GREENER AND ECONOMIC PROCESS FOR PREPARATION OF PREGABALIN

RELATED APPLICATION

This application claims the benefit to Indian Provisional Application No. 201911009439, filed on March 11, 2019, the contents of which are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to an economical, enzyme catalyzed and commercially viable greener process for manufacturing Pregabalin of formula (I) in high yield with high chemical and chiral purity.

BACKGROUND OF THE INVENTION

Pregabalin chemically known as 3 - (5 ) - a m i n o m c t h y 1 - 5 - m c t h y 1 hexanoic acid having structure formula (I), is known to treat several central nervous system disorders that include epilepsy, neuropathic pain, anxiety and social phobia.

(S)-Pregabalin has been found to activate GAD ( L-Glutamic Acid Decarboxylase) in a dose dependent manner and promote production of GABA (Gamma-Amino Butyric Acid), one of the major inhibitory neurotransmitters of brain. The discovery of anti-seizure activity was first disclosed in US Patent No. 5,563,175.

Pregabalin has been prepared in various ways. One of the common approaches involves synthesis of racemic Pregabalin typically a 50:50 mixture of R and S isomers and subsequent resolution through diastereomeric salt formation. Such an approach was found in PCT patent publications such as WO2009122215, W02009087674, W02009044409, WO 2008138874, WO2009125427 and W02009001372. The major difficulties associated with these approaches involve the loss of //-enantiomer along with a part of 5- isomer as well and this cannot be effectively recycled leading to increase in cost of production. Another approach has utilized resolution in the intermediate stage as a strategy. The scheme 1 outlines the approach described in PCT patent publication WO 9638405. The synthesis involves Knoevenagel condensation followed by Micheal addition and acidic hydrolysis which gives diacid. The diacid was converted to mono amide which was resolved by ( R )- phenylethylamine. After liberation of iCmono acid amide it was converted to (S)-Pregabalin by Hoffmann degradation. The overall yield was 12% and 99.8% enantiomeric excess (ee) over 8 steps. All commercial reagents were used, and the chiral auxiliary can be recovered.

Scheme 1:

Another approach as shown in scheme 2 is described in PCT patent publication WO2008137512, which involved the resolution of amide intermediate followed by Hoffmann degradation. A further modification was described in Org. Process Res. Dev., 2009, 13, 812-13, which involved the recovery of unwanted isomer i.e. racemization using organic base in solvent which lead to racemic acid amide intermediate.

Scheme 2:

The approach described in patent publications W02008062460 and U.S. Patent No. 6,046,353 is shown in scheme 3, which involved the condensation of diethyl malonate with isovaleraldehyde followed by cyanation. The product is selectively decarboxylated to cyano ester which on further hydrolysis gives cyano acid. The cyano acid was hydrogenated to racemic Pregabalin, which was resolved by using (S)-mandclic acid with overall yield of 15.5% and enantiomeric excess >99.5% over 6 steps.

Scheme 3:

Another commonly used scaffold was found to be 3-isobutylglutaric acid anhydride (IBG). In U.S. Patent publication No. 20090143615 and European Patent publication No. EP2067768 disclosed the synthesis of Pregabalin as shown in scheme 4 which involved the ring opening by hydrazine followed by conversion to urethane acid by Curtius rearrangement. This intermediate was resolved by using (S)-PEA (phenylethylamine). The release of chiral auxiliary followed by hydrolysis gave Pregabalin in overall yield of 12% over 4 steps with 99.8% enantiomeric excess. This method also suffers from the loss of unwanted iUisomer which cannot be efficiently recycled.

Scheme 4:

Asymmetric ring opening of 3-isobutylglutaric acid anhydride and subsequent chemical transformation to enantiopure Pregabalin constitute another approach. The PCT patent publication W02008118427 describes the synthesis of (S)-Pregabalin starting from 3- isobutyl-glutaric anhydride as depicted in scheme 5, which involve the stereo selective ring opening with (S)-PEA with good yield and enantiomeric excess. This was converted to amide by mixed anhydride approach. The amide was subjected to Hoffman degradation followed by PEA amide hydrolysis in two different approach to yield Pregabalin in 59.5% and 38.8% respectively with purity >99.5%.

Scheme 5:

A rather more efficient enzymatic route was described in U.S. patent publication US2005028302. In this, the cyano diacid diethyl ester was enzymatically hydrolyzed to ( S )- cyano ester monoacid potassium salt and the unwanted isomer was racemized. The salt was either reduced to a lactam acid followed by hydrolytic decarboxylation to Pregabalin with 34% overall yield over 3 steps with enantiomeric excess>99.5%. Alternatively, the (S)- cyano ester monoacid potassium salt was converted to cyano monoacid potassium salt which was hydrogenated to Pregabalin in 30% overall yield over 3 steps with 99.75% enantiomeric excess. Although it looks a reasonably good process however space vs time, yield may not be cost effective.

Scheme 6: Finally, there are some reports on asymmetric synthesis of Pregabalin mostly of academic interest due to the fact that they either involve longer sequence or provide Pregabalin with low enantiomeric excess. The scheme in J. Org. Chem., 2003, 68, 5731-34 (scheme 7) described the Bayllis-Hillman condensation and subsequent carbonate formation with chloroformate. The carbonate was subjected to CO insertion. The conjugated nitrile was hydrolyzed and converted to tert- butylamine salt that was stereo -selectively hydrogenated to cyano acid using [(R,R)-(Me-DuPHOS)Rh(COD)].BF4, followed by hydrogenation of CN with Ni to give Pregabalin in 41.5% overall yield with ee (enantiomeric excess) 99.8% over 6 steps.

Scheme 7:

The synthesis described in Org. Lett., 2007, 9, 5307-09 involves asymmetric Michael addition of nitromethane to a,b-unsaturated aldehyde using chiral catalyst. This catalyst needs to be prepared from D-proline that involve 5 steps. However, the number of steps are only three, it provides less enantiomeric purity, thus, it further needs additional resolution. Therefore, this approach cannot be economically viable. Scheme 8:

Scheme

In a similar fashion, Indian Journal of chemistry 2012, 51B, 1470-1488 described stereo selective synthesis of Pregabalin wherein the resolution was performed using lipases in phosphate buffer. In this process the pH of reaction was maintained between pH 7.2 to 7.5 at 10°C. However, it achieves limited selectivity moreover, preparation of phosphate buffer and reaction at lower temperature demands further improvement in the process.

Scheme 10:

The PCT patent publication WO2014072785 (henceforth WO’785) described the lipase catalyzed enzymatic process for the synthesis of Pregabalin. In this synthesis, genetically modified nitrilase enzyme was used as biocatalyst in cost effective and eco-friendly manner. The WO’785 discloses a process of preparation of Pregabalin wherein (i?)-3-cyano-5-methyl- hexanoic acid or salt thereof on racemisation provides corresponding /tfS'-cyanoacid which on esterification provides racemic alkyl-3-cyano-5-methyl-hexanoate (racemic ester). This racemic ester on hydrolysis gives the (S)-alkyl 3-cyano-5-methyl-hexanoateester and (R)- 3- cyano-5-methyl-hexanoic acid. The 5-cyano ester converted to Pregabalin by hydrolyzing ester group followed by hydrogenation. The process as described in WO’785 for racemic alkyl 3-cyano-5-methyl-hexanoateresults in the formation of two impurities i.e. succinimide impurity and amide ester impurity as shown below. The desired purity is achieved by high vacuum distillation by removing impurities and results the loss of yield.

Succinimide impurity Amide ester impurity

Accordingly, based on the drawbacks mentioned in all the prior arts involve synthesis containing chemical resolution method which are expensive, use of chiral reagents and chiral auxiliaries. It calls for an economically viable enzymatic synthesis for Pregabalin. Hence, the inventors are motivated to pursue the research for the improved process for the preparation of Pregabalin and the inventors have identified the formation of succinimide and amide ester impurities and have avoided the formation of these impurities for the first time.

Based on the drawbacks mentioned in all the prior arts above, there is an intense need to develop a process for the preparation of Pregabalin which is readily amenable to scale-up. Hence, we focused our research to simplify the process for the preparation of Pregabalin with simple process of using a genetically modified nitrilase enzyme and lipase enzyme to achieve greater yield, higher chemical and chiral purity. Thus, the process is eco-friendly, cost effective to obviate the formation of succinimide and amide ester impurities.

OBJECTIVES OF THE INVENTION

The main object of the present invention is to provide an improved process for the preparation of Pregabalin of formula (I), which is more economical, efficient, environment friendly and commercially viable.

Another objective of the present invention is to provide a process for the preparation of Pregabalin of formula (I), which would be easy to implement on commercial scale, and to avoid excessive use of reagent(s) which makes present invention eco-friendly.

Yet another objective of the present invention is to provide a process for the preparation of Pregabalin of formula (I) in a greater yield with higher chemical and chiral purity. Still another objective of the present invention is to provide a process for the preparation of Pregabalin of formula (I) where the process is cost efficient and effluent efficient in which the Co-products formed in the reaction is recoverable for reuse, thus, makes the process industrially more suitable.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an improved process for the preparation of Pregabalin of formula (I), which comprises the steps of

a) Obtaining 2-isobutyl-succinonitrile of formula (IV) by reacting compound of formula (II) with compound of formula (III) in presence of cyanide source, with or without phase transfer catalyst and with or without using a weak base or salts of weak acid and weak base in a solvent;

wherein Ri is linear or branched Ci to C4 alkyl;

b) converting 2-isobutyl-succinonitrile of formula (IV) to a racemic 3-cyano-5-methyl- hexanoic acid or salt thereof of formula (V) with a genetically modified nitrilase enzyme Nit 9N_56_2 in water adding with or without co-solvent at pH 7.5+ 0.8 and temperature 20°C to 45°C;

wherein R2 is hydrogen or a cation selected from the alkali metal, alkaline earth metal, ammonium, alkyl ammonium;

c) converting racemic 3-cyano-5-methyl-hexanoic acid or salt thereof of formula (V) to (R)- 3-cyano-5-methyl hexanoate of formula (VI) and (5)-3-cyano-5-mcthyl-hcxanoic acid or salt of formula (VII) by treatment with alcohol (R ₃ OH), alkyl acetate (R ₃ OAC) or alkyl halide (R ₃ X) in presence of lipase enzyme in a solvent or a mixture of solvents; wherein R2 is hydrogen or a cation selected from the alkali metal, alkaline earth metal, ammonium, alkyl ammonium;

R3 is linear or branched Ci to C ₄ alkyl;

d) converting a (R)-3-cyano-5-methyl hexanoate of formula (VI) (co-product) to its racemic compound of formula (V) where R2 is hydrogen, using a base, in an organic solvent followed by hydrolysis using an acid;

e) converting (5)-3-cyano-5-mcthyl-hcxanoic acid or salt of formula (VII) to Pregabalin of formula (I) in presence of a hydrogenation catalyst in a solvent selected from water or organic solvents or a mixture.

The above process is illustrated in the following general synthetic scheme.

Wherein

R _j is linear or branched C _j to C ₄ alkyl;

R ₂ is hydrogen or cation selected from alkali metal, alkaline earth metal, ammonium, alkyl ammonium;

R ₃ is linear or branched C _j to C ₄ alkyl

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter. The invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. As used in the specification, and in the appended claims, the singular forms“a”,“an”,“the”, include plural referents unless the context clearly indicates otherwise. The word“racemate(s)” or“racemic mixture(s)” means the 50:50 mixture of individual R and S enantiomers. The term“substantially pure S enantiomer(s)” indicates the presence of S enantiomer>>R enantiomer; preferentially the ratio of S:R can be in the range of 85:15 to 100:0; more preferably the ratio of S:R can be 95:5 to 100:0; while most preferably the ratio of S:R can be 99:1 to 100:0. The term“R enriched enantiomer” indicates the presence of R enantiomer »S enantiomer; preferentially the ratio of R:S can be in the range of 70:30 to 100:0.

In accordance with the objectives wherein, the present invention provides an improved process for the preparation of Pregabalin of formula (I) via selective enzymatic stereo specific synthetic approach.

Accordingly, in an embodiment of the present invention wherein, the said weak organic acid used in stage(a) is preferably selected from the group consisting of benzoic acid, succinic acid, maleic acid, fumaric acid, phthalic acid, acetic acid. The said organic weak base used in the stage (a) is preferably selected from the group consisting of triethylamine, diisopropyl ethylamine, pyridine, piperidine, l,8-diazabicyclo[5,4,0]undec-7-ene. The said salts in stage (a) is preferably selected from the group consisting of sodium acetate, ammonium acetate ammonium benzoate, ammonium succinate, alkyl ammonium acetate.

In another embodiment of the present invention wherein the said cyanide source of stage (a) is preferably selected from the group consisting of lithium cyanide, sodium cyanide, potassium cyanide, trimethylsilyl cyanide. In another embodiment of the present invention wherein, the reaction stage (a) the cyanide source optionally is used in 1-50% excess.

In another embodiment of the present invention wherein the phase transfer catalyst of said stage (a) is preferably selected from the group consisting of tetrabutyl ammonium bromide, tetrabutyl ammonium chloride, tetrabutyl ammonium iodide, tetrabutyl ammonium fluoride.

In another embodiment of the present invention wherein the said organic solvent in stage (a) is preferably selected from the group consisting of water, ethyl alcohol, methyl alcohol, isopropyl alcohol, «-propanol, «-butanol, ethyl acetate, dichloroethane, chloroform, tetrahydrofuran, 1,4-dioxane, dimethylformamide, dimethyl sulfoxide, dimethyl acetamide, methyl tert-butyl ether, cyclohexane, toluene and the mixture of solvents thereof.

In another embodiment of the present invention wherein the said reaction is carried out preferably at ambient temperature to reflux temperature.

The crude compound of formula (IV) disclosed in stage (a) is used as such or purified by distillation by different techniques well understood by those skilled in the art.

In another embodiment of the present invention wherein, in stage (b) 2-isobutyl- succinonitrile of formula (IV) is regio-selectively converted to a racemic 3-cyano-5-methyl- hexanoic acid or salt thereof of formula (V) with a genetically modified nitrilase enzyme Nit 9N_56_2 (sourced from c-LEcta GmbH, Germany). Further, this racemic 3-cyano-5-methyl- hexanoic acid or salt thereof of formula (V) in stage (c) is further treated stereo-selectively with /^-selective lipase to get (i?)-3-cyano-5-methyl hexanoate of formula (VI) and (5)-3- cyano-5-methyl-hexanoic acid or salt of formula (VII). The (5)-3-cyano-5-mcthyl-hcxanoic acid or salt of formula (VII) remains as such in aqueous media, which is then directly converted to Pregabalin of formula (I) in stage (e) by hydrogenation process using hydrogenation catalyst. The biological materials used in the present invention is not sourced from India.

In another embodiment of the present invention wherein, in stage (d), simultaneously, the co-product (i?)-3-cyano-5-methyl hexanoate of formula (VI) undergo a racemization followed by hydrolysis in presence of an acid to recover racemic 3-cyano-5-methyl- hexanoic acid of formula (V).

In another embodiment of the present invention of stage (b) wherein the said genetically modified nitrilase enzyme is Nit 9N_56_2 where the said enzyme provides greater selectivity, higher yields, minimum waste.

In another embodiment of the present invention wherein the said genetically modified nitrilase enzyme in stage (b) is Nit 9N_56_2 is used in pure form or with support. The said enzyme if used with support is recovered by filtration and is reused with or without treatment to mimic the comparable results. In another embodiment of the present invention wherein the loading of compound of formula (IV) for regio- selective hydrolysis in stage(b) preferably is chosen from 30 to 300 g per liter of water or water in combination of organic solvent; more preferably 50 to 200 g per liter of water or water in combination of organic solvent.

In another embodiment of the present invention wherein the loading of said genetically modified nitrilase enzyme Nit 9N_56_2 for the desired conversion of compound (IV) in stage (b) preferably is chosen from 4 to 25 U per g of compound (IV).

In another embodiment of the present invention wherein during the regio-selective hydrolysis of compound of formula (IV) in stage (b), the pH of the solution is maintained in the range of 7.5+ 0.8 and is maintained by a suitable buffer and are well known in the art; one of the most preferred way to achieve is to use a phosphate or acetate or bicarbonate buffer or maintain the pH with the addition of suitable acid chosen from among acetic, citric, tartaric, hydrochloric, sulfuric, phosphoric acid; and/or a base which is selected from the group consisting of ammonia, mono, di and tri alkyl amine, sodium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate.

In another embodiment of the present invention wherein the substrate is dispersed well with micronization or using dispersion stabilizing agent well known by those skilled in the art before loading of the said enzyme in stage (b).

In another embodiment of the present invention wherein the co-solvent used in stage (b) is selected from group consisting of water, ethyl alcohol, methyl alcohol, isopropyl alcohol, n- propanol, n-butanol, ethyl acetate, dichloroethane, chloroform, tetrahydrofuran, 1,4-dioxane, dimethylformamide, dimethyl sulfoxide, dimethyl acetamide, methyl tert-butyl ether, cyclohexane, toluene and the mixture of solvents thereof.

In another embodiment of the present invention wherein the regio-selective hydrolysis reaction of stage (b) preferably carried out at a temperature range between 20°C to 45°C.

The chemical esterification of racemic 3-cyano-5-methyl-hexanoic acid or salt thereof of formula (V) by following prior art processes results in the formation of two impurities i.e. succinimide impurity and amide ester impurity. So, these impurities are to be removed by high vacuum distillation, however, the removal of these impurities from synthesis by using high vacuum distillation also results in the loss of yield and purity of the product Pregabalin. Succinimide impurity Amide ester impurity

In another embodiment of the present invention wherein the racemic 3-cyano-5-methyl- hexanoic acid or salt thereof of formula (V) is directly subjected to //-selective esterification using lipases which results in formation of (R)-3-cyano-5- methyl hexanoate of formula (VI) and (S)-3-cyano-5-mcthyl-hcxanoic acid or salt of formula (VII). The (R)-3-cyano-5 -methyl hexanoate of formula (VI) is recycled by using racemization followed by hydrolysis to get racemic 3-cyano-5-methyl-hexanoic acid of formula (V).

In another embodiment of the present invention wherein the said enzymatic enantio selective esterification in stage (c) is performed by using commercially available esterification lipases enzymes which is selected from group consisting of esterases, lipolases, lipases. The said esterase or lipase enzymes preferably is selected from the group consisting of Candida Antarctica A, Candida Antarctica Bl, Candida Antarctica BY2, Rhizomucormeihei, Pseudomonas cepecia, Reinase HT, Novozymes 435, (commercially obtained from Chiralvision (Netherland), lipase 3.101 (commercially obtained from Evocatal, Germany), Amano AS, Amano ^(commercially obtained fromAmano, USA).

In another embodiment of the present invention wherein the loading of preferred enzymes in enzymatic esterification stage (c), is in the range of > 0.1 % to < 50% w/w compared to the substrate; more preferably the range is 0.5% to 25% w/w compared to the substrate.

In another embodiment of the present invention wherein the said alcohol (R3OH) in enzymatic esterification stage(c) is preferably selected from the group consisting of methyl alcohol, ethyl alcohol, isopropyl alcohol, n-propyl alcohol, n-butyl alcohol.

In another embodiment of the present invention wherein the said alkyl acetate (R3OAC) in enzymatic esterification of stage (c) is preferably selected from the group consisting of methyl acetate, ethyl acetate, isopropyl acetate, n-propyl acetate, n-butyl acetate. In another embodiment of the present invention wherein the said alkyl halide (R3X) in stage(c)is preferably selected from the group to C5 alkyl halides consisting of methyl iodide, ethyl chloride, ethyl bromide, ethyl iodide, n-propyl bromide, isopropyl chloride, isopropyl bromide.

In another embodiment of the present invention wherein the preferred enzymes in enzymatic esterification in stage (c), is recovered and reused for several times till almost full enzyme activity is retained; while during recycling of enzyme if the activity is less then additional amount of fresh enzyme is added and the additional amount is in the range of 5% to 50 % w/w with respect to initial enzyme loading.

In another embodiment of the present invention wherein the said organic solvent in enzymatic esterification stage (c)is selected from the group consisting of water, methyl alcohol, acetone, methyl isobutyl ketone, acetonitrile, methyl tert-butyl ether, diisopropyl ether, cyclohexane, heptane, toluene, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,4- dioxane, dimethyl sulfoxide, or a mixture of solvents thereof.

In another embodiment of the present invention wherein during enzymatic esterification of compound of formula (V) in stage (c), reaction is performed using with or without buffers. Buffer is prepared by adjusting the initial pH of the solution be kept in the range 7.0 + 1.0 preferably and most preferably in the range of 7.2 + 0.5 by using a suitable reagent selected from the group consisting of acetic acid, citric acid, boric acid, ethylenediaminetetraacetic acid, hydrochloric acid, sulfuric acid, triethyl amine, diisopropylamine, pyridine, sodium bicarbonate, potassium bicarbonate, sodium carbonate, potassium carbonate, calcium carbonate, sodium phosphate dibasic heptahydrate, sodium phosphate monobasic monohydrate, monobasic dihydrogen phosphate, dibasic monohydrogen phosphate, calcium hydroxide, magnesium hydroxide, magnesium oxide or suitable combination thereof. The selection of the amount of this suitable reagent is chosen in a manner so that final pH after completion of reaction does not exceed 8.0.

In another embodiment of the present invention wherein the conversion of racemic 3-cyano- 5-methyl-hexanoic acid or salt thereof of formula (V) to substantially enantiopure (5)-3- cyano-5-methyl-hexanoic acid or salt of formula (VII) in stage (c), the pH of the reaction mixture during the progress of the reaction is maintained at specific pH or canvary slowly in the range of 7 to 9. In another embodiment of the present invention wherein the conversion of racemic 3-cyano- 5-methyl-hexanoic acid or salt thereof of formula (V) to substantially enantiopure (S)-3- cyano-5-methyl-hexanoic acid or salt of formula (VII) in stage (c), the enzymatic step is optionally carried out in presence of salts which is selected from the group consisting of lithium chloride, sodium chloride, potassium chloride, calcium chloride, magnesium chloride or salt is generated in situ by neutralization of suitable acid and a suitable base.

In another embodiment of the present invention wherein the said enzymatic esterification reaction in stage (c) preferably carried out at a temperature range between 20°C to 80°C for 1 to 120 hours, more preferably 25°C to 60°C for 2 to 36 hours.

In an embodiment of the present invention wherein the said base used for racemization of co-product isomer in stage (d) is preferably selected from the group consisting of triethyl amine, diisopropylethyl amine, pyridine, piperidine, l,8-diazabicyclo[5.4.0]undec-7-ene, 1,4- diazabicyclo[2.2.2]octane in pure or supported form or sodium bicarbonate, potassium bicarbonate, sodium carbonate, potassium carbonate, alkali and alkaline earth metal, Ci to C alkoxide or mixture thereof.

In another embodiment of the present invention wherein the said racemization stage (d) optionally performed without solvent or in organic solvent which is selected from the group consisting of methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, acetone, methyl isobutyl ketone, cyclohexane, heptane, toluene, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,4-dioxane, dimethyl sulfoxide or a mixture thereof.

In another embodiment of the present invention wherein the said racemization in stage (d) preferably carried out at a temperature range between 10°C to 120°C for 1 to 12 hours, more preferably 25 °C to 60°C for 2 to 6 hours.

The crude compound of formula (VI) disclosed in stage (d) is used as such or purified by distillation by different techniques well understood by those skilled in the art.

In an embodiment of the present invention wherein the said base used in stage (d) is preferably selected in supported form and advance techniques like RBR, pack column bed is explored. In another embodiment of the present invention wherein, in the stage (e), the said preparation of Pregabalin of formula (I) comprises pH adjustment using bases like triethyl amine, diisopropylamine, pyridine, sodium bicarbonate, potassium bicarbonate, sodium carbonate, sodium hydroxide, potassium hydroxide, lithium hydroxide followed by catalytic hydrogenation while the base strength for hydrolysis is selected from 0. IN to 5N in solvents like water, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-propyl alcohol, n-butyl alcohol, isobutyl alcohol, tertiary butyl alcohol, cyclohexanol, ethyl acetate, heptane, toluene, tetrahydrofuran or a mixture thereof.

In another embodiment of the present invention wherein the said organic solvent in hydrogenation of stage (e)is selected from the group consisting of water, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-propyl alcohol, n-butyl alcohol, isobutyl alcohol, tertiary butyl alcohol, cyclohexanol, toluene, monochlorobenzene, dichlorobenzene, tetrahydrofuran, dioxane, dimethylformamide or a combination thereof.

In another embodiment of the present invention wherein the said hydrogenation catalyst in stage (e)is preferably selected from the group consisting of nickel, palladium, ruthenium, rhodium with or without support and their different chemical forms and grades optionally fresh or recovered or mixture of fresh and recovered catalyst.

In another embodiment of the present invention wherein hydrogenation reaction in the stage (e)preferably carried out at a temperature range between 10°C to 100°C, more preferably 15°C to 60°C.

In another embodiment of the present invention wherein catalytic hydrogenation in the stage (e) is preferably carried out with the hydrogen pressure in the range of 0.5 to 25 kg/cm or equivalent unit.

In yet another embodiment of the present invention for the preparation of Pregabalin of formula (I) comprises optional charcoalization of hydrogenation product and isolation of Pregabalin by preferably isoelectric focusing in the pH range of 6.5 to 7.5.

In yet another embodiment of the present invention for the preparation of Pregabalin of formula (I) comprises isolation of Pregabalin by isoelectric focusing wherein the pH is adjusted with any inorganic or organic acid such as hydrochloric acid, sulfuric acid, acetic acid, phosphoric acid, formic acid, trifluoroacetic acid. In yet another embodiment of the present invention for the preparation of Pregabalin of formula (I) comprises purification of Pregabalin by crystallization of crude from water, to C5 alcohol or a mixture thereof and recovering further amount of pure Pregabalin of formula (I) by recrystallization from concentrated mother liquor.

In still another embodiment of the present invention for the preparation of Pregabalin of formula (I) further comprises alternative recovery of Pregabalin of formula (I) from the mother liquor preferably as an amino protecting derivative such as tert-butyloxycarbonyl, carboxybenzyl, trityl and the like known in the art.

In still another embodiment of the present invention wherein the one or more stage from (a) to (e) of the instant invention may be performed in in-situ manner.

The key compounds involved in the above process are depicted below.

The following non-limiting examples are given by way of illustration of the present invention and therefore should not be construed as limitation of the invention scope.

EXAMPLES

Example 1.0: Preparation of 2-isobutyl-succinonitrile (IV)

The methyl cyanoacetate (450.0 g, 4.541 mol) and isovaleraldehyde (405 g, 4.541 mol) were added. A solution of sodium cyanide (225 g; 4.54 lmol) in water (850 ml) was added drop wise to the above mixture under stirring for 30 minutes. The reaction mixture was heated to reflux for 2 hours. The TBAB (2% w.r.t MCA quantity) in 1.5V of water was added in one lot. The methanol was continuously removed from reaction mixture until temperature of the reaction mixture reaches to 95°C and maintained the reaction temperature at 95°C to 98°C for 5 hours. The reaction mixture was cooled to room temperature and was extracted with (1 X 2V) of toluene. The combined organic layer was washed with (450ml x 2) water. The solvent was removed under reduced pressure to give 609 g of crude compound (IV). The crude compound (IV) was purified by high vacuum distillation to give 510 g of pure compound (IV) (yield 82.47 %, GC purity 99.46 %). Example 2.0: Preparation of racemic 3-cyano-5-methyl-hexanoic acid (V)

The crude compound (IV)(210.0 g, 4.405 mol) and 2793 ml of water (13.3 V) were added at room temperature. The pH of the solution was maintained at 7.5 ± 0.2 using solid NaHC0 ₃ . The reaction mixture was warmed to 30°C and nitrilase enzyme (21.26 g, 125 % loading) was added. The reaction mixture was stirred at 35°C for 24 hours and kept the pH at ~7.5. After 24 hours, the reaction mixture was filtered through celite bed and filtrate was cooled to 0°C to 5°C. The pH of reaction mixture was adjusted to <2.0 using cone hydrochloric acid and stirred for 30 minutes. The compound (V) was isolated by extraction with MTBE (2 x 1.5 V) followed by combined concentration under pressure for 3 to 4 hours to give 220 g of crude compound (V)(Yield 91.97%) (HPLC assay: 96.63%).

Example 3.0: Preparation of (S)-3-cyano-5-methyl-hexanoic acid (VII)

The compound (V) (50.0 g; 0.322 moles, 1.0 eq) and 750 ml (15 V) of toluene was added at 25°C. To this reaction solution lipase (Novozyme 435, 20 % w/w) was added, stirred for 10 minutes and (30 ml, 0.6 eq.) methanol was added drop wise. The reaction mixture was stirred at 25°C for 24 hours, filtered and washed with sat. NaHCCL solution. The organic layer was separated, and pH of aqueous layer maintained to < 2 using cone hydrochloric acid and further extracted with ethyl acetate. The combined organic layer was dried over sodium sulphate and solvent was removed under reduced pressure at 45°C to givel9.1 g of compound (VII, R ₂ is H, yield 38.2%, Chiral HPLC purity of 93%).

Example 3.1: Preparation of (S)-3-cyano-5-methyl-hexanoic acid (VII)

The compound V (10.0 g, 0.0644 moles, 1.0 eq) was suspended into 150 ml of toluene at room temperature. To this solution lipase (Novozyme 435, 20 % w/w) was added and stirred for 10 minutes and methanol (1.56 mL, 0.6 eq.) was added. The reaction mixture was heated to 50 °C for 13 hours, filtered and washed with sat. NaHCCL solution. The organic layer was separated, and pH of aqueous layer was maintained to <2 using cone hydrochloric acid and further extracted with ethyl acetate. The combined organic layer was dried over sodium sulphate and solvent was removed under reduced pressure at 45°C give4.8 g of compound (VII, R ₂ is H, yield 48%, Chiral HPLC purity of 97 %).

Example 4.0: Racemization process: Preparation of racemic 3-cyano-5-methyl-hexanoic acid (V) from enzymatic esterification step

The organic layer from example 3.1, and sodium methoxide (0.1 eq) was added. The reaction solution was heated to reflux temperature for 1 hour and cooled to room temperature. To the reaction mixture water (IV) was added, stirred for 10 minutes and layers were separated. The organic layer was concentrated under reduced pressure and aq. KOH solution was added. The reaction mixture was stirred for 1 hour and then acidified using acid to pH :2.0. The precipitated compound was filtered and dried to give compound (V, R2ISH).

Example 5.0: Preparation of (S)-3-aminomethyl-5-methyl-hexanoic acid (I)

In a methanol (2.5 V), aq. KOH solution (1.5 eq, 32% aq. KOH solution) was added drop wise and stirred for 10 minutes. The reaction mixture was transferred into an autoclave and 10% w/w Raney Nickel and 2.5 V methanol were added. The reaction mixture was kept under stirring in hydrogen pressure of 10+3 Kg/cm at 25°C to 30°C for 10 tol2 hours. After completion of reaction the reaction mixture was filtered through celite bed and washed the bed with methanol (1.0 V). The reaction mixture was again washed through celite bed and bed was washed with methanol (1.0 V). The combined organic layer was acidified using acetic acid to pH 7+0.3. The solvent was removed under pressure and residue was cooled to 5°C to 10°C and maintained for 2 to 5 hours. The compound was isolated by filtration. The filter cake was washed with (1.0 V) of methanol and dried for 30 minutes for to 2 hours to give 5.5g of crude wet cake (I). The crude compound (I) was purified using (11 V) purified water by dissolving and distillation of 8 V of water. The crude suspension was cooled to 40°C and pure compound (I) was obtained by filtration and washing the cake with 1 V IPA. The compound (I) was dried under vacuum for 3 to 5 hours to give 3.5g compound (I, 38% yield; HPLC purity 98.31%).

ADVANTAGES OF THE INVENTION:

1. The improved process for the preparation of Pregabalin as disclosed is more economical, efficient, environment friendly and commercially viable.

2. The improved process for the preparation of Pregabalin as disclosed would be easy to implement on commercial scale, and to avoid excessive use of reagent(s) and hazardous organic solvent(s) which makes present invention eco-friendly as well.

3. The improved process as disclosed gives Pregabalin in a greater yield with higher chemical and chiral purity.

4. The recycling of the (R)-alkyl 3-cyano-5-methyl-hexanoate (R-ester) of formula (VI) to (R,S)-3-cyano-5-mcthyl-hcxanoic acid or salt of formula (V) also reduces the number of synthetic steps. 5. The bi-product such as (A)-alkyl 3-cyano-5- methy l-hcxanoatc(A-cstcr) of formula (VI) formed in the improved process for the preparation of Pregabalin as disclosed is reusable and thereby recyclable, which makes the process industrially more suitable.

6. The improved process for the preparation of Pregabalin avoids the formation of succinimide and amide ester impurities.

The following list of some of abbreviations used in the present invention:

Aq. Aqueous

Boc Tertiary butyloxycarbonyl

cm ² Square centimeter

DM Demineralised

DCM Dichloromethane

DBU 1 ,8-Diazabicycloundec-7 -ene

DIPA Diisopropylamine

DMAP 4-Dimethylaminopyridine

DPA Diphenylamine

DMF Dimethyl formamide

DMSO Dimethyl sulfoxide

ee Enantiomeric excess

eqv. Equivalent

EtOAc Ethyl acetate

g Gram

GC Gas chromatography

h Hour

HPLC High pressure liquid chromatography

i.e. that is

IPA Isopropyl alcohol

Kg Kilogram

Kg/cm 2 Kilogram per Square Centimetre

KotBu Potassium t-butoxide

KU Kilo unit

Lit Liter

MDC Dichloromethane

MeOH Methy alcohol MIBK Methyl isobutyl ketone

ml Milliliter

mmol Milimole

mol Mole

MTBE Methyl tertiary-butyl ether

NMR Nuclear magnetic resonance spectroscopy

No. Number

N Normal

pH Power of hydrogen

RBF Round bottom flask

RPM Rotation per minute

THF T etrahydrofuran

TFC Thin layer chromatography

U Unit

U.S. United States

V Volume

w/w Weight / weight

w.r.t. With respect to