WO2010004577A1

WO2010004577A1 - Preparation of enantiomerically enriched gamma-nitro acid and pregabalin

Info

Publication number: WO2010004577A1
Application number: PCT/IN2009/000342
Authority: WO
Inventors: Lahiri Saswata; Mofazzal Hussain; Debashish Datta
Original assignee: Matrix Laboratories Ltd
Priority date: 2008-06-16
Filing date: 2009-06-12
Publication date: 2010-01-14

Abstract

The present invention provides a novel process for the preparation of enantiomerically enriched γ-nitro acid used an intermediate for the preparation of Pregabalin and further conversion to Pregabalin.

Description

PREPARATION OF ENANTIOMERICALLY ENRICHED GAMMA-NITRO ACID AND PREGABALIN

Field of the Invention

The invention in general relates to the novel process for the preparation of enantiomerically enriched γ-nitro acid used an intermediate for the preparation of Pregabalin. More particularly, the present invention is directed to a process which involves the enzymatic hydrolysis of racemic nitro ester to get enantiomerically enriched γ-nitro acid in high yields, which is further converted to Pregabalin by conventional means.

Background of the Invention

(S)-Pregabalin, (S)-(+)-3-(aminomethyl)-5-methylhexanoic acid, a compound having the chemical structure,

CH₃

is a γ-amino butyric acid or (S)-3-isobutyl (GABA) analogue. (S)-Pregabalin has been found to activate GAD (L-glutamic acid decarboxylase). (S)-Pregabalin has a dose dependent protective effect on-seizure, and is a CNS-active compound. (S)-Pregabalin is useful in anticonvulsant therapy, due to its activation of GAD, promoting the production of GABA, one of the brain's major inhibitory neurotransmitters, which is released at 30 percent of the brains synapses. (S)-Pregabalin has analgesic, anticonvulsant, and anxiolytic activity. Pregabalin has been prepared in various ways. Typically, a racemic mixture of 3- aminomethyl-5-methyl-hexanoic acid is synthesized and subsequently resolved into its R- and S-enantiomers. Such methods may employ an azide intermediate, a malonate intermediate, or Hofmann synthesis. See, respectively, U. S. Patent No. 5,563,175 to R. B. Silverman et al.; U. S. Patent Nos. 6,046,353; 5,840,956 and 5,637,767 to T. M. Grote et al.; and U. S. Patent Nos. 5,629,447 and 5,616,793 to B. K. Huckabee & D. M. Sobieray, which are herein incorporated by reference in their entirety and for all purposes. In each of these methods, the racemate is reacted with a chiral acid (a resolving agent) to form a pair of diastereoisomeric salts, which are separated by known techniques, such as fractional crystallization and chromatography. These methods thus involve significant processing beyond the preparation of the racemate, which along with, the resolving agent, adds to production costs. Moreover, the undesired R-enantiomer is frequently discarded since it cannot be efficiently recycled, thereby reducing the effective throughput of the process by 50%.

Pregabalin has also been synthesized directly using a chiral auxiliary,(4R,5S)-4- methyl - 5-phenyl-2-oxazolidinone. See e.g., U.S. Patent Nos. 6,359,169; 6,028,214; 5,847,151; 5,710,304; 5,684,189; 5,608,090 and 5,599,973 all to R. B. Silverman et al, which are herein incorporated by reference in their entirety and for all purposes. Although these methods provide Pregabalin in high enantiomeric purity, they are less desirable for large- scale synthesis because they employ comparatively costly reagents (e.g. the chiral auxiliary) that are difficult to handle, as well as special cryogenic equipment to reach required operating temperatures, which can be as low as -78⁰C.

U.S. patent application 2003/0212290 Al to Burk et al., discusses a method of making Pregabalin via asymmetric hydrogenation of a cyano-substituted olefin to produce a chiral cyano precursor of (S)-3-aminomethyl-5-methylhexanoic acid. The cyano precursor is subsequently reduced to give Pregabalin. The asymmetric hydrogenation employs a chiral catalyst that is comprised of a transition metal bound to a bisphosphine ligand, such as (R₅R)- Me-DUPHOS. The method results in substantial enrichment of Pregabalin over (R)-3-(aminomethyl)-5-methylhexanoic acid. The method discussed in U.S. Patent Application No. 2003/0212290 Al represents a commercially viable method for preparing Pregabalin, but further improvements would be desirable for various reasons. For example, bisphosphine ligands, including the proprietary ligand (R₅R)-Me-DUPHOS, are often difficult to prepare because they possess two chiral centers, which adds to their cost. Furthermore, asymmetric hydrogenation requires the use of special equipment capable of handling H₂, which adds to capital costs.

Therefore there is a need to develop a process which make the large scale production of Pregabalin eco-friendly, safe and yet economically feasible. Present invention bridges this gap and discloses the novel process, which is environmental friendly as well as eliminates the use of costly chiral ligands or auxiliary, column chromatography and suitable for industrial scale up.

Object of the invention

The main object of the present invention is to provide a novel process for the preparation of enantiomerically enriched (3S)-5-methyl-3-nitromethyl-hexanoic acid (γ-nitro acid) from racemic 5-methyl-3-nitromethyl-hexanoic acid ester.

Another object of the present invention is to provide an improved process for the preparation of (S)-Pregabalin from 5-methyl-3(S)-nitromethyl-hexanoic acid

Yet another object of the present invention is to provide (3S)-5-methyl-3-nitromethyl hexanoic acid having enantiomeric purity more than 96.0 %.

Summary of the Invention

It is therefore, a principal object of the present invention to provide a novel way for producing Pregabalin overcoming the limitations faced in the prior art. These and other objects are attained in accordance with the present invention wherein there is provided several embodiments of the process for producing Pregabalin employing eco-friendly reaction conditions.

In accordance with one preferred embodiment of the present invention, there is provided a novel process for producing γ-nitro acid (2) from racemic nitro ester (1) employing eco- friendly enzymatic conditions in a way to improve the yield and obviate the formation of by-products.

In accordance with another preferred embodiment of the present invention, there is provided a novel process for producing enantiomerically enriched γ-nitro acid (2) from racemic nitro ester (1) without using chiral auxiliaries, which should lead to lower unit costs.

In accordance with yet another preferred embodiment of the present invention there is provided a novel process for producing enantiomerically enriched (3S)-5-methyl-3- nitromethyl-hexanoic acid i.e γ-nitro acid (2) from racemic 5-methyl-3-nitromethyl- hexanoic acid ester (1) with an enzyme, wherein enzyme is adapted to enantioselective hydrolysis of the nitro ester.

In accordance with yet another preferred embodiment of the present invention there is provided a novel process for producing Pregabalin, wherein the racemic nitro ester (1) is first converted to enantiomerically enriched γ-nitro acid (2) using enzymatic hydrolysis and then undergo reduction by conventional means.

Detailed Description of the Invention

The disclosed embodiment of the present invention deals with a novel process for producing Pregabalin by using eco-friendly enzymatic reaction conditions.

The present invention in its aspect is a new, improved, economical and industrially feasible method for producing Pregabalin starting from racemic nitro ester. The nitro ester (1) is first converted to enantiomerically enriched γ-nitro acid (2) which on reduction by conventional means give Pregabalin as shown in scheme 1.

aralkyl

wherein R is alkyl or aralkyl

Reduction

(S)-Pregabalin

Scheme 1

The process for preparing enantiomerically enriched γ-nitro acid (2) includes the step of contacting or combining a reaction mixture, which is comprised of racemic nitro ester (1) and water, with an enzyme. The enzyme (or biocatalyst) may be any protein that will catalyze the hydrolysis of its opposite enantiomer to yield the (S)-γ-nitro acid (2). Useful enzymes for enantioselectively hydrolyzing the racemic nitro ester (1) to (S)-γ-nitro acid (2) may thus include hydrolases, including lipases, certain proteases, and other enantioselective esterases. Such enzymes may be obtained from a variety of natural sources, including animal organs and microorganisms preferably hydrolase is lipase which is Candida antarctica lipase B (Novozyme-435).

The reaction mixture may comprise a single phase or may comprise multiple phases. Thus, for example the enantioselective hydrolysis may take place in a single aqueous phase, which contains the enzyme, the initially racemic substrate (1), the undesired optically active nitro ester (R-I) and the desired optically active nitro acid (2). Alternatively, the reaction mixture may comprise a multi-phase system that includes an aqueous phase in contact with a solid phase (e.g., enzyme or product), an aqueous phase in contact with an organic phase, or an aqueous phase in contact with an organic phase and a solid phase. For example, the enantioselective hydrolysis may be carried out in a two-phase system comprised of a solid phase, which contains the enzyme, and an aqueous phase, which contains the initially racemic substrate (1), the undesired optically- active nitro ester (R-I), and the desired optically-active nitro acid (2).

The amounts of the racemic substrate (1) and the biocatalyst used in the enantioselective hydrolysis will depend on, among other things, the properties of the particular racemic nitro ester and enzyme. Generally, however, the reaction may employ a substrate having an initial concentration of about 0.1 M to about 3.0 M, and in many cases, having an initial concentration of about 1.5 M to about 3.0 M. Additionally, the reaction may generally employ an enzyme loading of about 1% to about 25%, and in many cases, may employ an enzyme loading of about 3% to about 8% (v/v or w/w).

The enantioselective hydrolysis may be carried out over wide ranges of temperature and pH. For example, the reaction may be carried out at a temperature of about 1O⁰C to a temperature of about 5O⁰C, but is typically carried out at about RT. Such temperatures generally permit substantially full conversion (e.g., about 42 % to about 50 %) of the racemate in a reasonable amount of time (about 2 h to about 24 h) without deactivating the enzyme. Additionally, the enantioselective hydrolysis may be carried out at a pH of about 5 to a pH of about 10, more typically at a pH of about 6 to a pH of about 9, and often at a pH of about 6.5 to a pH of about 7.5. In the absence of pH control, the reaction mixture pH will decrease as the hydrolysis of the substrate proceeds because of the formation of the optically active nitro acid. To compensate for this change, the hydrolysis reaction may be run with internal pH control (i.e., in the presence of a suitable buffer) or may be run with external pH control through the addition of a base. Suitable buffers include potassium phosphate, sodium phosphate, sodium acetate, ammonium acetate, calcium acetate, BES, BICINE, HEPES, MES, MOPS, PIPES, TAPS, TES, TRICINE, Tris, TRIZMA@, or other buffers having a pKa of about 6 to a pKa of about 9. The buffer concentration generally ranges from about 5 mM to about 1 mM, and typically ranges from about 50 mM to about 200 mM. Suitable bases include aqueous solutions comprised of KOH, NaOH, NH₄OH etc., having concentrations ranging from about 0.5 M to about 15 M, or more typically, ranging from about 5 M to about 10 M. Other inorganic additives such as calcium acetate may also be used.

Following or during the enzymatic conversion of the racemate, the desired optically active nitro acid (2) is isolated from the product mixture using standard techniques. For example, in the case of a single (aqueous) phase batch reaction, the product mixture may be extracted one or more times with a non-polar organic solvent, such as hexane or heptane, which separates the desired nitro acid and the unreacted nitro ester in aqueous and organic phases, respectively.

According to one embodiment, the present invention provides, process for the preparation of Pregabalin starting from racemic nitro ester of formula 1 where in R is ethyl.

According to one embodiment, the present invention provides (35)-5-methyl-3- nitromethyl hexanoic acid having enantiomeric purity more than 96 %.

The substantially enantiopure nitro acid may be converted to a (S)-Pregabalin by reducing the nitro group via reaction with H₂ in the presence of catalytic amounts of Raney nickel, palladium, platinum, and the like, or through reaction with a reducing agent, such as LiAlH₄, BH₃-Me₂S, and the like. Having thus described the various methods for the preparation of Pregabalin of the present invention, the following examples are provided to illustrate specific embodiments of the present invention. They are however, not intended to be limiting the scope of present invention in any way:

Example: 1

Preparation of γ-Nitro acid (2) from racemic nitro ester (1):

A round bottom flask equipped with overhead stirring was charged with potassium phosphate buffer (50 mL, 1OmM, pH 7.0) and (i?/<S)-5-methyl-3-nitromethyl hexanoic acid ethyl ester 1 (5 gram). The mixture was stirred at 850 RPM for 5 min and NaOH (10% solution) was added to adjust the pH 7.0. Novozyme-435 (125 mg) was added and the resulting mixture was treated with NaOH (10%) during hydrolysis to maintain the pH 7.0. The extent of reaction was monitored by HPLC. As about 45 % nitro acid 2 was formed (e.g: after 2 to 5 hrs), the enzyme was filtered off and the reaction mixture was transformed into separating funnel. The aqueous mixture was extracted with hexane (100% v/v) and the aqueous and organic layer was separated. The extraction step was repeated twice to obtain an aqueous layer containing (35)-5-methyl-3-nitromethyl hexanoic acid sodium salt. The organic layer containing (i?)-5-methyl-3-nitromethyl hexanoic acid ethyl ester are combined, dried and concentrated. The resulting nitro ester was again used for hydrolysis.

The aqueous layer which contains (3iS)-5-methyl-3-nitromethyl hexanoic acid sodium salt was acidified (pH 4.0) with dil HCl and was extracted with dichloromethane (100% v/v). The extraction step was repeated twice to obtain an organic layer containing (3S)-5- methyl-3-nitromethyl hexanoic acid 2. The organic layer was combined, dried and concentrated to the (35)-5-methyl-3-nitromethyl hexanoic acid 2 as viscous oil. The optical purity of the (3S)-5-methyl-3-nitromethyl hexanoic acid is -96.0 % ee.

Example 2

Reduction of the (3S)-5-methyl-3-nitromethyl hexanoic acid 2 into (S)-Pregabalin To a MeOH (10 ml) solution of (3S)-5-methyl-3-nitromethyl hexanoic acid 2 (1 g) was added 5% Pd/C (600mg) at room temperature and the reaction mixture was stirred for 48 h under H2 atmosphere. The reaction mixture was filtered through a pad of Celite and concentrated under reduced pressure to afford solid. The solid was dissolved in IPA: H2O after some time white solid precipitated out which was filtered and washed with IPA to afford (S)-(+) Pregabalin (600 mg) with >98% ee.

Claims

We Claim:

1. A process for preparing a Pregabalin intermediate of the following formula (S)-2

comprising:

(i) reacting by enantioselective hydrolysis of (i?/5)-5-methyl-3-nitromethyl hexanoic acid ester of formula I

wherein R is alkyl or aralkyl

I using hydrolase and controlling pH..

2. The process of claim 1, wherein the enantioselective hydrolysis reaction is carried out using hydrolase selected from lipase, protease and esterase.

3. The process of claim 2, wherein the hydrolase is Lipase which is Candida antarctica lipase B (Novozyme-435).

4. The process of claim 1, wherein the enantioselective hydrolysis reaction is carried out by controlling the pH of the reaction.

5. The process of claim 4, wherein pH of the reaction can be controlled either internally in presence of suitable buffer or externally by addition of base.

6. The process of claim 5, wherein the buffer used for controlling the pH of the reaction is selected from potassium phosphate, sodium phosphate, sodium acetate, ammonium acetate, calcium acetate, BES, BICNE, HEPES, MES, MOPS, TAPS, TES, TRICINE, Tris, TRIZMA@, or other buffers having a pKa of about 6 to a pKa of about 9.

7. The process of claim 5, wherein the base used for controlling the pH of the reaction is selected from potassium hydroxide, sodium hydroxide, ammonium hydroxide.

8. (35)-5-methyl-3-nitromethyl hexanoic acid having enantiomeric purity more than 96.0 %.

9. The process according to claim 1, further comprising the steps of:

(i) subjecting the obtained enantiopure (3S)-5-methyl-3-nitromethyl hexanoic acid sodium salt to hydrogenation reaction; and

(ii) isolating (S)-Pregabalin.

10. The process of claim 9, wherein (35)-5-methyl-3-nitromethyl hexanoic acid is subjected to hydrogenation reaction using hydrogen in the presence of catalyst selected from Raney nickel, palladium, platinum or a reducing agent such as LiAlH₄ or BH₃-Me₂S.