DESC:FIELD OF INVENTION:
The invention relates to a commercially viable green process for manufacturing (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid. Particularly this invention relates to an enzymatic process of making (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid in high yield with high chemical and chiral purity.

BACKGROUND OF THE INVENTION:
Pregabalin, also known as (S)-3-isobutyl GABA is marketed under the name LYRICA®. It has been found to treat epilepsy, fibromyalgia, neuropathic pain, fibromyalgia, restless leg syndrome, and generalized anxiety disorder. Pregabalin is a gabapentinoid and acts by inhibiting certain calcium channels and is a CNS-active compound. It has been known to stimulate the brain expression of L-glutamic acid decarboxylase (GAD), the enzyme responsible for promoting the production of GABA (inhibitory neurotransmitter).

An important intermediate in the preparation of Pregabalin is 3-(carbamoylmethyl)-5-methylhexanoic acid. As can be seen in EP 828704, which discloses the usage of 3-(carbamoylmethyl)-5-methylhexanoic acid in the preparation of pregabalin.

Considering the importance of Pregabalin and therefore the importance of (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic acid, Multiple efforts have been made in the industry to invent a process for preparing (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic acid. A few examples can be seen hereunder:

US 5,616,793 discloses a process (scheme-1) to prepare (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic acid , which comprises, forming the anhydride of 3-isobutylglutaric acid; reacting the anhydride with ammonia to form (±)-3-(carbamoylmethyl)-5-methylhexanoic acid; reacting (±)-3-(carbamoylmethyl)-5-methylhexanoic acid with (R)-(+)-a-phenylethylamine to obtain the (R)-(+)-a-phenylethylamine salt of (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic acid; combining the salt with an acid to obtain (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic acid.
Scheme-1

CN104496832discloses a process (scheme-2) to prepare (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic acid, which comprises, forming 4-isobutylpiperidine-2,6-dione of 3-isobutylglutaric acid; reacting 4-isobutylpiperidine-2,6-dione with aqueous sodium hydroxide to form (±)-3-(carbamoylmethyl)-5-methylhexanoic acid; reacting (±)-3-(carbamoylmethyl)-5-methylhexanoic acid with (R)-(+)-a-phenylethylamine to obtain the (R)-(+)-a-phenylethylamine salt of (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic acid; combining the salt with an acid to obtain (R)-(-)-3-(carbamoylmethyl)-5-methylhexanoic acid.

However, above mentioned optical resolution methods, the yield cannot be expected to reach 50 % or greater.

U.S. Pat. No. 7,462,738 discloses (scheme-3) the asymmetric alcoholysis of 3-isobutylglutaric anhydride to afford hemiester. Enantioselective alcoholysis of 3-isobutylglutaric anhydride is obtained utilizing either chiral alcohol or achiral alcohol in combination with a chiral amine. Amidation of enantioenrichedhemiester with aqueous ammonia yields (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid.

Scheme-3

Jung, Jae-Hoon et al, Organic & Biomolecular Chemistry, 11(22), 3635-3641; 2013, reported (scheme-4) lipase catalyzed desymmetrization of 3-isbutylglutaric acid diester to prepare optically active (S)-3-isobutylglutaric acid monoester in 93 % ee, followed by conversion to (S)-(+)-3-aminomethyl-5-methylhexanoic acid with Mg3N2 and methanol.

Scheme-4
Nojiri, M. et al, Journal of Applied Microbiology, 115(5), 1127-1133; 2013, reported (scheme-5) synthesis of a (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid by desymmetrization of 3-isobutyl glutaric acid diamide with amidase enzyme with >99 % e.e.

Scheme-5

The yield reported for 3-isobutyl glutaric acid diamide is only 28 % from 3-isobutyl glutaric acid.

Evidently, there is a lack of an innovative process for manufacturing (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid which ensures a high yield.

Considering the importance of the chemical product and considering the multiple efforts to manufacture (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid, it is evident that there is a need in the art for an advanced process that guarantees a higher yield than presently available, which is eco-friendly, safe and economically & industrially feasible for a large scale production and which successfully eliminates the use of expensive reagents.

Therefore, there is a need for a process to overcome at least one of the abovementioned shortcomings.

SUMMARY OF THE INVENTION:
Before the present process, is described, it is to be understood that this application is not limited to the particular disclosure, and details described, as there can be multiple possible embodiments that are not expressly illustrated in the present disclosure. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions or embodiments only and is not intended to limit the scope of the present application. This summary is provided to introduce concepts related to theprocess of making (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid and are further described below in the detailed description. This summary is not intended to identify essential features of the subject matter nor is it intended for use in limiting the scope of the subject matter.

An aspect of the present invention is to provide a process for making (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid. More specifically, the present invention discloses a process for an eco-friendly, safe and economically feasible process and moreover, a process that results in a much higher yield than processes available in the market with high chemical and chiral purity.

A further aspect of the present invention is to provide an enzymatic process for the preparation of (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid having >99 % chiral purity. Wherein, the process comprises of the following steps:
i. Adding 4-isobutylpiperidine-2,6-dione (1) to a buffer forming a primary reagent mix;
ii. Adding to the primary reagent mix, an enzyme;
iii. Reacting the reagents to form a reaction mass;
iv. Adding a base to maintain the pH between 6.5-8.5pH throughout the reaction process;
v. Filtering the reaction mass to remove enzyme as solid mass from the filtrate;
vi. Performing solvent extraction to extract the filtrate;
vii. Adjusting the reaction mass pH to acidic by isolating the optically active (R)-(-)-3-(Carbamoylmethyl)- 5-methylhexanoic acid (2)
viii. Separating (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid (2) as the precipitate, by filtering or by other industrially acceptable methods;
ix. Washing the precipitate with chilled water resulting in a wet cake; and
x. Drying the wet cake under a vacuum produces the product in the desired form.
Wherein, the enzyme is a hydrolase enzyme and the structures of reagents involved in the process are as mentioned below:

4-isobutylpiperidine-2,6-dione (1)

(R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid (2)
The above process is illustrated in the following general synthetic scheme (scheme-6)

Scheme-6

BRIEF DESCRIPTION OF DRAWINGS:
The detailed description of embodiments is better understood when read in conjunction with the appended figure. For the purpose of illustrating some of the embodiments of the present document, the figure is shown; however, the disclosure is not limited to the process disclosed in the document and the figure.

Fig. 1: Shows a chromatogram showing the chiral purity tested of the product made using the present invention in accordance with an exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION:
Some embodiments of this disclosure, illustrating all its features, will now be discussed in detail. The words "comprising," "having," "containing," and "including," and other forms thereof, are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. It must also be noted that as used herein and in the appended claims, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. Although any systems and methods similar or equivalent to those described herein can be used in the practice, the exemplary, systems and methods are now described. The disclosed embodiments are merely exemplary of the disclosure, which may be embodied in various forms.

Various modifications to the embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. However, one of ordinary skill in the art will readily recognize that the present disclosure is not intended to be limited to the embodiments illustrated but is to be accorded the widest scope consistent with the principles and features described herein.

According to various embodiments of the present invention that are described below a process for manufacturing (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid is disclosed. More specifically, the present invention relates to a process for an eco-friendly, safe and economically feasible process and moreover, a process that results in a much higher yield than processes available in the market.

In an embodiment of the present invention, an enzymatic process for the preparation of (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid as illustrated in Scheme 6 (see below) is disclosed. Wherein the process comprises of at least the following steps:
i. Adding 4-isobutylpiperidine-2,6-dione (1) to a buffer forming a primary reagent mix;
ii. Adding to the primary reagent mix, an enzyme;
iii. Reacting the reagents to form a reaction mass;
iv. Adding a base to maintain the pH between 6.5-8.5pH throughout the reaction process;
v. Filtering the reaction mass to remove enzyme as solid mass from the filtrate;
vi. Performing solvent extraction to extract the filtrate;
vii. Adjusting the reaction mass pH to acidic and isolating the optically active (R)-(-)-3-(Carbamoylmethyl)- 5-methylhexanoic acid (2)

Wherein, the structures of reagents involved in the process are as mentioned below:

4-isobutylpiperidine-2,6-dione (1)

(R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid (2)
The above process is illustrated in the following general synthetic scheme (scheme-6)

Scheme-6

In a further embodiment of the present invention the process further includes:
i. Separating (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid (2) as the precipitate, by filtering or by other industrially acceptable methods;
ii. Washing the precipitate with chilled water resulting in a wet cake; and
iii. Drying the wet cake under a vacuum produces the product in the desired form.
In an embodiment of the present invention, theenzyme is added to the reagent mix along with at least a buffer in the presence of a base forming a final reagent mix.

In an embodiment of the present invention, the enzymatic process yields exceed 89% under moderate temperature of about 40°C. Wherein the enzyme is preferably a hydrolase enzymecapable of asymmetrically hydrolyzing 4-isobutylpiperidine-2,6-dione in a stereoselective manner.

REAGENT ADDITION
In an embodiment of the present invention, reagent 4-isobutylpiperidine-2,6-dione is added to a buffer to form a primary reagent mix. A hydrolase enzyme is added to the primary reagent mix to form a final reagent mix. Wherein, the hydrolase enzyme is added to the primary reagent mix at ambient temperature of about 40°C. As the stereoselectivehydrolysis reaction of 4-isobutylpiperidine-2,6-dione progresses, the pH goes towards an acidic side, which is readjusted to 8pH by addition of 3 M potassium hydroxide solution or such other base. The base is added throughout the reaction process to maintain the pH between 6.5-8.5pH, preferably 8pH.
The buffer is selected from but not limited to ammonium phosphate buffer or potassium phosphate buffer or mixtures thereof

REACTION
In one embodiment, the stereoselective hydrolysis reaction is carried out by stirring the reagents contained in the final reagent mix at 40°C, under controlled pH ranging from 6.5pH to 8.5pH, preferably ranging from 7.0pH to 8.0pH and most preferably 8.0pH. The pH is controlled by adding a base. The base used in this process is selected from sodium hydroxide or potassium hydroxide, preferably 3 M potassium hydroxide solution or such other industrially acceptable base and wherein the base maintains a reaction pH of 6.5 - 8.5ph, preferably 8pH. Time taken for the reaction ranges from 24 to 120 hours. In an embodiment of the present invention, the reaction is carried out at a temperature range of 25?C-45?C with constant stirring.

The step of adding the enzyme is performed while stirring the reaction mixture at an ambient temperature and under controlled pH ranging from 6.5pH to 8.5pH. Throughout such reaction time the stirring is conducted continuously and the base is added to maintain a controlled pH. At the completion of reaction time, the process culminates in a reaction mass containing enzyme (solid mass), buffer solution (liquid form) and the optically active product (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid as a potassium salt (liquid form).

FILTRATION
In an embodiment of the present invention the reaction mass obtained after the completion of the stipulated reaction time, is a heterogeneous mixture wherein the enzyme is present in a solid state while the product (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid is in the liquid part as a potassium salt. The enzyme is separated from the filtrate as a solid mass using an industrially acceptable method, selected from but not limited to Nutsche filtration, centrifugation, agitated nutsche filtration. The filtrate thus obtained is free of the enzyme and contains (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid in a soluble form as a potassium salt.

The filtrate obtained after the filtration process is subjected to solvent wash followed by neutralization with the use of a hydrochloric acid or the like to remove the starting material or any organic impurities present.
The resulting precipitate of interest is filtered for separation. Wherein the solvent is selected from but not limited to Methylene chloride, chloroform, toluene, ethyl acetate. methyl t-butyl ether

ISOLATION
In an embodiment of the present invention, the (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid present as a soluble salt in the filtrate is isolated from the reaction solution by lowering the pH to a range of 5pH to 2pH and preferably 2pH. The pH of the reaction is lowered by adding an acid selected from sulphuric acid or hydrochloric acid or their mixtures thereof, preferably hydrochloric acid.
The precipitated solid form of the product ((R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid) is separated by filtration or such other acceptable methods and washed with chilled water. The obtained cake is dried under a vacuum to produce the desired product. The filtrate left behind is to be discarded. The process of the present invention produces >89% yield of (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid having optical purity >99 %.
UTILITY
The process disclosed in this invention has at least the following advantages and utility:
• Isolation of product in high yields is possible with ease.
• Isolation of product in enantiomeric excess from the reaction mixture is possible with ease.
• The process is eco-friendly
• The process is safe, economically, industrially feasible for large scale productions
• The process successfully eliminates use of expensive reagents

UNIQUE ELEMENTS OF INVENTION
The process disclosed in the present invention is unique in at least the following manner:
• Use of enzyme for production of optically active (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid
• The stereoselective reaction is carried out at ambient temperatures
• Steps involved in the process are greatly simplified as compared to processes currently available
• There is no waste of the starting material as all of it is reacted with the enzyme
• In the stereoselective process of the present invention there is no need for optical resolution
• The process successfully eliminates use of expensive reagents
• The process results in impressively high yields

INDUSTRIAL APPLICABILITY
It is apparent that the process disclosed in this document provides a cost-effective solution for an eco-friendly, safe & sustainable means of producing (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid. It is also evident that the yields produced by this process are astonishingly higher than available in the prior art. Further, the use of enzyme greatly reduces the requirement of multiple expensive reagents that require extensive steps and conditions to produce (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid.

The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.

Example 1:
Process for the preparation of R (-)-3-(Carbamoylmethyl)-5-methyl hexanoic acid
4-isobutylpiperidine-2,6-dione (100 g) was added to 700 ml 200 mM ammonium phosphate buffer (pH 8) at 40°C. To this reaction mixture 7 g hydrolase enzyme was added and pH was adjusted to 8 by the addition of 3 M potassium hydroxide solution for 120 hours. The reaction mass was filtered to remove enzyme. The filtrate was extracted with 250 ml chloroform. The pH of the filtrate was adjusted to 2pH by adding hydrochloric acid. The reaction mass was filtered and washed with chilled water. The obtained wet cake was dried under vacuum to yield R(-)-3-(Carbamoylmethyl)-5-methyl hexanoic acid (98.6 g). Yield 89.6 % , purity by HPLC 99.54 % with 99.8 % e.e.
EXPERIMENTAL DATA:

Enantiomeric excess is a measurement of purity used for chiral substances. It reflects the degree to which a sample contains one enantiomer in greater amounts than the other. A racemic mixture has an ee of 0%, while a single completely pure enantiomer has an ee of 100%

A purity test was conducted for the product made using the process of the present invention. The resultant enantiomeric excess of >99% and a purity of >99% were observed.

Please see Figure 1 for the corresponding graph to the experimental results found after conducting the above-mentioned tests.

Therefore, the quality of the product made using the present invention was established as exceptionally desirable.

The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to a person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.
,CLAIMS:

[Claim 1.] A process for making (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid comprising the following steps:
i. Adding 4-isobutylpiperidine-2,6-dione (1) to a buffer forming a primary reagent mix;
ii. Adding to the primary reagent mix, an enzyme;
iii. Reacting the reagents to form a reaction mass;
iv. Adding a base to maintain the pH between 6.5-8.5pH throughout the reaction process;
v. Filtering the reaction mass to remove enzyme as solid mass from the filtrate;
vi. Performing solvent extraction to extract the filtrate;
vii. Adjusting the reaction mass pH to acidic by isolating the optically active (R)-(-)-3-(Carbamoylmethyl)- 5-methylhexanoic acid (2)
[Claim 2] The process as claimed in claim 1 wherein the enzyme is a hydrolase enzyme
[Claim 3] The process as claimed in claim 1 wherein the enzyme is added to the reagent mix along with a buffer in the presence of a base forming a final reagent mix
[Claim 4] The process as claimed in claim 1 wherein the solvent extraction is performed by a solvent being ethyl acetate, chloroform, methylenechloride, toluene or methyl-t-butyl ether and wherein the filtrate is Neutralized using hydrochloric acid or the like

[Claim 5] The process as claimed in claim 1 wherein the buffer having an 8 pH is selected from ammonium phosphate buffer or potassium phosphate buffer or mixtures thereof

[Claim 6] The process as claimed in claim 1 wherein the base is 3 M potassium hydroxide solution or such other industrially acceptable base and wherein the base maintains a reaction pH of 6.5 - 8.5ph, preferably 8pH.

[Claim 7] The process as claimed in claim 1 whereinthe process produces >89% yield of (R)-(-)-3-(carbamoylmethyl)-5-methyl hexanoic acid having optical purity >99%.
[Claim 8] The process as claimed in claim 1 wherein the reaction is carried out over a period of 24 hours – 120 hours, at a pH of 6.5-8.5, at a temperature range of 25?C-45?C,with continuous stirring
[Claim 9] The process as claimed in claim 1 wherein the process further includes:
i. Separating (R)-(-)-3-(Carbamoylmethyl)-5-methylhexanoic acid (2) as the precipitate, by filtering or by other industrially acceptable methods;
ii. Washing the precipitate with chilled water resulting in a wet cake; and
iii. Drying the wet cake under vacuum producing the desired product.

[Claim 10] The process as claimed in claim 1 wherein the process includes Isolating (R)-(-)-3-(Carbamoylmethyl)- 5-methylhexanoic acid (2) at an acidic pH range of 5pH-2pH, preferably 2 pH and wherein such acidic pH is achieved by employing hydrochloric acid, alternatively by sulfuric acid or a mixture thereof.