, 1H), 7.30 (s, 1H), 11.6

(s, 1H), Chiral purity : R-enantiomer 100 % : S-enantiomer not detected. Anal calculated

for C9H17NO3 : C, 57.73; H, 9.15; N, 7.48. Found C, 57.85; H, 9.07; N, 7.50.

Preparation of (S)-3-aminomethyl-5-methylhexanoic acid (1).

R-3-(carbamoylmethyl)-5-methylhexanoic acid (13) (30 g, 0.160 mol) was

dissolved in water (28 mL), 50 % sodium hydroxide solution (12.6 g, 0.315 mol) and

cooled to 0-5 °C. The sodium hypobromite solution (prepared by using water (85 mL),

sodium hydroxide (53 g, 1.32 and bromine (30.6 g, 0.17 mol) was added to the reaction

mass and stirred for 3 h. After absence of (13), the reaction mass was warmed to 50 °C

and quenched with HCl (42 mL). The solids obtained were filtered, washed with water

(30 mL) and dried under vacuum at 50-55 °C to afford Pregabalin (1). [Analysis of

Pregabalin (1) was similar to the analysis reported in section A].

58

NH2

COOH

Isolation of S-3-(carbamoylmethyl)-5-methylhexanoic acid (14) from mother liquor

of (12).

OH O

NH2

O

Mother liquor of R-(+)-α-methylbenzylamine salt of R-3-(carbamoylmethyl)-5-

methylhexanoic acid (12) obtained after resolution step was concentrated completely

under vacuum at 50-55 °C and acidified to pH 1-2 using dil HCl. The mixture was stirred

for 30 min at 25 to 30 °C. The precipitated solids were filtered and washed with cold

water and dried at 50-55 °C to afford (S) 3-(carbamoylmethyl)-5-methyl hexanoic acid

(14) as a white solid (105 g, 98 %). IR (KBr) : 699, 1053, 1133, 1215, 1242, 1278, 1340,

1428, 1461, 1586, 1669, 1701 cm-1

, 1H NMR (DMSO-d6) : δ 0.82-0.90 (d, 6H, J = 1.9

Hz) 1.09 (m, 2H), 1.55-1.65 (m, 1H), 1.97-2.25 (m, 5H), 6.72 (s, 1H), 7.26 (s, 1H), 11.97

(s, 1 H), Chiral purity : R- enantiomer 16.13 % : S- enantiomer 83.86 %. Anal calculated

for C9H17NO3 : C, 57.73; H, 9.15; N, 7.48. Found C, 57.75; H, 9.40; N, 7.50.

Racemization of S-3-(carbamoylmethyl)-5-methylhexanoic acid (14) to (11) via the

formation of 3-isobutyl glutarimide (15).

OH O

NH2

O

A mixture of piperidine (0.68 g), (S)-3-(carbamoylmethyl)-5-methylhexanoic acid

(14) (40.0 g, 0.214 mol) (containing ~15 % of R-enantiomer) and toluene (200 mL) was

heated under reflux for 10 h. The reaction mass was cooled to 60 °C and diluted with 10

% sodium hydroxide solution (200 mL). After 1 h at 60 °C, the reaction mixture was

cooled to ambient temperature and layers were separated. The aqueous layer was cooled

to 0-5 °C and acidified with dil HCl (64 mL) to pH 1 to 2. The solids precipitated were

filtered and washed with cold water and dried at 50-55 °C to give crude 3-

(carbamoylmethyl)-5-methyl hexanoic acid (11). Recrystallization from ethyl acetate

gave the racemized compound 11 as a white solid (31 g, 78 %). M.P. 108-110 °C 10

,

59

Chiral purity : R-enantiomer 50.86 % : S-enantiomer 49.14 %, HPLC purity : 99.71 %.

IR, 1H NMR, elemental analysis of compound 11 isolated after racemization was similar

as mentioned above in experimental section.

60

SPECTRA

1H NMR spectrum of Compound 9 in DMSO-d6

13C NMR spectrum of Compound 9 in DMSO-d6

61


13C NMR spectrum of Compound 10 in DMSO-d6

62



63


Chiral purity by HPLC chromatogram of compound 11

64

Purity by HPLC chromatogram of compound 11


65

5



66



67



68



69



70



71

1H NMR spectrum of Compound 15 in CDCl3

1H NMR spectrum of Compound 15 in CDCl3

72



73

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