Ind. J. Chem. 51B, 1470-1488 (2012)-Ecofriendly Process for Pregabalin

Indian Journal of Chemistry Vol. 51B, October 2012, pp 1470-1488

Eco-friendly, industrial process for synthesis of (S)-3-(aminomethyl)-5-methylhexanoic acid [pregabalin]

Bhairab Nath Roy*, Girij Pal Singh, Piyush Suresh Lathi, Manoj Kunjabihari Agrawal, Rangan Mitra &Vijay Sadashiv Pise

Lupin Research Park, Survey No 46/A & 47/A, Nande Village, Mulshi Taluka, Pune 411 042, India

E-mail: [email protected]

Received 14 May 2012; accepted (revised) 21 August 2012

In the present work pregabalin has been synthesized by four novel routes, which have been broadly categorized into two approaches. In the first strategy, it has been synthesized through a hemiaminal intermediate. In the second approach, (RS)-ethyl-3-cyano-5-methylhexanoate, a key intermediate, has been synthesized through novel and cost effective routes, followed by either lipase catalyzed kinetic resolution to optically pure (S)-ethyl-3-cyano-5-methylhexanoate (>99% ee, 85% yield) or resolution via diastereomeric salt formation with cinchonidine to obtain optically pure (S)-3-cyano-5-methyl-hexanoic acid (>99% ee, 85% yield), which has been subsequently converted to pregabalin (>99% ee). In addition, for improvement of atom economy as well as cost effectiveness, an efficient process for complete racemization of (R)-3-cyano-5-methyl-hexanoic acid has been developed.

Keywords: Pregabalin, cyano mono ester, resolution, lipase, cinchonidine

Pregabalin, (S)-3-(aminomethyl)-5-methylhexanoic acid, also known as -isobutyl--aminobutyric acid, or isobutyl-GABA, is a potent anticonvulsant1, currently sold under trade name of Lyrica. The drug, pregabalin has also been found to be useful for treatment of various other conditions, like pain, fibromyalgia, physiological conditions associated with psychomotor stimulants, inflammation, gastro-intestinal damage, insomnia, alcoholism and various psychiatric disorders, including mania and bipolar disorder2.

Syntheses of pregabalin via asymmetric as well as achiral pathways are well reported. Generally, synthesis of (S)-3-(aminomethyl)-5-methylhexanoic acid is broadly categorized into three major approaches: (i) resolution of (RS)-3-(aminomethyl)-5-methylhexanoic acid via diastereomeric salt formation3 (ii) synthesis via (S)-ethyl 3-cyano-5-methylhexanoate3-7 and (iii) chiral or non-chiral desymmetrization of 3-isobutyl glutaric anhydride8-10.

Some of the other methods for synthesis of (S)-3-(aminomethyl)-5-methylhexanoic acid include reductive amination of mucohalic acid and its derivatives11, stereo-selective synthesis using chiral auxiliaries such as (+)-4-methyl-5-phenyl-2-oxazo-lidinone12 and also through 2,2-dichloro-3-isobutyl-cyclobutanone13. Most of the above reported methods

are not ecologically friendly and non-benign chemicals such as alkali cyanides, nitro compounds or azides have been employed.

Improvement of environmental profile of manufacturing processes for synthesis of active pharmaceutical ingredients (APIs) is one of the important goals in pharmaceutical industry14. This includes use of eco-friendly benign chemicals, enhancing efficiency and yield in order to minimize waste, reduction of solvent usage and reactants, and / or to develop operational friendly processes. A number of statistical co-relations with respect to greenness of chemistry and process efficiency have been established and are currently practiced15,16.

The present work focuses on two major approaches for the synthesis of pregabalin. In the first approach, desired product i.e. pregabalin was obtained through a hemiaminal intermediate whereas, the second approach involved the development of novel process for the synthesis of (S)-3-cyano-5-methyl hexanoic acid, which was subsequently converted to pregabalin. In addition to this, an attempt has been made to assess the greenness of the developed routes vis--vis desirability for scale-up for industrial manufacture through the use of prevalent yard-sticks for measurement of greenness and efficiency.

ROY et al.: ECO-FRIENDLY, INDUSTRIAL PROCESS FOR PREGABALIN

1471

Results and Discussion In Scheme I, four reaction sequences for the

synthesis of pregabalin have been shown.

First Approach: Synthesis of pregabalin from 5-hydroxy-4-iso-butyl-5H-furan-2-one [Route 1]

5-Hydroxy-4-iso-butyl-5H-furan-2-one 117 on reaction with ammonia gave the hemiaminal

intermediate 2, which on subsequent hydrogenation gave racemic pregabalin. It can be further resolved to obtain pregabalin as per reported method3. Moreover, asymmetric synthesis of pregabalin was attempted by replacing ammonia with chiral amines such as (S)-alpha methyl benzylamine and (S)-phenylglycinol, which resulted in 60% enantiomerically enriched pregabalin, as depicted in Scheme II, which could be

Scheme I

INDIAN J. CHEM., SEC B, OCTOBER 2012

1472

resolved as per reported method3 to obtain optically pure pregabalin.

Compound 4 was obtained from compound 3 probably because compound 3 existed in equilibrium with compound 3a i.e. the dienamine intermediate, which underwent hydrogenation and hydrogenolysis.

Second Approach: Synthesis of pregabalin from (S)-3-cyano-5-methyl-hexanoic acid

The first approach for synthesis of pregabalin yielded racemic/enantionmerically enriched product which needed further resolution to obtain pregabalin. Currently, in the literature there are no reports for racemization and recycling of undesired isomer of pregabalin. Hence, the strategy was re-directed towards the synthesis of the key intermediate of pregabalin i.e. (S)-3-cyano-5-methyl hexanoic acid which consisted of the following strategies:

(i) a) synthesis of (RS)-ethyl 3-cyano-5-methyl-hexanoate and b) resolution of (RS)-3-cyano-5-

methyl-hexanoic acid / ethyl ester to eventually obtain (S)-3-cyano-5-methyl-hexanoic acid.

(ii) Asymmetric synthesis of (S)-3-cyano-5-methyl-hexanoic acid/ ester, and

(iii) Racemization of undesired isomer i.e. (R)-3-cyano-5-methyl-hexanoic acid/ ester.

Synthesis of (RS)-ethyl 3-cyano-5-methylhexanoate from succinic anhydride [Route 2]

Succinic acid diesters 6a-d were converted to corresponding products 7a-d via Stobbe condensation with iso-butyraldehyde. Hydrogenation of compounds 7a-c gave the compounds 8a-c which were attempted upon to convert to the corresponding amide 8d. However, amidification of the half esters i.e. compounds 8a-c through ammonolysis failed even under drastic conditions such as 10 kg/cm2 ammonia pressure at 50C (Scheme III).

Hence, compound 7d (R=CH2Ph) was converted to compound 9 by esterification with ethanol in presence

Scheme II Reagents and conditions: (i) NH3, MeOH, 25C, 1.5 h; (ii) NH2CH(R)Ph, MeOH, 25C, 1 h; (iii) a) H2 (5 atm), 10 mol % Pd/C (50 % wet and 10 % Pd loading), MeOH, RT, 8 h; (iv) H2 (25 atm), 10 mol % Pd/C (50 % wet and 10 % Pd loading), MeOH, RT,

8 h; (v) resolution as per reported method3


1473

of p-toluenesulfonic acid (pTSA), which on subsequent hydrogenation and hydrogenolysis gave the compound 10. Compound 10 was converted to compound 11 via the corresponding acid chloride and subsequently reacting with ammonia. Dehydration of compound 11 in presence of thionyl chloride gave (RS)-ethyl 3-cyano-5-methylhexanoate 14 (Scheme IV).

Another objective was to convert ester functionality of the compound 11 to acid through

alkaline hydrolysis and subsequent selective reduction18 of amide group to obtain 3-(amino-methyl)-5-methylhexanoic acid. Surprisingly, it was observed that during hydrolysis of compound 11 in presence of bases such as potassium hydroxide, sodium hydroxide or lithium hydroxide, only compound 12 was obtained through migration of amine functionality presumably via a cyclic imide intermediate. Structure of compound 12 was confirmed by reduction of compound 12 with lithium borohydride to yield compound 1319 (Scheme V).

Scheme III Reagents and conditions: (i) ROH, pTSA, toluene, reflux; 5 h; (ii) iso-butyraldehyde, potassium tert-butoxide, tert-butanol, RT, 24 h; (iii) H2 (5 atm), 5 mol % Pd/C (50% wet and 10% Pd loading, ethanol, RT, 5 h

Scheme IV Reagents and conditions: (i) Ethanol, pTSA, reflux; 12 h; (ii) H2 (10 atm), 5 mol % Pd/C (50% wet and 10% Pd loading), Methanol, RT, 10 h; (iii) a) Thionyl chloride, 60C, 10 h; b) DCM, NH3, 1 h; (iv) Thionyl chloride, reflux; 12 h

Scheme V Reagents and conditions: (i) Aq. KOH, RT, 10 h; (ii) LiBH4, THF, reflux; 4 h


1474

Synthesis of enantiomerically enriched (S)-ethyl-3-cyano-5-methylhexanoate from L-leucine [Route 3]

(S)-2-Bromo-4-methyl-pentanenitrile 18 was prepared from L-leucine 15 as per reported method20, which on SN2 displacement with diethyl malonate gave enantiomerically enriched 2-[(S)-1-cyano-3-methyl-butyl]-malonic acid diethyl ester 19. Krapcho decarboxylation21 of enantiomerically enriched 2-[(S)-1-cyano-3-methyl-butyl]-malonic acid diethyl ester 19 gave enantiomerically enriched (S)-ethyl 3-cyano-5-methylhexanoate (60% ee) 20 (Scheme VI).

Synthesis of (RS)-ethyl 3-cyano-5-methylhexanoate from cyano acetic acid alkyl ester [Route 4]

2-Cyano-4-methyl-pentanoic acid alkyl ester 21 was obtained via Knoevenagel condensation of 2-methyl-propionaldehyde with cyanoacetic acid alkyl ester 20, followed by hydrogenation using palladium-on-charcoal22 (Scheme VII).

2-Cyano-4-methyl-pentanoic acid ethyl ester on reaction with ethyl chloroacetate in presence of alkali metal carbonates such as potassium carbonate or cesium carbonate gave diethyl 2-cyano-2-isobutyl

Scheme VI Reagents and conditions: (i) NaNO2, H2SO4, KBr, water, 14C, 4 h (ii) a) SOCl2, 60C, 10 h; b) DCM, NH3,1 h; (iii) P2O5, 80C, 12 mm Hg; iv) diethyl malonate, NaH, DMF, RT, 24 h; (v) H2O (1 equiv.), KCl, DMSO, 150C, 6 h.

Scheme VII Reagents and conditions: (i) iso-butyraldehyde, piperidine/acetic acid, H2 (5 atm) 2 mol % Pd/C (50% wet and 10% Pd loading), methanol, RT, 5 h; (ii) Cs2CO3, ethyl chloro acetate, 65C, 2 h; or K2CO3, ethyl chloro acetate, 90C, 3 h (iii) CsCl, DMSO, 170C, 4 h; (iv) Cs2CO3, thiophenol, DMF, 130C, 4 h.


1475

succinate 22. It was observed that the rate of reaction with cesium carbonate was two times faster than with potassium carbonate; moreover colour of the product obtained with cesium carbonate was light yellow as compared to light brown obtained with potassium carbonate.

Compound 22 was decarboxylated in presence of cesium chloride or potassium chloride to yield (RS)-ethyl 3-cyano-5-methylhexanoate 14. Compound 22 was also decarboxylated at much lower temperature in presence of cesium carbonate/thiophenol23 (Scheme VII).

Enzymatic resolution of (RS)-ethyl-3-cyano-5-methylhexanoate 14 to obtain (S)-ethyl-3-cyano-5-methylhexanoate 23

A number of lipase enzymes were screened and Table I lists the result and stereo-selectivity for the

screened enzymes. Only Candida Antarctica Lipase B showed desired stereo-selectivity towards the hydrolysis of (RS)-ethyl 3-cyano-5-methylhexanoate to (S)-ethyl 3-cyano-5-methylhexanoate in high enantiomeric purity (99% ee) (Scheme VIII).

Ratio of rate of hydrolysis of (R)-ethyl-3-cyano-5-methylhexanoate to that of (S)-ethyl-3-cyano-5-methylhexanoate was found to be 2.85 and 10 at 25C and 10C respectively (Figure 1).

Resolution of (RS)-3-cyano-5-methyl-hexanoic acid via diastereomeric salt formation

In order to resolve (RS)-3-cyano-5-methyl-hexanoic acid, diastereomeric salts of optically pure (S)-3-cyano-5-methyl-hexanoic acid and (R)-3-cyano-5-methyl-hexanoic acid were prepared with chiral amines such as (S)--methyl benzylamine, D-phenyl glycinol and cinchonidine. (S)--Methyl benzylamine, and D-phenyl glycinol did not form crystalline solid either with (S) or (R)-3-cyano-5-methyl hexanoic acid. Cinchonidine produced nice crystalline high melting salt with both (S) and (R)-3-cyano-5-methyl-hexanoic acid.

The temperature dependence solubility for cinchonidine salts of (R)-3-cyano-5-methyl-hexanoic acid and cinchonidine salt of (S)-3-cyano-5-methyl hexanoic acids was found to be 0.12 g/cm3,C and 0.056g/cm3,C respectively.

It was observed that difference in the solubility of cinchonidine salt of (R)-3-cyano-5-methyl-hexanoic acid over cinchonidine salt of (S)-3-cyano-5-methyl-hexanoic acid was more than 6 times at 55C. Hence, resolution of (RS)-3-cyano-5-methyl hexanoic acid

Table I Screening of lipases

Enzyme Trade Name Supplier % ee

Candida Antarctica Lipase B Novozym 435 Novozyme A/S 99% Thermomyces langinous Lipozyme TL IM Novozyme A/S @ Rhizomucor miehei Lipozyme RM- IM Novozyme A/S @ Candida antarctica Lipase B CALB C-LETA 96% Candida Antarctica Lipase B CALB-lyophilized C-LETA 99% Aspergillus niger Amano AS Amano enzyme Japan @ Pseudomonas fluorescens Amano AK Amano enzyme Japan @ Burkholderia cepacia Amano PS IM Amano enzyme Japan @ Burkholderia cepacia Amano PS SD Amano enzyme Japan @ Candida rugosa Amano AYS Amano enzyme Japan @ Candida Antarctica Lipase A CLEA CLEA Techologies 7% Rhizomucor miehei Rhizomucor miehei Sigma @ Thermomyces langinous @: No specificity observed

Thermomyces langinous Sigma @

(S)N

O

O

(R)N

OH

O

N

O

O

+

14

23 24

(i)

Scheme VIII Reagents and conditions: (i) Phosphate buffer (40 mmol, pH 7.2), Lipase, 10C, 4 h.


1476

with cinchonidine carried out at about 55C gave 97% optically pure (S)-3-cyano-5-methyl-hexanoic acid, which was further re-crystallized to obtain 99% optically pure (S)-3-cyano-5-methyl-hexanoic acid.

Racemization of undesired isomer i.e. (R)-3-cyano-5-methyl-hexanoic acid to (RS)-3-cyano-5-methyl-hexanoic acid (Scheme IX)

It is needless to mention, that the process economics could be very significantly improved if the undesired stereoisomer was recycled i.e. (R)-3-cyano-5-methyl-hexanoic acid/ester was racemized. However, base (sodium ethoxide) catalyzed racemiza-tion in absolute alcohol did not give satisfactory results.

Racemization of nitriles such as (+)-2-methyl-3-phenyl propionitrile, (-)-2-phenylbutyronitrile and (-)-2,2-diphenylcyclopropylnitrile24,25 has been reported in dry dimethyl sulfoxide doped with 2% ethanol with 1.25 eq of sodium ethoxide at 70C.

In compound 28, pKa of the proton adjacent to carboxylate functionality was apparently lower as compared to pKa of the proton adjacent to nitrile group, which was evident from D2O exchange reaction26 and formation of acylated product resulting from generation of anion alpha to the carboxylate group27.

Surprisingly, smooth racemization of (R)-ethyl-3-cyano-5-methylhexanoate to (RS)-3-cyano-5-methyl-hexanoic acid in practically quantitative yield was obtained by employing similar conditions i.e. dimethyl sulfoxide doped with 2% ethanol with 1.25 eq of sodium ethoxide at 70C. Interestingly, although reaction medium was completely anhydrous during the course of racemization, the sole product obtained was (RS)-3-cyano-5-methylhexanoic acid and not its corresponding ester.

Moreover, rate of racemization in case of tert-butyl ester of (R)-3-cyano-5-methylhexanoic acid was considerably slower than that for the corresponding

020406080

100120140160

0 50 100 150 200Time, min

% co

nv

ersi

on

o

f es

ter

to ac

id

(S)-3-Cyano-5-methylhexanoic acid at 10C; (R)-3-Cyano-5-methylhexanoic acid at 10C; (S)-3-cyano-5-methylhexanoic acid at 25C; (R)-3-cyano-5-methylhexanoic acid at 25C

Figure 1 Rate of formation of (R)-ethyl 3-cyano-5-methylhexanoate and (S)-ethyl 3-cyano-5-methylhexanoate vs. time

O

O

N

O

OH

N

(i) 2 % Ethanol

NaOEt (1.25 eq)

Dry solvent

70 0C

(ii) Acetic acid, RT

(iii) Water, RT

Yield=95%

2526

Scheme IX


1477

ethyl ester. This could be rationalized by involving formation of a three member cyclopropanone intermediate or its equivalent28.

Also, it is to be noted that the solvent system reported in the literature i.e. dimethyl sulfoxide/ 2% ethanol could be replaced with other solvents doped with 2% ethanol such as 2-methyl tetrahydrofuran, methyl tert-butyl ether, dimethoxy ethane, dimethyl formamide, and N-methyl-2-pyrrolidone, having water content in the range of 0.05 to 0.65% to give efficient racemization of the above mentioned substrate.

Process metrics for harmonizing green chemistry and industrial scalability29-31

Having developed a number of alternative novel processes for the synthesis of (S)-3-(aminomethyl)-5-methylhexanoic acid, it would be worthwhile to

assess environmental aspect, greenness, efficiency and assign relative merits to the developed routes and based on these, recommend them for further scale up.

A number of criteria or metrics as given below were used for quick assessment of greenness of reaction protocols29-31. Table II summarizes the outcome of the following metrics which are generally followed for the assessment of a process and Figure 2 gives the graphical representation of metrics calculations for the four routes developed.

1. E-factor:

Product of Kg(Kg) WasteTotalfactor-E =

In pharmaceutical industry E-factor is normally in the range to 25-250; lower the value for E-factor more benign the process.

Table II Metrics values for the four routes

Route Metrics

Route 1 Route 2 Route 3 Route 4

E-Factor 31.06 163.02 154.2 22.23 E-Factor (assuming 80% solvent recovery) 9.16 29.07 55.60 4.72 Atom economy 49.93 16.23 15.09 51.70 Atom efficiency 39.94 12.21 11.32 36.20 Reaction Mass efficiency 22.53 3.98 2.40 29.22 Carbon efficiency (without considering recycling of undesired isomer) 35.33 6.31 14.37 42.37

0

20

40

60

80

100

120

140

160

180

E-Factor E-Factor(SolventRecovery)

Atom Economy

Atom Efficiency

ReactionMass

Efficiency

Carbon Efficiency

Route 4 Route 1 Route 3 Route 2

Figure 2 Graphical representation of metrics calculation for routes 1, 2, 3 and 4


1478

2. Atom economy is defined as amount of the reactants incorporated in the final product.

3. Atom efficiency = Atom economy Over all yield (%)

4. Reaction mass efficiency

reactants theof masses theof Sum100product theof Mass

efficiency massReaction

=

5. Carbon efficiency

reactantsin present carbon Total100 product in carbon ofAmount

efficiencyCarbon

=

Higher the value for atom economy, atom efficiency, reaction mass efficiency and carbon efficiency, more desirable and efficient the process.

In routes 1 and 4, E-factors are the least whereas their atom economy, atom efficiency, reaction mass efficiency and carbon efficiency are also better than those for routes 2 and 3. According to these factors, routes 1 and 4 should be the routes of choice from greener aspects for the synthesis of (S)-3-(aminomethyl)-5-methylhexanoic acid. From the practical point of view also, routes 1 and 4 are easier to carry out and easier to scale up as both involve less number of steps and lower amount of reagents per kg of product and also do not require any special treatment for the waste generated.

Experimental Section The enantiomeric excess (ee) for pregabalin was

determined by HPLC using a Shimadzu LC 2010 system equipped with a chiral column [Purosphere star RP-18e (4.6 150 mm), 5 m], column oven temperature 25C and UV-Vis detector (UV at 340 nm). Mobile phase was phosphate buffer (10 mmol) : acetonitrile (55:45) with flow rate 1.0 mL/min, injection volume 20 L. The enantiomeric excess (ee) was determined by derivatizing with Marfeys reagent (retention time for R-isomer is 8.4 min and for S-isomer is 11.4 min). NMR spectra were obtained using 200 MHz Bruker instruments, with CDCl3, MeOD and DMSO-d6 as solvents. Chemical shifts () are given in ppm relative to tetramethyl-silane ( = 0 ppm). IR spectra were recorded on Perkin-Elmer Spectrum (Model: Spectrum 100)

instrument and absorption bands are given in cm-1. The enantiomeric excess (ee) for (S)-ethyl-3-cyano-5-methyl-hexanoate was determined by Gas-Liquid chromatography using a Shimadzu GC 2010 system equipped with a chiral column [Chiraldex (20 m 0.25 mm 0.12 mm)], and FID detector, retention time for (R)-ethyl-3-cyano-5-methyl-hexanoate is 21 min and for (S)-ethyl-3-cyano-5-methyl-hexanoate is 24 min. Mass analyses were performed on Shimadzu LCMS 2010A instrument. Powder X-ray diffraction was recorded on PANalytical B. V. Netherlands model PN3040/60X Part Pro. DSC was recorded on Perkin-Elmer model Diamond DSC at the rate of 10C/min, and endothermic peak was recorded in C. All chemicals mentioned were obtained from commercial sources and were used without any further purification.

5-Hydroxy-4-iso-butyl -5H-furan-2-one17 1 n-Heptane (75.0 mL) and morpholine (17.8 g, 0.20

mol) were introduced in a reactor while stirring at ambient temperature. The resultant mixture was cooled to 0C and glyoxylic acid (29.6 g, 0.40 mol, 50 wt % in water) was added slowly over 20 min at 0C. The mixture was stirred at 20C for 1 h and 4-methylvaleraldehyde (20.0 g, 0.20 mol) was added. The reaction mixture was heated to 45C and stirred for additional 20 h, after which it was cooled to 20C and 37% aqueous solution of hydrochloric acid (30 mL) was carefully added and stirring continued for 2 h. n-Heptane phase was removed and aqueous phase was washed with n-heptane (2 50 mL). The aqueous phase was extracted with di-iso-propyl ether (3 50 mL). The di-iso-propyl ether layers were combined, washed with brine, dried and concentrated under reduced pressure to obtain 5-hydroxy-4-iso-butyl-5H-furan-2-one 1 (13.0 g, 60%) as light yellow oil. IR (neat): 3371 and 1738 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.87-0.99 (t, 6H), 1.87-2.01 (m, 1H), 2.28-2.32 (d, 2H), 5.82 (s, 1H) and 6.11 (s, 1H); EI-MS: m/z 155.09 (M+-H. C8H12O3 requires 156.06).

5-Hydroxy-4-iso-butyl-1,5-dihydro-pyrrol-2-one, 2 Ammonia gas was purged through a solution of 5-

hydroxy-4-iso-butyl-5H-furan-2-one (15.0 g, 0.096 mol) in methanol (50 mL) for 30 min at 25C. The reaction mixture was stirred for 1 h at 25C and solvent was evaporated under reduced pressure to afford 5-hydroxy-4-iso-butyl-1,5-dihydro-pyrrol-2-one 2 (14.3 g, 95%) as a dark yellow oil. IR (neat):


1479

3243, 2957, 1749, 1574, 1030 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.93-1.03 (m, 6H), 1.88-2.00 (m, 1H), 2.18-2.26 (t, 2H), 5.57-5.64 (d, 1H), 5.98 (s, 1H); EI-MS: m/z 155.85 (M+ + H. C8H13NO2 requires 155.09).

5-Hydroxy-1-[(S)-phenyl-ethyl]-4-iso-butyl-1,5-di-hydro-pyrrol-2-one, 3a

To a solution of 5-hydroxy-4-iso-butyl-5H-furan-2-one (10.0 g, 64.1 mmol) in iso-propanol (100 mL) was added (S)--methyl benzyl amine (7.8 g, 64.1 mmol) at 25C. The mixture was stirred at 25C for 1 h after which solvent was evaporated from the reaction mass under reduced pressure to afford 5-hydroxy-1-[(S)-phenyl-ethyl]-4-iso-butyl-1,5-dihydro-pyrrol-2-one 3a (15.5g, 93%) as a dark yellow oil. IR (neat): 3319, 2959, 1751, 1166 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.86-0.94 (t, 3H), 0.96-0.99 (t, 3H), 1.34-1.38 (d, 2H), 1.49-1.53 (d, 1H), 1.75-1.85 (m, 1H), 2.24-2.27 (d, 2H), 4.27-4.30 (q, 1H), 5.17 (s, 1H), 5.88 (s, 1H), 7.26-7.37 (m, 5H); EI-MS: m/z 260.30 (M+ + H. C16H21NO2 requires 259.0).

5-Hydroxy-1-(2-hydroxy-1-(S)-phenyl-ethyl)-4-iso-butyl-1,5-dihydro-pyrrol-2-one, 3b

To a solution of 5-hydroxy-4-iso-butyl-5H-furan-2-one (10.0 g, 64.1 mmol) in methanol (100 mL) was added (S)-(+) phenylglycinol (8.83 g, 64.1 mmol) at 25C. The mixture was stirred at 25C for 1 h after which solvent was evaporated from the reaction mass under reduced pressure to afford 5-hydroxy-1-(2-hydroxy-1-phenyl-ethyl)-4-iso-butyl-1,5-dihydro-pyr-rol-2-one 3b (14.0 g, 80%)as a dark yellow oil. IR(neat): 3337, 2933, 1740, 1167, 757 cm-1; EI-MS: m/z 274.30 (M+ - H. C16H21NO3 requires 275.15).

3-(Aminomethyl)-5-methylhexanoic acid, 4 from 5-hydroxy-4-iso-butyl-1,5-dihydro-pyrrol-2-one, 2

In a Parr autoclave, a solution of 5-hydroxy-4-iso-butyl-1,5-dihydro-pyrrol-2-one (10.0 g, 0.064 mmol) in methanol (50 mL) and 50% wet palladium-on-carbon (1.0 g) was charged. Reactor was purged with hydrogen gas twice and 5 atm. Hydrogen pressure was maintained for 8 h. The reaction mixture was filtered through a Celite pad and solvent from filtrate was evaporated under reduced pressure to leave a semi-solid material, which was re-crystallized from iso-propyl alcohol : water mixture (94:06, 25 mL) to obtain 3-(aminomethyl)-5-methylhexanoic acid 4 (6.5 g, 64%), as a white solid. IR(neat): 3367, 2956, 1661,

1544, 1409, 1389 cm-1; 1H NMR (200 MHz, D2O, Me4Si): 0.87-0.88 (m, 6H), 1.18-1.21 (t, 2H), 1.60-1.65 (q, 1H), 2.12-2.32 (m, 3H), 2.90-3.01 (m, 2H); EI-MS: m/z 160.30 (M+ + H. C8H17NO2 requires 159.23); m.p. 182-83C

Synthesis of enantiomerically enriched (S)-3-(aminomethyl)-5-methylhexanoic acid, 4 from compound 3a and 3b

A Parr autoclave was charged with a solution of compound 3 (0.06 mol) in methanol (50 mL) at 25C and 50% wet palladium-on-carbon (1.0 g) was carefully added. Reactor was purged with hydrogen gas twice and 20 atm. Hydrogen pressure was maintained for 8 h. The reaction mixture was filtered through a Celite pad and solvent from filtrate was evaporated under reduced pressure to leave a semi-solid material, which was re-crystallized from iso-propyl alcohol:water mixture (94:06, 25 mL) to obtain (S)-3-(aminomethyl)-5-methylhexanoic acid (60% ee by chiral HPLC analysis), as a white solid. IR(neat): 3367, 2956, 1661, 1544, 1409, 1389 cm-1; 1H NMR (200 MHz, D2O, Me4Si): 0.87-0.88 (m, 6H), 1.18-1.21 (t, 2H), 1.60-1.65 (q, 1H), 2.12-2.32 (m, 3H), 2.90-3.01 (m, 2H) (matches with reference 3); EI-MS: m/z 160.30 (M+ + H. C8H17NO2 requires 159.23); m.p. 187-88C.

General method for synthesis of succinic acid diester 6a and 6b

To a solution of succinic anhydride (1.0 mol) in ethanol (5 v/w of substrate) was added p-toluene-sulfonic acid (10% w/w of succinic anhydride) and alcohol (5 v/w of substrate) at 25C. The mixture was heated and maintained at a temperature of 75-85C for 12 h after which it was cooled to RT. Solvent was evaporated under reduced pressure to leave a semi-solid residue. The residue was extracted with ethyl acetate (500 mL) and the combined organic layer was washed with 10% aqueous solution of sodium bicarbonate (250 mL), dried and solvent was evaporated under reduced pressure to obtain product.

6a: 85% yield, colorless oil; IR (neat): 2983, 2938, 1735, 1394, 1159, 1031, 857 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 2.61 (s, 4H), 3.72 (s, 6H) (matched with reference 32); EI-MS: m/z 146.95 (M+ + H. C6H10O4 requires 146.06).

6b: 85% yield, colorless oil; IR (neat): 2984, 2938, 1733, 1393, 1159, 1031, 857 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 1.14-1.22 (t, 6H), 2.54


1480

(s, 4H), 4.02-4.12 (q, 4H) (matched with Ref. 32); EI-MS: m/z 174.95 (M+ + H. C8H14O4 requires 174.09).

General method for synthesis of succinic acid diester 6c and 6d

To a solution of succinic anhydride (1.0 mol) in toluene (5v/w of the substrate) was added p-toluene-sulphonic acid (10% w/w of succinic anhydride) and alcohol (2.0 mol) at 25C. The mixture was heated to a temperature of 135C for 12 h after which the reaction mass was cooled to RT. Solvent was evaporated under reduced pressure to leave a semi-solid residue. The residue was extracted with ethyl acetate (500 mL) and combined organic layer was washed with 10% aqueous solution of sodium bicarbonate (250 mL), dried and solvent was con-centrated under reduced pressure to afford product.

6c: 90%, white crystalline solid; IR (neat): 2959, 2927, 2862, 1729, 1214, 1167, 983 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.71-1.05 (m, 24H), 1.29-1.40 (t, 4H), 1.62-1.69 (d, 4H), 1.81-1.99 (m, 4H), 2.58 (s, 4H), 4.61-4.74 (m, 2H); EI-MS: m/z 417.30 (M++ Na. C24H42O4 requires 394.30); m.p. 63-64C.

6d: 86%, off white crystalline solid; IR (neat): 3088, 3031, 1732, 1498, 1156, 1003, 733, 698 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 2.73 (s, 4H), 5.16 (s, 4H), 7.38 (s, 10H); EI-MS: m/z 299.05 (M++ H. C18H18O4 requires 298.0); m.p. 49-50C.

General method for synthesis of 7a-d A solution of the succinic acid diester(0.67 mol)

and iso-butyraldehyde (48.2 g, 0.67 mol) in tert-butanol (250 mL) was added cautiously and slowly over 60 min to a solution of potassium tert-butoxide (82.8 g, 0.74 mol) in tert-butanol (250 mL) at 50C. The reaction mixture was stirred for 2 h at 70C and cooled to 25C after which it was further stirred for another 12 h. Solvent was evaporated under reduced pressure to obtain a semi-solid residue, which was dissolved in water (5 v/w of residue). Aqueous layer was washed with ethyl acetate (2 100 mL) to remove any un-reacted succinic acid diester. Aqueous layer was acidified with hydrochloric acid (6 M, 200 mL) and extracted with ethyl acetate (3 100 mL). Combined organic layer was dried and concentrated under reduced pressure to afford product.

7a: White crystalline solid; (yield 88%); IR (neat): 2952, 1712, 1416, 1157, 983 cm-1; EI-MS: m/z 186.9 (M+ + H. C9H14O4 requires 186.0); m.p. 72-74C.

7b: Colorless oil (yield 75%); IR(neat): 2965, 2963, 1713,1652, 1447, 1155 cm-1; EI-MS: m/z 240.0 (M+ + K. C10H16O4 requires 200.10).

7c: Yellow color oil; (yield 73%); IR (neat): 2958, 1714, 1256, 1178, 986 cm-1; EI-MS: m/z 333.3 (M+ + Na. C18H30O4 requires 310.4).

7d: Yellow color oil; (yield 66%); IR (neat): 3500, 2964, 1735, 1708, 1948, 1497, 1379, 1268, 1076, 991 cm-1; EI-MS: m/z 262.9 (M+ + H. C15H18O4 requires 262.0).

General method for hydrogenation of Stobbe product, 8a-c

A solution of the Stobbe product (0.15 mol) in ethanol (100 mL) was charged into a Parr reactor followed by addition of 10 mol% palladium-on-carbon (10% Pd loading). Reactor was purged with hydrogen gas twice and 10 atm. hydrogen pressure was maintained in the Parr autoclave until hydrogen consumption ceased. The reaction mass was filtered through a Celite pad to remove Pd/C and solvent from the filtrate was removed under reduced pressure to obtain a semi-solid residue. The residue was dissolved in 1 M aqueous sodium hydroxide solution (150 mL). Aqueous layer was washed with ethyl acetate to remove any un-reacted material and was acidified with aqueous hydrochloric acid (50%, 30 mL) and extracted with di-iso-propyl ether (3 250 mL). Combined organic layer was dried and solvent was evaporated under reduced pressure to obtain the product.

8a: Colorless oil (yield 90%); IR (neat): 2965, 1734, 1711, 1469, 1177, 1015 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.87-1.01 (m, 6H), 1.23-1.27 (m, 1H), 1.49-1.53 (m, 2H), 2.39-2.44 (m, 1H), 2.63-2.70 (m, 1H), 2.81-2.83 (m, 1H), 3.62 (s, 3H); EI-MS: m/z 189.0 (M+ -H. C10H18O4 requires 188.0).

8b: Colorless oil (yield 87%); IR(neat): 2960, 1732, 1713, 1469, 1178, 1015 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.87-1.01 (m, 6H), 1.23-1.32 (m, 4H), 1.52-1.61 (m, 2H), 2.42-2.47 (m, 1H), 2.60-2.84 (m, 2H), 4.11-4.16 (m, 2H); EI-MS: m/z 203.0 (M+ -H. C10H18O4 requires 202.0).

8c: Colorless oil (yield 80%); IR (neat): 2927, 1732, 1714, 1455, 1369, 1175, 983 cm-1; EI-MS: m/z 313.4 (M+ -H. C18H32O4 requires 312.2).

3-[(Benzyloxy)carbonyl]-5-methylhex-3-enoic acid ethyl ester, 9

To a solution of 3-[(benzyloxy)carbonyl]-5-methylhex-3-enoic acid (78.0 g, 0.29 mol) in ethanol


1481

(500 mL) was added p-toluenesulfonic acid (7.8 g, 10% w/w of substrate) at 25C. The mixture was heated to 90C and stirred for 12 h after which it was cooled to RT. Solvent was evaporated under reduced pressure to obtain a semi-solid residue which was dissolved in 5% aqueous sodium carbonate solution (120 mL) and extracted with di-iso-propyl ether (3 100 mL). Combined organic layer was dried and concentrated under reduced pressure to obtain 3-[(benzyloxy) carbonyl]-5-methylhex-3-enoic acid ethyl ester 9 (61.8 g, 72%) as yellow oil. IR (neat): 3065, 3033, 2962, 1736, 1711, 1649, 1498, 1264, 1171, 1149, 1073, 993, 771, 697 cm-1; 1H NMR (200 MHz, DMSO-d6, Me4Si): 1.02 (d, 3H), 1.03 (d, 3H), 1.20 (t, 3H), 2.58 (m, 1H), 3.37 (s, 2H), 4.10 (t, 2H), 4.5.13 (s, 2H), 6.85(d, 1H), 7.33-7.34 (m, 5H); EI-MS: m/z 290.85 (M++H, C17H22O4 requires 290.0).

2-[(Ethoxycarbonyl)methyl]-4-methylpentanoic acid, 10

A Parr autoclave was charged with a solution of 3-[(benzyloxy) carbonyl]-5-methylhex-3-enoic acid ethyl ester (43.0 g, 0.15 mol) in ethanol (100 mL) followed by addition of 10 mol% palladium-on-carbon (10% Pd loading). The reactor was purged with hydrogen gas twice and charged with hydrogen gas; 10atm.hydrogen pressure was maintained in the Parr autoclave until hydrogen consumption ceased. Reaction mixture was filtered through a Celite pad to remove Pd/C and solvent from the mother liquor was evaporated under reduced pressure to leave an oily residue which was dissolved in 1 M aqueous sodium hydroxide solution (150 mL). Aqueous layer was washed with ethyl acetate to remove any un-reacted material. Aqueous layer was acidified with aqueous hydrochloric acid (50%, 30 mL) and extracted with di-iso-propyl ether (3 250 mL). Combined organic layer was dried and solvent was evaporated under reduced pressure to obtain 2-[(ethoxycarbonyl)-methyl]-4-methylpentanoic acid (23.0 g, 77%) as a light yellow oil. IR(neat): 3451, 2959, 2872, 1735, 1468, 1176, 1033 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.82 (d, 3H), 0.86 (d, 3H), 0.98-1.22 (m, 4H), 1.35-1.59(m, 2H), 2.46 (t, 2H), 2.57-2.71(m, 1H), 4.01(t, 2H); EI-MS: m/z 202.90 (M+ + H. C10H18O4 requires 202.25).

Ethyl 3-carbamoyl-5-methylhexanoate, 11 A solution of 2-[(ethoxycarbonyl)methyl]-4-methyl-

pentanoic acid (21.0 g, 0.1 mol) in cyclohexane

(50 mL) under nitrogen atmosphere was heated to 60C and thionyl chloride (18.6 g, 0.15 mol) was added carefully over 1 h at 60C. The mixture was further heated at 80C and stirred for an additional 12 h, after which it was cooled to 25C and dichloromethane (1000 mL) was added. To the reaction mixture, ammonia gas was purged for 1-1.5 h and ammonia solution (500 mL) was added. Organic layer was separated and aqueous layer was extracted with dichloromethane (500 mL). Combined organic layer was dried and solvent was evaporated under reduced pressure to obtain ethyl 3-carbamoyl-5-methylhexanoate as yellow oil (18.0 g, 86%). IR(neat): 3428, 3354, 2958, 2873, 1733, 1674, 1468, 1414, 1373, 1179, 1034, 787 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.88 (dd, 6H), 1.12-1.24 (m, 4H), 1.51-1.60 (m, 2H), 2.33 (dd, 1H), 2.56-2.78 (m, 2H), 4.05(t, 2H), 6.08 (s, 1H),6.17 (s,1H); EI-MS: m/z 201.95 (M++H C10H19NO3 requires 201.0).

2-(Carbamoylmethyl)-4-methylpentanoic acid 12 and corresponding tert-butyl amine salt

To an aqueous solution of potassium hydroxide (11%, 100 mL) was added ethyl 3-carbamoyl-5-methylhexanoate (15.0 g, 74.6 mmol) at RT. The reaction mixture was stirred for 10 h at RT. Reaction mass was extracted with ethyl acetate (100 mL) and the aqueous layer was acidified to pH 1.5 by addition of dilute HCl solution. The aqueous layer was further extracted with ethyl acetate (3 100 mL). Combined organic layer was concentrated under reduced pressure to leave 2-(carbamoylmethyl)-4-methyl-pentanoic acid) as a white solid (13.6 g). To a solution of the compound 12 (1.6 g, 9.24 mmol) in ethyl acetate (15 mL) was added tert-butyl amine (0.68 g, 9.24 mmol) in a dropwise manner over 15 min at RT and stirred for 30 min. The precipitate obtained was filtered off and sucked dry to leave the tert-butyl amine salt of compound 12 as a white crystalline solid (2.1 g, 92%). IR(neat): 3356, 2956, 1678, 1527, 1409, 1276 cm-1; EI-MS: m/z 247.05 (M+ + H. C12H26N2O3 requires 246.19).

2-(2-Aminoethyl)-4-methylpentanoic acid, 13 To a solution of lithium borohydride (0.76 g, 34.7

mmol) in dry tetrahydrofuran (20 mL) at 0C,was added slowly in a dropwise manner a solution of 2-(carbamoylmethyl)-4-methylpentanoic acid (6.0 g, 34.7 mmol) in dry THF (30 mL). Temperature of the reaction mass was increased to 65C and was refluxed


1482

for 6 h. After cooling the reaction mixture to RT, it was quenched carefully by adding a solution of ammonium chloride (1.8 g, 34.7 mmol) in methanol (30 mL). The organic layer was concentrated under reduced pressure and the residue was purified by column chromatography with the product eluting out with 15% ethyl acetate in cyclohexane as a white crystalline solid (0.35g, 49%). IR (neat): 3399, 2956, 1659, 1569, 1415, 1029 cm-1; 1H NMR (200 MHz, D2O, Me4Si): 0.88 (dd, 6H), 1.17-1.30 (m, 1H), 1.42-1.55 (m, 3H), 2.32-2.51 (m, 3H), 2.69-2.80 (m, 1H); EI-MS: m/z 159.95 (M+ + H. C8H17NO2 requires 159.0)22.

(RS)-Ethyl-3-cyano-5-methylhexanoate, 14 from ethyl 3-carbamoyl-5-methylhexanoate, 11

A reactor equipped with overhead stirrer was charged with ethyl 3-carbamoyl-5-methylhexanoate (15.1 g, 74.6 mmol) and heated to 80C. Thionyl chloride (10 mL, 137.4 mmol) was added carefully to the above reaction mixture over 1 h at 80C. The reaction mixture was stirred for 12 h at 80C, after which it was cooled to 25C and quenched by carefully adding water, maintaining the reaction temperature below 25C. The aqueous layer was extracted with di-iso-propyl ether (2 50 mL). Combined organic layer was dried and concentrated under reduced pressure to obtain crude (RS)-ethyl-3-cyano-5-methylhexanoate (11.0 g, 80%) as a yellow oil. IR (neat): 2961, 2242, 1738, 1469, 1182, 1023 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.22-1.24 (m, 4H), 1.58 (m, 1H), 1.83 (m, 1H), 2.49 (dd, 1H), 2.65 (dd, 1H), 2.98-3.06 (m, 1H), 4.17 (q, 2H); EI-MS: m/z 201.05 (M+ + H2O. C10H17NO2 requires 183.0).

2-(S)-Bromo-4-methyl-pentanoic acid, 16 To a solution of potassium bromide (2.72 g, 22.9

mmol) in sulphuric acid (162.0 g in 1100 mL of water) L-leucine (86.0 g, 0.66 mol) was added and the mixture was cooled to 14C. A solution of sodium nitrite (70.0 g, 1.01 mol) in water (200 mL) was added slowly over 2 h and mixture was stirred for 3 h at 14C, after which it was warmed to 20C and stirred for another 1.5 h. The reaction mixture was extracted with dichloromethane (5 500 mL). Combined organic layers were dried and concentrated under reduced pressure to obtain 2-(S)-bromo-4-methyl-pentanoic acid (108.0 g, 85%) as a light yellow oil. IR (neat): 3583, 2959, 1718, 1468, 1258

cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.72-1.86 (m, 1H), 1.93 (dd, 2H), 4.29 (t, 1H); EI-MS: m/z 192.80/194.80 (M+ - H. C6H11BrO2requires 193.0/195.0).

2-(S)-Bromo-4-methyl-pentanoic acidamide, 17 A solution of 2-(S)-bromo-4-methyl-pentanoic acid

in cyclohexane (50 mL) was heated to 60C in an atmosphere of nitrogen and thionyl chloride was added carefully over 1 h while stirring. The mixture was further heated to 80C for 12 h after which it was cooled to 25C and dichloromethane (1000 mL) was added. To the reaction mixture, ammonia gas was purged for 1-1.5 h and ammonia solution (500 mL) was added. Organic layer was separated and aqueous layer was extracted with dichloromethane (500 mL). Combined organic layer was dried and concentrated under reduced pressure to obtain 2-(S)-bromo-4-methyl-pentanoic acidamide as a white solid (61.4 g,77%). IR (neat): 3363, 3188, 2957, 2871, 2364, 1666, 1419, 616 cm-1; 1H NMR (200 MHz, DMSO-d6, Me4Si): 0.83 (d, 3H), 0.88 (d, 3H), 1.54-81 (m, 3H), 4.35 (t, 1H), 7.26 (s, 1H), 7.76 (s, 1H); EI-MS: m/z 193.85/195.75 (M+. C6H12BrNO requires 192.0/194.0); m.p. 114-15C.

2-(S)-Bromo-4-methyl-pentanenitrile, 18 2-(S)-Bromo-4-methyl-pentanoic acid amide (50.0

g, 0.25 mol) and phosphorous pentoxide (80.0 g, 0.56 mol) were mixed thoroughly in a round-bottomed flask and kept for vacuum distillation at 80C and 12 mm of Hg for 4-5 h to obtain 2-(S)-bromo-4-methyl-pentanenitrile (43.0 g, 94%) as a colorless oil. IR (neat): 2963, 2936, 2248, 1756, 1470, 1372, 746 cm-1; 1H NMR (200 MHz, DMSO-d6, Me4Si): 0.97 (d, 3H), 1.01 (d, 3H), 1.88-1.97 (m, 3H), 4.44 (t, 1H); EI-MS: m/z 175.85 (M+. C6H10BrN requires 175.0).

2-(1-Cyano-3-methyl-butyl)-malonic acid diethyl ester, 19

A reactor was charged with dimethyl formamide (200 mL) and sodium hydride (5.76 g, 0.145 mol; 60% emulsion in paraffin) was added carefully in small portions under an atmosphere of nitrogen. The mixture was cooled to 10-15C and a solution of diethyl malonate(23.1 g, 0.145 mol) in dimethyl formamide(50 mL) was added slowly over 30 min by maintaining temperature below 15C, after which it was heated to 25C and stirred for 1 h. To the above reaction mixture was added a solution of 2-bromo-4-


1483

methyl-pentanenitrile (28.0 g, 0.145 mol) in dimethyl formamide (50 mL) over 30 min and stirred for 24 h, after which the reaction mixture was quenched by adding water (1000 mL). Aqueous layer was extracted with dichloromethane (3 500 mL). Combined organic layer was dried and solvent was evaporated under reduced pressure to yield 2-(1-cyano-3-methyl-butyl)-malonic acid diethyl ester as a yellow oil (40.0 g, 98%). 1H NMR (200 MHz, CDCl3, Me4Si): 0.98-1.01 (m, 6H), 1.22-1.32 (m, 6H), 1.62-1.69 (m, 1H), 1.77-1.90 (m, 2H), 3.25-3.31 (m, 1H), 3.50-3.52 (d, 1H), 4.15-4.23 (m, 4H); EI-MS: m/z 253.75 (M+ - H. C13H21NO4 requires 255.15).

Enantiomerically enriched (S)-ethyl-3-cyano-5-methylhexanoate, 14 from 2-(1-cyano-3-methyl-butyl)-malonic acid diethyl ester, 19

To a solution of 2-(1-cyano-3-methyl-butyl)-malonic acid diethyl ester (30.0 g, 0.117 mol), in dimethyl sulfoxide (300 mL) were added potassium chloride (9.65 g, 0.13 mol), water (10 mL) and heated to 150-160C for 6 h after which it was cooled to 30-40C and methyl tert-butyl ether (200 mL) was added. The mixture was further cooled to 0-5C and quenched with water (1000 mL) maintaining temperature below 40C and stirred for 30 min. The aqueous phase was extracted with methyl tert-butyl ether (3 800 mL). The organic layer was decolorized by treating with 7.0 g of activated charcoal. The resultant mixture was filtered and solvent was evaporated to give enantiomerically enriched (S)-ethyl-3-cyano-5-methylhexanoate as a yellow oil (78:22, S:R) (17.5 g, 81%). IR (neat): 2961, 2242, 1738, 1469, 1182, 1023 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.22-1.24 (m, 4H), 1.58 (m, 1H), 1.83 (m, 1H), 2.49 (dd, 1H), 2.65 (dd, 1H), 2.98-3.06 (m, 1H), 4.17 (q, 2H); EI-MS: m/z 201.05 (M+ + H2O. C10H17NO2 requires 183.0).

Methyl-2-cyano-4-methylpentanoate, 21a Methyl cyano acetate (113.0 g, 1.14 mol) was

dissolved in methanol (125 mL), iso-butyraldehyde (98.0 g, 1.36 mol) and glacial acetic acid (12 mL) were added to it at RT. The mixture was cooled to 4C and a solution of acetic acid (12 mL) and piperidine (4 mL) in 50 mL of methanol was added slowly over a period of 20 min by maintaining temperature below 20C. The reaction mixture was transferred into a Parr autoclave reactor followed by

addition of 2% catalyst palladium-on-carbon [50% wet (10% Pd loading)]. Reactor was purged with hydrogen gas two times and charged with hydrogen, 3 kg/cm2 pressure was maintained in the Parr autoclave until hydrogen consumption ceased. Reaction was monitored by TLC. After completion of reaction, the mixture was filtered through Celite bed to remove Pd/C and filtrate was concentrated under reduced pressure to remove solvent. Residue was suspended in 100 mL water. Organic layer was separated to obtain methyl-2-cyano-4-methylpentanoate as light yellow oil (170 g, 90% yield). IR (neat): 2958, 2872, 2642, 2250, 1751, 1468, 1185, 1131, 1010 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.92 (d, 3H), 0.99 (d, 3H), 1.74-1.98 (m, 3H), 3.53 (t, 1H) 3.81 (s, 3H).

2-Cyano-4-methyl-valeric acid ethyl ester, 21b To a solution of ethyl cyano acetate (56.5 g, 0.5

mol) in methanol (100 mL) was added iso-butyraldehyde (43.2 g, 0.6 mol) at 25C after which it was cooled to 4C. A solution of acetic acid (6 mL) and piperidine (2 mL) in methanol (50 mL) was added slowly to above reaction mixture over 20 min without allowing the temperature to increase above 20C. The reaction mixture was transferred into a Parr autoclave reactor followed by addition of 2 mol% loading of palladium-on-carbon (10% Pd). Reactor was purged with hydrogen gas twice and charged with hydrogen; 3 atm pressure was maintained in the Parr autoclave until hydrogen consumption ceased, after which it was filtered through a Celite pad to remove Pd/C and solvent from the filtrate was evaporated under reduced pressure to obtain an oily residue, which was suspended in water (100 mL) and extracted with di-iso-propyl ether (3 250 mL). Combined organic layer was dried and solvent was evaporated under reduced pressure to obtain 2-cyano-4-methyl-valeric acid ethyl ester aslight yellow oil (80.0 g, 95%). IR (neat): 2962, 2249, 1746, 1469, 1186 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.28 (t, 3H), 1.17-1.87 (m, 3H), 3.49 (q, 1H), 4.22 (q, 2H); EI-MS: m/z 186.85 (M+ + H2O. C9H15NO2 requires 169.0).

Synthesis of 4-ethyl 1-methyl 2-cyano-2-isobutyl-succinate, 22a in presence of cesium carbonate

A reactor was charged with methyl 2-cyano-4-methylpentanoate 21 (41.0 g, 265.0 mmol), ethyl chloro acetate (35.7 g, 291 mmol) and benzyl triethyl ammonium chloride (0.6 g) and the resulting reaction


1484

mixture was stirred for 15-20 min at RT. To above reaction mixture activated fine powder of cesium carbonate (47.3 g, 145.5 mmol) was added slowly in small portions while stirring over a period of 10-15 min. Addition of cesium carbonate resulted in rise in the reaction temperature upto 65 to 70C. After complete addition of cesium carbonate, reaction mixture was stirred further for 1 h at 60C. Reaction was monitored by TLC for complete consumption of starting materials and after completion of reaction, it was quenched by adding 100 mL water and organic layer was separated to obtain 4-ethyl 1-methyl 2-cyano-2-isobutylsuccinate as light yellow oil (57.5 g, 90% yield). IR(neat): 2958, 2248, 1741, 1637, 1467, 1199, 1025 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.88 (d, 3H), 0.92 (d, 3H), 1.05 (t, 3H), 1.70-1.89 (m, 3H), 2.79 (d, 1H), 3.03 (d, 1H), 3.84 (s, 3H), 4.18 (q, 2H); EI-MS: m/z C9H15NO2: 241; [M+H2O]+: 259.05.

Synthesis of 4-ethyl 1-methyl 2-cyano-2-isobutyl-succinate, 22a in presence of potassium carbonate

A reactor was charged with methyl 2-cyano-4-methylpentanoate (41.0 g, 265.0 mmol), ethyl chloroacetate (35.7, 291 mmol) and benzyl triethyl ammonium chloride (0.6 g) and resulting reaction mixture was stirred for 15-20 min at RT. To above reaction mixture activated fine powder of potassium carbonate (20.3 g, 145.5 mmol) was added slowly in small portions while stirring over a period of 10-15 min. After complete addition of potassium carbonate, the reaction mixture was stirred further for 3 h at 90C. Reaction was monitored by TLC for complete consumption of starting materials and after completion of reaction, it was quenched by adding 100 mL water and organic layer was separated and concentrated to obtain 4-ethyl 1-methyl 2-cyano-2-isobutylsuccinate as light brown oil (57.5 g, 90% yield).

Diethyl 2-cyano-2-isobutylsuccinate, 22b A reactor equipped with overhead stirrer was

charged with dimethoxy ethane (50 mL) and sodium hydride (22.0 g, 0.550 mol: 60% emulsion in paraffin) was added in small portions under nitrogen atmosphere. The mixture was cooled to 10-15C and solution of 2-cyano-4-methyl-valeric acid ethyl ester (80.0 g, 0.474 mol) in dimethoxy ethane (500 mL) was added slowly over 1 h by maintaining temperature below 20C after which the reaction

mixture was heated to 50C and stirred further for 1 h. A solution of ethyl chloro acetate (74.0 g, 0.6 mol) in dimethoxy ethane (300 mL) was added slowly to the reaction mixture over 1 h. After complete addition of the ethyl chloro acetate solution, the reaction mixture was cooled to RT and stirred for an additional 24 h, after which reaction mixture was filtered to remove sodium chloride and filtrate was concentrated under reduced pressure to obtain 2-cyano-2-isobutyl-succinate as light yellow oil (102.0 g, 84%). IR (neat): 2963, 2248, 1743, 1469, 1195, 1025 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.23 (t, 3H), 1.28 (t, 3H), 1.70-1.89 (m, 3H), 2.80 (d, 1H), 3.02 (d, 1H), 4.16 (q, 2H), 4.28 (q, 2H); EI-MS: m/z C9H15NO2: 255, [M+H2O] +: 273.05.

(RS)-Ethyl 3-cyano-5-methylhexanoate, 14 A reactor was charged with diethyl 2-cyano-2-

isobutylsuccinate (102.0 g, 0.4 mol), cesium chloride (72.5 g, 0.43 mol) in dimethyl sulfoxide (500 mL) and heated to 150-160C for 4 h after which the reaction mixture was cooled to 20 to 30C and methyl tert-butyl ether (200 mL) was added. The mixture was further cooled to 0-5C and water (1000 mL) was added in small portions without allowing the temperature to increase above 40C and was stirred for 30 min. The aqueous phase was extracted with methyl tert-butyl ether (3 800 mL), The organic layer was decolorized by treating with 7.0 g of activated charcoal. The resultant mixture was filtered to remove charcoal and solvent was evaporated to afford(RS)-ethyl 3-cyano-5-methylhexanoate as light brown oil (66.1 g, 98.5% purity by GC). IR (neat): 2961, 2242, 1738, 1469, 1182, 1023 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.22-1.24 (m, 4H), 1.58 (m, 1H), 1.83 (m, 1H), 2.49 (dd, 1H), 2.65 (dd, 1H), 2.98-3.06 (m, 1H), 4.17 (q, 2H); EI-MS: m/z 201.05 (M+ + H2O. C10H17NO2 requires 183.0).

Decarboxylation of diethyl 2-cyano-2-isobutyl-succinate, 22 to obtain (RS)-ethyl 3-cyano-5-me-thylhexanoate, 14 in presence of thiophenol/cesium carbonate in dimethylformamide

A 250 mL reactor was charged with diethyl 2-cyano-2-isobutylsuccinate (13.1 g, 51.3 mmol), thiophenol (8.47, 77.0 mmol), cesium carbonate (5.0 g, 15.4 mmol) and N,N-dimethylformamide (40 mL). The resulting reaction mixture was heated at 130C and maintained at that temperature for 4 h.


1485

Reaction was monitored by GC for conversion of (RS)-ethyl 3-cyano-5-methylhexanoate.

Screening of enzymes for stereo-selective hydroly-sis of (RS)-ethyl-3-cyano-5-methylhexanoate, 14

Screening of enzymes was conducted using HLC Heating-ThermoMixer (Model No. MHR 11) having 14 vial (14 10 mL) chamber blocks with orbital shaking. Each 10 mL vial was charged with phosphate buffer (5 mL) of pH 7.2, (RS) 3-cyano-5-methylhexanoic acid ethyl ester (0.5 g, 2.7 mmol) and enzyme (10% w/w of substrate) as mentioned in Table I. The reaction mixture was stirred for 4 h, after which it was extracted with dichloromethane (2 5 mL) and monitored on chiral GC analysis for stereo-selectivity of enzymes.

Stereo-selective hydrolysis of (RS)-ethyl-3-cyano-5-methylhexanoate 14 in presence of Novozym 435 at 10C

A reactor equipped with overhead stirring was charged with sodium phosphate buffer (500 mL) (40 mM, pH 7.2) and (RS)-3-cyano-5-methylhexanoic acid ethyl ester (100 g) at 25C and was cooled to 10C. Novozym 435 (6.0 g, 6% w/w of substrate) immobilized enzyme was added and the resulting heterogenous reaction mixture was titrated with 1 M solution of sodium hydroxide to maintain a pH of 7.2. The extent of reaction was monitored on GC for chiral purity. After complete hydrolysis of (R)-3-cyano-5-methylhexanoic acid ethyl ester, reaction was stopped by filtering the enzyme. Aqueous layer was extracted with di-iso-propyl ether (3 100 mL). Combined organic layer was dried over anhydrous sodium sulphate and solvent was evaporated under reduced pressure to obtain (S)-ethyl-3-cyano-5-methyl-hexanoate as brown oil (40.0 g, 40%, and 99% ee).

Stereo-selective hydrolysis of (RS)-ethyl 3-cyano-5-methylhexanoate 14 in presence of Novozym 435 at 25C

A reactor equipped with overhead stirring was charged with sodium phosphate buffer (500 mL) (40 mM, pH 7.2) and (RS)-3-cyano-5-methylhexanoic acid ethyl ester (100 g) at 25C. Novozym 435 (6.0 g, 6% w/w of substrate) immobilized enzyme was added and the resulting heterogenous reaction mixture was titrated with 1 M solution of sodium hydroxide to maintain a pH of 7.2. The extent of reaction was monitored on GC for chiral purity. After complete

hydrolysis of (R)-3-cyano-5-methylhexanoic acid ethyl ester, reaction was stopped by filtering the enzyme. Aqueous layer was extracted with di-iso-propyl ether (3 100 mL). Combined organic layer was dried over anhydrous sodium sulphate and solvent was evaporated under reduced pressure to obtain (S)-ethyl-3-cyano-5-methylhexanoate as brown oil (20.0 g, 20%, and 99% ee).

Stereo-selective hydrolysis of enantiomerically enriched (S)-ethyl-3-cyano-5-methylhexanoate 14 in presence of Novozym 435 at 10C

A reactor equipped with overhead stirring was charged with sodium phosphate buffer (500 mL) (40 mM, pH 7.2) and enantiomerically enriched (S)-3-cyano-5-methylhexanoic acid ethyl ester (78:22) (100 g) at 25C and was cooled to 10C. Novozym 435 (6.0 g, 6% w/w of substrate) immobilized enzyme was added and the resulting heterogeneous reaction mixture was titrated with 1 M solution of sodium hydroxide to maintain a pH of 7.2. The extent of reaction was monitored on GC for chiral purity. After complete hydrolysis of (R)-3-cyano-5-methylhexanoic acid ethyl ester, reaction was stopped by filtering the enzyme. Aqueous layer was extracted with di-iso-propyl ether (3 100 mL). Combined organic layer was dried over anhydrous sodium sulphate and solvent was evaporated under reduced pressure to obtain (S) ethyl-3-cyano-5-methylhexanoate as brown oil (50.0 g, 50%, and 99% ee).

(S)-Ethyl-3-cyano-5-methylhexanoate A reactor was charged with (S)-ethyl-3-cyano-5-

methylhexanoate (52.0 g) and water (250 mL). A solution of lithium hydroxide (15.0 g) in water (25 mL) was added slowly while stirring. The reaction mixture was stirred further for 12 h at 60C. Thereafter, the reaction mixture was cooled to RT and un-reacted (RS)-3-cyano-5-methylhexanoic acid ethyl ester, if any was extracted with di-iso-propyl ether. Aqueous layer was acidified with dilute hydrochloric acid upto pH 2 and extracted with dichloromethane (3 150 mL). Combined organic layer was dried over anhydrous sodium sulfate and solvent was evaporated under reduced pressure to obtain (RS)-3-cyano-5-methylhexanoic acid as yellow oil (31.1 g). IR (neat): 3118, 2961, 2935, 2875, 2642, 2244, 1715, 1470, 1174, 1113 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.36-1.38 (d, 1H), 1.59-1.66 (m, 1H), 1.79-1.85 (m, 1H), 2.59-2.61 (dd, 1H),


1486

2.69-2.75 (dd, 1H), 2.98-3.04 (m, 1H); EI-MS: m/z C8H13NO2: 155.19, [M-H] -: 154.00, [M+H] +: 156.15.

(S)-3-(Aminomethyl)-5-methylhexanoic acid 4 from (S)-3-cyano-5-methylhexanoic acid

A solution of (S)-3-cyano-5-methylhexanoic acid(20.0 g, 0.13 mol) in methanol:water (50:50) (100 mL) was added to a solution of potassium hydroxide (7.2 g, 0.13 mol) in water (20 mL) at 25C and was stirred at RT for 2 h. The mixture was then transferred into a Parr autoclave reactor and 50% wet palladium-on-carbon (1.0 g) was added carefully. Reactor was purged with hydrogen gas twice and then 10 atm. hydrogen pressure was maintained for 24 h. The reaction mixture was filtered through a Celite pad and solvent from filtrate was evaporated under reduced pressure to leave a semi-solid material, which was re-crystallized from iso-propyl alcohol:water mixture (94:06, 25 mL) to obtain (S)-3-(aminomethyl)-5-methylhexanoic acid as a white solid (12.0 g, 60% and 99% ee).

Resolution of (RS)-3-cyano-5-methylhexanoic acid through diastereomeric salt formation with cincho-nidine (1:1 mol ratio) in ethyl acetate

A reactor was charged with cinchonidine (37.9 g, 129 mmol) and ethyl acetate (500 mL) and resulting reaction mixture was heated to 70C. A solution of (RS)-3-cyano-5-methylhexanoic acid (20.0 g, 129.0 mmol) in ethyl acetate (200 mL) was added to above reaction mixture over a period of 15-20 min and reaction mixture was further stirred for 5 h at reflux temperature, after which reaction mixture was cooled to RT and stirred further for 12 h. (S)-3-Cyano-5-methylhexanoic acid salt of cinchonidine precipitated out during this period. The resultant mixture was filtered to give (S)-3-cyano-5-methylhexanoic acid salt of cinchonidine as a white solid (28.3 g, 97% ee for (S)-ethyl-3-cynao-5-methylhexanoate by GC area%), which was further re-crystalized to obtain (S)-3-cyano-5-methylhexanoic acid salt of cinchonidine (25.3 g, 99% ee for (S)-ethyl-3-cyano-5-methyl-hexanoate by GC area %). Spectral data is given below.

IR (neat): 3413, 3071, 2955, 2234, 1639, 1595, 1508, 1394, 1102, 915, 785, 759, 619 cm-1; 1H NMR (200 MHz, DMSO-d6, Me4Si): 0.89 (d, 3H), 0.91 (d, 3H), 1.30-1.45 (m, 1H), 1.52-1.59 (m, 3H), 1.69-1.82 (m, 4H), 2.38 (bs, 1 H), 2.43-2.55 (dd, 3H), 2.60-2.68 (m, 2H), 2.97-3.07 (m, 2H), 3.25 (s, 1H), 3.49 (s, 1H),

4.93 (d, 1H), 4.99 (d, 1H), 5.64 (d, 1H), 5.78-5.87 (m, 1H), 7.60-7.64 (m, 2H), 7.75 (t, 1H), 8.04 (d, 1H), 8.36 (d, 1H), 8.86 (d, 1H); 13C NMR (DMSO-d6, 50 MHz): 21.6, 22.1, 23.2, 26.2, 26.3, 26.4, 27.6, 38.3, 38.9, 42.3, 55.1, 60.5, 69.4, 115.3, 119.4, 123.0, 124.4, 125.9, 126.9, 129.3, 130.1, 141.6, 148.2, 149.6, 150.5, 172.9; Powder X-ray diffraction pattern PXRD [2] (Cu K1 = 1.54060 , K2 = 1.54443 , K = 1.39225 ; 40 mA, 45 kV): 5.84, 7.27, 7.69, 10.72, 11.65, 13.79, 14.92, 15.39,15.73, 16.69, 17.31, 17.41, 17.58, 17.99, 19.48, 20.03, 20.71, 21.18, 21.92, 23.18, 24.93, 25.29, 25.95, 26.38, 27.07, 27.91, 28.79, 31.06, 31.65, 35.36, 38.00 and 39.35; DSC Value (10C/min): Peak = 152.49C, Onset = 149.86C.

Racemization of (R)-ethyl-3-cyano-5-methylhexa-noateto (RS)-3-cyano-5-methylhexanoic acid in dimethyl sulfoxide and 2% ethanol

A reactor equipped with overhead stirring was charged with (R)-ethyl-3-cyano-5 methylhexanoate (15.8 g, 0.086 mol), dimethyl sulfoxide (158 mL), ethanol (4 mL) and sodium ethoxide (7.35 g, 0.10 mol) and the resulting reaction mixture was stirred for 4 h at 75C, after which the reaction mixture was cooled to RT, neutralized with acetic acid and treated with water (200 mL) in small portions to maintain the temperature below 30C. The aqueous phase was extracted with methyl tert-butyl ether (3 200 mL). Organic phases were combined and dried over anhydrous sodium sulfate and solvent was evaporated under reduced pressure to give (RS)-3-cyano-5-methylhexanoic acid as light brown oil (12.5 g, 95% yield and analyzed by chiral GC by converting to the corresponding ethyl ester).

IR (neat): 3118, 2961, 2935, 2875, 2642, 2244, 1715, 1470, 1174, 1113 cm-1; 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.36-1.38 (d, 1H), 1.59-1.66 (m, 1H), 1.79-1.85 (m, 1H), 2.59-2.61 (dd, 1H), 2.69-2.75 (dd, 1H), 2.98-3.04 (m, 1H); EI-MS: m/z C8H13NO2: 155.19, [M-H] -: 154.00, [M+H] +: 156.15.

Racemization of (R)-ethyl-3-cyano-5-methyl-hexanoate to (RS)-3-cyano-5-methylhexanoic acid in N-methyl pyrrolidone and 2% ethanol

A reactor equipped with overhead stirring was charged with (R)-ethyl-3-cyano-5 methylhexanoate (3.0 g), N-methyl pyrrolidone (30 mL), ethanol (0.6 mL) and sodium ethoxide (1.4 g) and the resulting reaction mixture was stirred for 4 h at 75C, after


1487

which the reaction mixture was cooled to RT, neutralized with acetic acid and treated with water (200 mL) in small portions to maintain the temperature below 40C. The aqueous phase was extracted with methyl tert-butyl ether (3 100 mL). Organic phases were combined and dried over anhydrous sodium sulfate and solvent was evaporated under reduced pressure to give (RS)-3-cyano-5-methylhexanoic acid as brown oil (2.4 g, analyzed by chiral GC by converting to corresponding ethyl ester).

Racemization of (R)-ethyl-3-cyano-5-methyl-hexanoate to (RS)-3-cyano-5-methylhexanoic acid in dimethyl formamide and 2% ethanol

A reactor equipped with overhead stirring was charged with (R)-ethyl-3-cyano-5 methylhexanoate (3.0 g), dimethyl formamide (30 mL), ethanol (0.6 mL) and sodium ethoxide (1.4 g) and the resulting reaction mixture was stirred for 4 h at 75C, after which the reaction mixture was cooled to RT, neutralized with acetic acid and treated with water (200 mL) in small portions to maintain the temperature below 40C. The aqueous phase was extracted with methyl tert-butyl ether (3 100 mL). Organic phases were combined and dried over anhydrous sodium sulfate and solvent was evaporated under reduced pressure to give (RS)-3-cyano-5-methylhexanoic acid as brown oil (2.1 g, analyzed by chiral GC by converting to corresponding ethyl ester).

Racemization of (R)-ethyl-3-cyano-5-methylhexa-noate to (RS)-3-cyano-5-methylhexanoic acid in dimethoxy ethane and 2% ethanol

A reactor equipped with overhead stirring was charged with (R)-ethyl-3-cyano-5 methylhexanoate (3.0 g), dimethoxy ethane (30 mL), ethanol (0.6 mL) and sodium ethoxide (1.4 g) and the resulting reaction mixture was stirred for 4 h at 75C, after which the reaction mixture was cooled to RT, neutralized with acetic acid and treated with water (200 mL) in small portions to maintain the temperature below 40C. The aqueous phase was extracted with methyl tert-butyl ether (3 100 mL). Organic phases were combined and dried over anhydrous sodium sulfate and solvent was evaporated under reduced pressure to give (RS)-3-cyano-5-methylhexanoic acid as brown oil (2.2 g, analyzed by chiral GC by converting to corresponding ethyl ester).

Racemization of (R)-ethyl-3-cyano-5-methylhexa-noate to (RS)-3-cyano-5-methylhexanoic acid in methyl-tert-butyl ether and 2% ethanol

A reactor equipped with overhead stirring was charged with (R)-ethyl-3-cyano-5 methylhexanoate (3.0 g), methyl-tert-butyl ether(30 mL), ethanol (0.6 mL) and sodium ethoxide (1.4 g) and the resulting reaction mixture was stirred for 4 h at 75C, after which the reaction mixture was cooled to RT, neutralized with acetic acid and treated with water (200 mL) in small portions to maintain the temperature below 40C. Organic layer was separated and aqueous phase was extracted with methyl tert-butyl ether (100 mL). Organic phases were combined and dried over anhydrous sodium sulfate and solvent was evaporated under reduced pressure to give (RS)-3-cyano-5-methylhexanoic acid as brown oil (2.2 g, analyzed by chiral GC by converting to corresponding ethyl ester).

Racemization of enantiomerically enriched (R)-ethyl-3-cyano-5-methylhexanoate to (RS) 3-cyano-5-methylhexanoic acid in 2-methyl tetrahydro-furan and 2% ethanol

A reactor equipped with overhead stirring was charged with (R)-ethyl-3-cyano-5 methylhexanoate (3.0 g), 2-methyl tetrahydrofuran (30 mL), ethanol (0.6 mL) and sodium ethoxide (1.4 g) and resulting reaction mixture was stirred for 4 h at 75C, after which the reaction mixture was cooled to RT, neutralized with acetic acid and solvent was evaporated under reduced pressure to obtain residue. Residue was further suspended in water (200 mL) and aqueous phase was extracted with methyl tert-butyl ether (3 300 mL). Organic phases were combined and dried over anhydrous sodium sulfate and solvent was evaporated under reduced pressure to give (RS)-3-cyano-5-methylhexanoic acid as brown oil (2.0 g, analyzed by chiral GC by converting to corresponding ethyl ester).

Conclusion Four novel routes for the synthesis of (S)-3-

(aminomethyl)-5-methylhexanoic acid have been developed which appear to be benign, efficient, eco-friendly and do not employ any explosive or toxic chemicals. All the developed routes have been assessed for the greenness and efficiency as per criteria generally used for such assessment and based on that, route 4 has been recommended for scale-up.


1488

References 1 Silverman R B, Andruszkiewicz R, Yuen P, Sobieray D M,

Franklin L C & Schwindt M A, US Patent 5563175, University Northwestern and Warner Lambert Co, 1966.

2 (a) Bueno L, Chovet M, Diop L, Guglietta A, Little H J, Rafferty M F, Ren J, Taylozr C P & Watson W P, US Patent 6242488, University of Oklahoma and Warner Lambert Co, 2001; (b) Magnus L & Segal C A, US Patent 6326374, Warner Lambert Co, 2001; (c) Singh L, US Patent 6001876, Warner Lambert Co, 1999; (d) Akunne H C, Corbin A E, Dooley D J, Green A L & Heffner T G, US Patent 6194459, Warner Lambert Co, 2000; (e) Schrier D, Taylor C P & Westlund H K N, US Patent 6329429, Warner Lambert Co, 2001; (f) Bueno L, Chovet M, Diop L, Guglietta A, Little H, Rafferty M F, Ren J, Taylor C P & Watson W P, US Patent 6127418, Warner Lambert Co, 2000; (g) Bueno L, Chovet M, Diop L, Guglietta A, Little H, Rafferty M F, Ren J, Taylor C P & Watson W P, US Patent 6426368, Warner Lambert Co, 2001; (h) Magnus L & Segal C A, US Patent 6306910, Warner Lambert Co, 2001; (i) Pande A C, US Patent 6359005, Warner Lambert Co, 2002.

3 Grote T M, Huckabee B K, Mulhern T, Sobieray D M & Titus R D, US Patent 5637767, Warner Lambert Co, 1997.

4 Shanghui H, Carlos A M, Junhua T, William E T, Patrick K & Yves D, US Patent 0283023 (A1), Warner Lambert Co, 2005.

5 Burk M J, Goel O, Hoekstra M S, Mich T F, Thomas, Mulhern J & Ramsden A, US Patent Appl 0212290 A1, Warner Lambert Co, 2003.

6 Burns M P, Weaver J K & Wong J W, US Patent Appl 0196905(A1), Warner Lambert Co, 2007.

7 Sammis G M & Jacobsen E N, J Am Chem Soc, 125, 2003, 4442.

8 Kansal V K, Chaurasia B P, Rao V G & Tiwari A P, PCT Int Appl WO 035890 (A1), Teva Pharmaceutical Industries Ltd, 2007.

9 Kansal V K, Chaurasia B P & Tiwari A P, PCT Int Appl WO 035789 (A1), Teva Pharmaceutical Industries Ltd, 2007.

10 Kansal V K, Chaurasia B P, Tiwari A P & Shelke S H, US Patent Appl 0306292, Teva Pharmaceutical Industries Ltd, 2008.

11 Blazecka P G, Davidson J G & Zhang J, US Patent 6924377, Warner Lambert Co, 2005.

12 Silverman R B, Andruszkiewicz R, Yuen P W, Sobieray D M, Franklin L C & Schwindt M A, US Patent 5563175, University Northwestern and Warner Lambert Co, 1996.

13 Rasparini M, Tufaro R, Marras G, Giovenzana G & Castaldi G, PCT Int Appl WO 053446 (A2), Chemo Ibrica S.A, 2009.

14 Anastas P T & Warner J C, in Green Chemistry: Theory and Practice (Oxford University Press, Oxford), 1998.

15 Trost B M, Science, 254, 1991, 1471. 16 Sheldon R A, Pure Appl Chem, 72, 2000, 1233. 17 Bourguignon J J & Wermuth C G, J Org Chem, 46, 1981,

4889. 18 (a) Brown H C & Narasimhan S, J Org Chem, 47, 1982,

1604; (b) Schlesinger H I & Brown H C, J Am Chem Soc, 62, 1940, 342.

19 Belliotti T R, Capiris T, Ekhato I V, Kinsora J J, Field M J, Heffner T G, Meltzer L T, Schwarz J B, Taylor C P, Thorpe A J, Vartanian M G, Wise L D, Zhi-Su T, Weber M L & Wustrow D J, J Med Chem, 48, 2005, 2294.

20 Ichimura K & Ohta M, Bull Chem Soc Japan, 43, 1970, 1443. 21 Krapcho A P, Glynn G A & Grenon B J, Tetrahedron Lett, 8,

1967, 215. 22 Alexander E R & Cope A C, J Am Chem Soc, 66, 1944, 886. 23 Keinan E & Eren D, J Org Chem, 51, 1986, 3165. 24 Cram D J, Rickborn B, Kingsbury C A & Haberfield P, J Am

Chem Soc, 83, 1961, 3678. 25 Youssef A A & Sharaf S M, J Org Chem, 39, 1974, 1705. 26 D2O exchange experimental condition: In a NMR tube, 3-

cyano-5-methyl hexanoic acid ethyl ester (50 mg), D2O (50 L), ethanol (50 L), dimethyl sulfoxide (1 mL) and sodium ethoxide (23.2 mg) were heated to 70C and NMR was recorded. 1H NMR (200 MHz, CDCl3, Me4Si): 0.95 (d, 3H), 0.96 (d, 3H), 1.22-1.24 (m, 4H), 1.58 (m, 1H), 1.83 (m, 1H), 2.49 (d, 1H), 2.98-3.06 (m, 1H), 4.17 (q, 2H).

27 To rationalize the acidity of protons, an experiment was carried out by treating 3-cyano-5-methyl hexanoic acid ethyl ester with diethyl oxalate under identical conditions and it was observed that only one acylation product was obtained viz. diethyl 2-(1-cyano-3-methylbutyl)-3-oxosuccinate, i.e. acylation to carboxyl functionality and not acylation at cyano functionality (Isolated yield: 70%); IR (Neat): 3656, 3475, 2961, 2244, 1757, 1733, 1657, 1469, 1370, 1254, 1092, 1023, 858 cm-1; 1H NMR (CD3OD, 200 MHz, equilibrium existed for keto and enol form): 0.82-0.88 (m, 3H), 0.95 (d, 3H), 0.97 (d, 3H) 1.22-1.62 (m, 11H), 1.77-1.90 (m, 2H), 3.33 (dd, 1H), 4.24 (q, 2H), 4.35 (q, 2H), 13.02 (enolic OH); EI-MS: C14H21NO5: m/z 283.32, [M] +m/z = 283.95, [M-H]-m/z = 282.05.

28 One might rationalize the formation of (RS)-3-cyano-5-methylhexanoic acid from (R)-ethy-3-cyano-5-methyl-hexanoic acid ester, through a cyclopropanone intermediate (Tetrahedron: Asymmetry, 13, 2002, 563-567).

29 Constable D J C, Curzons A D & Cunningham V L, Green Chem, 4, 2002, 521.

30 Curzons A D, Constable D J C, Mortimer D N & Cunningham V L, Green Chem, 3, 2001, 1.

31 Watson W J W, Green Chem, 14, 2012, 251. 32 Yan J, Travis B R & Borhan B, J Org Chem, 69, 2004, 9299.

Documents

Ind. J. Chem. 51B, 1470-1488 (2012)-Ecofriendly Process for Pregabalin