DESC:
FIELD OF THE INVENTION:
The present invention relates to a process for preparation of tau-fluvalinate. The present invention also relates to the process for purification of tau-fluvalinate.

BACKGROUND OF THE INVENTION:
Fluvalinate is an active ingredient belonging to the pyrethroid group of compounds and is used as an insecticide for the control of wide range of insects by acting on the nervous system of insects and disturbing the function of neurons by interaction with the sodium channel. Fluvalinate is later replaced by Tau-fluvalinate which is a diastereomeric mixture of RR and SR fluvalinate. It is chemically known as (SR)-a-cyano-3-phenoxybenzyl N-(2-chloro-4-trifluoro- p-tolyl)-D-valinate and is a 1:1 mixture of (R)-a-cyano-, 2-(R)- and (S)-a-cyano-, 2-(R)- diastereoisomers.

US4260633 discloses the preparation of tau-fluvalinate by reacting (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoic acid with racemic a-cyano-3-phenoxybenzyl alcohol. The product obtained is further purified by preparative thin layer chromatography (TLC) method employing 10% ether/hexane mixture. The product obtained after purification gives diastereomer pair consisting of (S)-a-cyano-3-phenoxybenzyl (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoate (42%) and (R)-a-cyano-3-phenoxybenzyl (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoate (52%). the process as disclosed in the '633 has several disadvantages. The '633 process results in a product with a low diastereomeric purity. In order to comply with the regulatory requirement, further efforts are required for improving the diastereomeric isomer ratio of tau -fluvalinate (RR : SR) - 50 : 50 (±2) . Further, the byproducts, such as dicyclohexylurea, formed during the reaction of the process as disclosed in the US'633 patent are not separable from the reaction mixture during the work up due to their similar solubility in the reaction solvent. This affects the overall purity and yield of the product. Additionally, purification of the crude product by thin layer chromatography as disclosed in above patent is also not commercially suitable.

Therefore, there is a need to develop a commercially viable process which gives tau-fluvalinate with desired chiral purity as well as an improved chemical purity. The inventors of the present invention have developed one such industrially suitable process, which addresses the drawbacks of the prior art process.

OBJECTIVES OF THE INVENTION:
A primary objective of the present invention is to provide a process for purification of tau-fluvalinate.

Another objective of the present invention is to provide a process for preparation of tau-fluvalinate with desired chiral purity.

Yet another objective of the present invention is to provide tau-fluvalinate substantially free of impurities.

SUMMARY OF THE INVENTION:
A primary aspect of the present invention is to provide a process for purification of tau-fluvalinate, the process comprising treating tau-fluvalinate with a hydrocarbon solvent system.

Another aspect of the present invention is to provide a process for preparation of tau-fluvalinate with desired chiral purity, wherein the said process comprises, reacting (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoic acid with hydroxy(3-phenoxyphenyl)acetonitrile in presence of a solvent system comprising a mixture of a halogenated hydrocarbon and a hydrocarbon solvent.

In yet another aspect, the present invention provides a process for preparation of tau-fluvalinate with desired chiral purity wherein the said process comprises:
a) reacting (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoic acid with hydroxy(3-phenoxyphenyl)acetonitrile in presence of a solvent system comprising a mixture of a halogenated hydrocarbon and a hydrocarbon solvent to obtain crude tau-fluvalinate having purity less than about 90 %; and
b) treating the crude tau-fluvalinate with a hydrocarbon solvent system comprising a hydrocarbon solvent and an adsorbent.

DETAILED DESCRIPTION OF THE INVENTION:
In order to provide a clear and consistent understanding of the terms used in the present specification, a number of definitions are provided below. Moreover, unless defined otherwise, all technical and scientific terms as used herein have the same meaning as commonly understood by the person of ordinary skill in the art to which this invention pertains.

As used in this specification the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “include” and “includes”) or “containing” (and any form of containing, such as “contain” and “contains”), are inclusive or open-ended and do not exclude additional, unrecited elements or process steps.

The term “impurities” refers to unreacted synthetic intermediates, reagents, solvents, organic and/or inorganic products of side reactions, organic and/or inorganic salts and/or other undesired materials.

The term “room temperature” unless stated otherwise, refers to a temperature in the range of about 20°C to about 35°C.

The term “crude tau-fluvalinate” unless stated otherwise, refers to tau-fluvalinate having purity less than about 90%.

The term “desired chiral purity” refers to the desired chiral ratio of diastereoisomers (R)-a-cyano-, 2-(R)-, designated as (RR) and (S)-a-cyano-, 2-(R)-, designated as (SR) isomers of Tau-fluvalinate. With respect to the present invention, the desired chiral ratio of (RR: SR) isomers is in a range of about 50 (±2):50 (±2).

For the purpose of present invention, the term “chiral ratio” is interchangeably used for diastereomeric ratio of (RR) and (SR) diastereoisomers of tau-fluvalinate, which are structurally represented as,

(R)-a-cyano-3-phenoxybenzyl (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoate; and

(S)-a-cyano-3-phenoxybenzyl (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoate.

The ratio “(RR: SR) - 50 (±2):50 (±2)” includes RR isomer in a range of about 48 to 52% and SR isomer is in a range of about 48 to 52%.

The ratio “(RR: SR) - 50 (±1):50 (±1)” includes RR isomer in a range of about 49 to 51% and SR isomer in a range of about 49 to 51%.

The term “hydrocarbon solvent system” as used herein comprises a hydrocarbon solvent and an adsorbent, which is used in the purification process for tau-fluvalinate.

The term “solvent system” as used herein comprises a mixture of a halogenated hydrocarbon and a hydrocarbon solvent, which is used in the preparation process for tau-fluvalinate.

Accordingly, the present invention is now described in connection with certain embodiments of the invention which will be apparent to a person skilled in the art from the examples provided herewith.

Accordingly, an aspect of the present invention is to provide a process for purification of tau-fluvalinate, the process comprising treating tau-fluvalinate with a hydrocarbon solvent system.

In another embodiment, the present invention provides a process for purification of tau-fluvalinate, the process comprising treating tau-fluvalinate having purity less than about 90% with a hydrocarbon solvent system comprising a hydrocarbon solvent and an adsorbent.

In another embodiment, said treatment with a hydrocarbon solvent system does not involve treating tau-fluvalinate by using column chromatography or preparative thin layer chromatography. In another embodiment, the treatment with a hydrocarbon solvent system comprises treating tau-fluvalinate with methods other than column chromatography or preparative thin layer chromatography.

In an embodiment, said treatment of tau-fluvalinate avoids use of column chromatography or preparative thin layer chromatography.

In an embodiment, tau-fluvalinate subjected to purification according to the present invention is prepared by any known method. In an embodiment, tau-fluvalinate subjected to purification comprises a purity of less than about 90%.

In an embodiment, the hydrocarbon solvent system comprises a hydrocarbon solvent and an adsorbent.

In an embodiment, the hydrocarbon solvent of the hydrocarbon solvent system is selected from saturated or unsaturated hydrocarbons such as n-hexane, n-heptane, cyclohexane, toluene, xylene, benzene, or mixtures thereof. In a preferred embodiment, the hydrocarbon solvent is n-hexane. In a preferred embodiment, the hydrocarbon solvent is n-heptane.

In an embodiment, the adsorbent of the hydrocarbon solvent system is selected from silica, activated carbon or resin. In a preferred embodiment the adsorbent is silica.

In an embodiment, the adsorbent used for the treatment of tau-fluvalinate is reusable. In another embodiment, said treatment with hydrocarbon solvent may be repeated by re-slurring.

In an embodiment, in the hydrocarbon solvent system, a weight ratio of the adsorbent to the hydrocarbon solvent is in a range from about 1:10 to about 1:15. In a preferred embodiment, the weight ratio of the adsorbent to the hydrocarbon solvent is in a range from about 1:13 to about 1:15. In a preferred embodiment, the weight ratio of adsorbent to the hydrocarbon solvent is in a range of about 1:10.

In another embodiment, the adsorbent and tau-fluvalinate are present in a weight ratio ranging from about 0.1:1 to about 1:1.

In another embodiment, the treatment of tau-fluvalinate with the hydrocarbon solvent system is performed at a temperature ranging from about 10°C to about80 °C. In another embodiment, the treatment of tau-fluvalinate with the hydrocarbon solvent system is performed at a temperature ranging from about 20°C to about 40 °C. In another embodiment, the treatment of tau-fluvalinate with the hydrocarbon solvent system is performed at a temperature ranging from about 20°C to about 30 °C.

In another embodiment, the present invention provides a process for preparation of tau-fluvalinate with desired chiral purity, the process comprising reacting (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoic acid with hydroxy(3-phenoxyphenyl)acetonitrile in a solvent system comprising a mixture of a halogenated hydrocarbon and a hydrocarbon solvent.

In accordance with above embodiment, the halogenated hydrocarbon is selected from methylene dichloride, ethylene dichloride, mono-chlorobenzene or mixtures thereof. In accordance with above embodiment, the halogenated hydrocarbon is methylene dichloride. In a preferred embodiment, the halogenated hydrocarbon is ethylene dichloride.

In an embodiment, the hydrocarbon solvent is selected from saturated or unsaturated hydrocarbon such as n-hexane, n-heptane, cyclohexane, toluene xylene, benzene or mixtures thereof. In a preferred embodiment, the hydrocarbon solvent is n-heptane. In a preferred embodiment, the hydrocarbon solvent is toluene.

In an embodiment, the mixture of the halogenated hydrocarbon and the hydrocarbon solvent comprises methylene dichloride and n-heptane, respectively. In an embodiment, the mixture of the halogenated hydrocarbon and the hydrocarbon solvent comprises ethylene dichloride and n-heptane, respectively. In an embodiment, the mixture of the halogenated hydrocarbon and the hydrocarbon solvent comprises methylene dichloride and toluene, respectively. In an embodiment, the mixture of the halogenated hydrocarbon and the hydrocarbon solvent comprises ethylene dichloride and toluene, respectively.

In an embodiment, a weight ratio of the halogenated hydrocarbon and the hydrocarbon solvent is in a range from about 20:80 to about 80:20, preferably from about 40:60 to about 60:40, preferably from about 30:70 to about 70:30, and more preferably of about 50:50.

In an embodiment, the reaction is carried out in presence of a coupling agent and a base.

In an embodiment, the reaction is carried out in presence of a base. In an embodiment, the base is selected from 4-Dimethylaminopyridine (DMAP), Triethylamine (TEA) or diisopropylethylamine. In an embodiment, the base is 4-Dimethylaminopyridine.

In an embodiment, the reaction is carried out in presence of a coupling agent. In an embodiment, the coupling agent is selected from N, N-Dicyclohexylcarbodiimide (DCC), O-(Benzotriazol-1-yl)-N,N,N',N'-tetramethyluronium tetrafluoroborate (TBTU), 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) or N,N'-Diisopropylcarbodiimide (DIC). In an embodiment, the coupling agent is N, N-Dicyclohexylcarbodiimide. In another embodiment, the coupling agent is 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC).

In an embodiment, the reaction is carried out at a temperature ranging from about -20°C to about 10°C. In an embodiment, the reaction is carried out at a temperature ranging from about -15°C to about 10°C.

In another embodiment, the process according to the present invention provides tau -fluvalinate comprising diastereomeric pair of (S)-a-cyano-3-phenoxybenzyl (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoate 50(±2)% and (R)-a-cyano-3-phenoxybenzyl (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoate 50(±2)%.

In an embodiment, the preferred diastereomeric ratio of (S)-a-cyano-3-phenoxybenzyl (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoate and (R)-a-cyano-3-phenoxybenzyl (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoate is about 50(±1):50(±1).

Accordingly, the present invention provides the process for preparation of tau-fluvalinate with 50(±1):50(±1) diastereomeric ratio, wherein the said process comprises: reacting (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoic acid with hydroxy(3-phenoxyphenyl)acetonitrile in a solvent system comprising a mixture of a halogenated hydrocarbon and a hydrocarbon solvent.

Advantageously, the hydrocarbon solvent system used in the purification process of the present invention effectively removes undesired impurities, byproducts, such as dicyclohexylurea, during the work up process and provides improved purity compound.

In another embodiment, the present invention provides a process for preparation of tau-fluvalinate with desired chiral purity more than about 90%; wherein the said process comprises:
a) reacting (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoic acid with hydroxy(3-phenoxyphenyl)acetonitrile in a solvent system comprising a mixture of a halogenated hydrocarbon and a hydrocarbon solvent to obtain crude tau-fluvalinate having purity less than about 90 %; and
b) treating the crude tau-fluvalinate with a hydrocarbon solvent system comprising a hydrocarbon solvent and an adsorbent.

In an embodiment, the hydrocarbon solvent system of step b) comprises a hydrocarbon solvent and an adsorbent.

In an embodiment, the tau-fluvalinate comprises diastereomeric pair of (RR) and (SR) in a ratio of about 50(±2):50 (±2).

In an embodiment the hydrocarbon solvent of the hydrocarbon solvent system of step b) is selected from saturated or unsaturated hydrocarbon such as n-hexane, n-heptane, cyclohexane xylene, benzene, toluene or mixtures thereof. In an embodiment, the hydrocarbon solvent is n-hexane. In an embodiment, the hydrocarbon solvent is n-heptane.

In an embodiment, the adsorbent of the hydrocarbon solvent system is selected from silica, activated carbon or resin. In an embodiment, the adsorbent is silica.

In another embodiment, the present invention provides tau-fluvalinate with chemical purity of more than about 90% w/w by high-performance liquid chromatography (HPLC).

EXAMPLES:
The embodiments of the present invention are illustrated by below given examples. However, the scope of the present invention is not limited by the examples.

Analytical methods:
Samples were analyzed with high performance liquid chromatography (HPLC) with UV detector using Zorbax SB – C18 (150 x 4.6 mm i.d., 3.5 micron) & Chiralpak OD-H (250 X 4.6 mm i.d.).

Example 1: Purification of tau-fluvalinate
Tau-fluvalinate (100 g) (Purity of crude product: 86.16%), (was charged in a reactor with n-hexane (1400 ml). The solution was stirred at room temperature for 20-30 minutes. To this solution, silica (50 g) was added to obtain the slurry, and this slurry was stirred for 4 hours at room temperature. The slurry was then filtered and washed with n-hexane (150 ml). The filtrate thus obtained was distilled at 45-55°C under vacuum to obtain the desired product (Purity: 93.97% w/w by HPLC). Chiral ratio of tau-fluvalinate was 50.69:48.91.

Example 2: Preparation and purification of tau-fluvalinate
a) Preparation of tau-fluvalinate:
In a reactor (2R)-2-{[2-chloro-4-(trifluoromethyl)phenyl]amino}-3-methylbutanoic acid (145.7 g; 0.475 moles), hydroxy(3-phenoxyphenyl)acetonitrile (106.8 g; 0.46 moles) in methylene dichloride (200 ml), 4-Dimethylaminopyridine (5.61 g; 0.046 moles) and n-heptane (311 ml) were added and the reaction mixture was cooled to -10°C to 0°C. A solution of N, N-Dicyclohexylcarbodiimide (111.5 g; 0.541 moles) in n-heptane (155 ml) was added slowly to the said reaction mixture in 1 hour and maintained for 4 hours. After the completion of reaction, the reaction mass was then filtered, followed by washing with n-heptane (155 ml). The filtrate thus obtained, was washed with diluted HCl followed by water. The organic layer was distilled off at 45-55°C under vacuum to obtain the tau-fluvalinate (230 g) (Purity:85.35% w/w by HPLC).

b) Purification of tau-fluvalinate:
Crude mass obtained in step a) was added in a reactor charged with n-hexane (3220 ml) and the obtained solution was stirred at room temperature for 20-30 minutes. To this solution, silica (230 g) was added to get the slurry, and this slurry was stirred for 4 hours at room temperature. The slurry mass was then filtered and washed with n-hexane (345 ml). The filtrate thus obtained was distilled at 45-55°C under vacuum to obtain the desired product (Purity: 92.54% w/w by HPLC). Chiral ratio of tau-fluvalinate was 49.56:49.26.

Example 3: Purification of tau-fluvalinate by recycling the adsorbent (silica)
a) Purification of crude tau-fluvalinate:
Crude tau-fluvalinate (100 g) (Purity of crude product: 86.27%), was added in a reactor charged with n-hexane (1400 ml) and the solution was stirred at room temperature for 20-30 minutes. To this solution, silica (100 g) was added to get the slurry, and this slurry was stirred for 4 hours at room temperature. The slurry mass was then filtered and washed with n-hexane (150 ml). The filtrate thus obtained was taken for distillation at 45-55°C under vacuum to get the product (65 g) (Purity: 95.68% w/w by HPLC). Silica remaining after filtration (145 g) was re-used for next purification batch. Chiral ratio of tau-fluvalinate was 50.58:48.68.

b) Recycling of silica for next purification batch:
Crude tau-fluvalinate (100 g) (Purity of crude product: 86.27%), was added in a reactor charged with n-hexane (1400 ml) and the solution was stirred at room temperature for 20-30 minutes. To this solution, silica (116 g) from above example, (3-a), along with fresh silica (20 g) was added to obtain the slurry and this slurry was stirred for 4 hours at room temperature. The slurry mass was then filtered and washed with n-hexane (345 ml). The filtrate thus obtained was taken for distillation at 45-55°C under vacuum to get the desired product (88 g) (Purity: 94.54% w/w by HPLC). Chiral ratio of tau-fluvalinate was 50.73:48.91. Combined yield was 76.5%.

Example 4: Purification of crude tau-fluvalinate
Crude tau-fluvalinate (112 g) (Purity of crude product: 86.75%), was added in a reactor charged with n-heptane (1568 ml) and the solution was stirred at room temperature for 20-30 minutes. To this solution, silica (112 g) was added to obtain the slurry, and this slurry was stirred for 4 hours at room temperature. The slurry mass was then filtered and washed with heptane (168 ml). The filtrate thus obtained was taken for distillation at 45-55°C under vacuum to get the desired product (75 g) (Purity: 94.01 % w/w by HPLC).

Example 5: Preparation and purification of crude tau-fluvalinate
a) Preparation of Tau-fluvalinate:
In a reactor (2R)-2-{[2-chloro-4-(trifluoromethyl)phenyl]amino}-3-methylbutanoic acid (35.29g; 0.11 moles), hydroxy(3-phenoxyphenyl)acetonitrile (25.77 g; 0.110 moles ) in ethylene dichloride (49 ml), 4-Dimethylaminopyridine (1.36 g; 0.011 moles) and n-heptane (75 ml) were added and the reaction mixture was cooled to -10°C to 0°C. A solution of N, N-Dicyclohexylcarbodiimide (26.9 g; 0.13 moles) in n-heptane (37 ml) was added slowly to the said reaction mixture in 1 hour and maintained for 4 hours. After the completion of reaction, the reaction mass was then filtered, followed by washing with n-heptane (37 ml). The filtrate thus obtained, was washed with diluted HCl followed by water. The organic layer was distilled off at 45-55°C under vacuum to obtain tau-fluvalinate (55 g) (Purity: 86.17% w/w by HPLC).

b) Purification of crude tau-fluvalinate:
Crude mass obtained as above was added in a reactor charged with n-Hexane (777 ml) and the obtained solution was stirred at room temperature for 20-30 minutes. To this solution, silica (55 g) was added to obtain the slurry, and this slurry was stirred for 4 hours at room temperature. The slurry mass was then filtered and washed with n-hexane (83 ml). The filtrate thus obtained was taken for distillation at 45-55°C under vacuum to get the desired product (Purity: 92.99% w/w by HPLC). Chiral ratio of tau-fluvalinate was 50.73:48.60.

Example 6: Preparation of and purification of crude tau-fluvalinate
a) Preparation of Tau-fluvalinate:
In a reactor (2R)-2-{[2-chloro-4-(trifluoromethyl)phenyl]amino}-3-methylbutanoic acid (35.29g; 0.11 moles), hydroxy(3-phenoxyphenyl)acetonitrile (25.77 g; 0.110 moles ) in methylene dichloride (82 ml), 4-Dimethylaminopyridine (1.36 g; 0.011 moles) and toluene (44 ml) were added and the reaction mixture was cooled to -10 to 0°C. A solution of N, N-Dicyclohexylcarbodiimide (26.9 g; 0.13 moles) in toluene (37 ml) was added slowly to the said reaction mixture in 1 hour and maintained for 4 hours. After the completion of reaction, the reaction mass was filtered, followed by washing with toluene (37 ml). The filtrate thus obtained, was washed with diluted HCl followed by water. The organic layer was distilled off at 45-55°C under vacuum to obtain tau-fluvalinate (52 g) (Purity: 85.58% w/w by HPLC).

b) Purification of crude tau-fluvalinate:
Crude mass obtained as above was added in a reactor charged with n-hexane (777 ml) and the obtained solution was stirred at room temperature for 20-30 minutes. To this solution, silica (55 g) was added to obtain the slurry, and this slurry was stirred for 4 hours at room temperature. The slurry mass was then filtered and washed with n-hexane (83 ml). The filtrate thus obtained was taken for distillation at 45-55°C under vacuum to get the desired product (Purity: 93.84% w/w by HPLC). Chiral ratio of tau-fluvalinate was 50.79:48.71.

Example 7: Preparation of and purification of crude tau-fluvalinate
a) Preparation of tau-fluvalinate:
In a reactor (2R)-2-{[2-chloro-4-(trifluoromethyl)phenyl]amino}-3-methylbutanoic acid (35.27g; 0.11 moles), hydroxy(3-phenoxyphenyl)acetonitrile (25.77 g; 0.11 moles) in methylene dichloride (50 ml), 4-Dimethylaminopyridine (1.35 g; 0.011 moles) and n-heptane (75 ml) were added and the reaction mixture was cooled to -10°C to 0°C. A solution of 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (25.1 g; 0.13 moles) in n-heptane (37 ml) was added slowly to the said reaction mixture in 1 hour and maintained for 4 hours. After the completion of reaction, the reaction mass was then filtered, followed by washing with n-heptane (37 ml). The filtrate thus obtained, was washed with diluted HCl followed by water. The organic layer was distilled off at 45-55°C under vacuum to obtain tau-fluvalinate (50 g) (Purity: 85.86% w/w by HPLC).

b) Purification of crude tau-fluvalinate:
Crude mass obtained as above was added in a reactor charged with n-hexane (777 ml) and the obtained solution was stirred at room temperature for 20-30 minutes. To this solution, silica (55 g) was added to obtain the slurry, and this slurry was stirred for 4 hours at room temperature. The slurry mass was then filtered and washed with n-hexane (83 ml). The filtrate thus obtained was taken for distillation at 45-55°C under vacuum to get the desired product (Purity: 93.86% w/w by HPLC). Chiral ratio of tau-fluvalinate was 50.83:48.65.

Advantages of the present invention:
1. The present invention provides tau-fluvalinate with desired isomeric mixture
2. The present invention is effective for removal of undesired impurities and byproducts such as dicyclohexylurea, during the work up process
3. The present invention provides improved chemical purity of tau-fluvalinate
4. The present invention provides purification of tau-fluvalinate; wherein the adsorbent used can be recycled/reused.
,CLAIMS:
1. A process for purification of tau-fluvalinate, the process comprising treating tau-fluvalinate having purity less than about 90% with a hydrocarbon solvent system comprising a hydrocarbon solvent and an adsorbent.

2. The process as claimed in claim 1, wherein a weight ratio of the adsorbent to the hydrocarbon solvent is in a range from about 1:10 to about 1:15.

3. The process as claimed in claim 1, wherein the adsorbent is selected from silica, activated carbon or resin.

4. The process as claimed in claim 1, wherein the treatment of tau-fluvalinate with the hydrocarbon solvent system is performed at a temperature ranging from about 10°C to about 80°C.

5. The process as claimed in claim 1, wherein said treatment of tau-fluvalinate avoids use of column chromatography or preparative thin layer chromatography.

6. A process for preparation of tau-fluvalinate, the process comprising reacting (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoic acid with hydroxy(3-phenoxyphenyl)acetonitrile in a solvent system comprising a mixture of a halogenated hydrocarbon and a hydrocarbon solvent.

7. The process as claimed in claim 6, wherein a weight ratio of the halogenated hydrocarbon and the hydrocarbon solvent is in a range from about 20:80 to about 80:20.

8. The process as claimed in claim 6, wherein reaction is carried out at a temperature ranging from about -20°C to about 10°C.

9. The process as claimed in claim 6, wherein reaction is carried out in presence of a coupling agent and a base.

10. The process as claimed in claim 9, wherein the coupling agent is selected from N, N-Dicyclohexylcarbodiimide, O-(Benzotriazol-1-yl)-N,N,N',N'-tetramethyluronium tetrafluoroborate, 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride or N,N'-Diisopropylcarbodiimide.

11. The process as claimed in claim 9, wherein the base is selected from 4-Dimethylaminopyridine, Triethylamine or diisopropylethylamine.

12. A process for preparation of tau-fluvalinate with purity more than about 90%; wherein the said process comprises:
a) reacting (R)-2-(2-chloro-4-trifluoromethylphenylamino)-3-methylbutanoic acid with hydroxy(3-phenoxyphenyl)acetonitrile in a solvent system comprising a mixture of a halogenated hydrocarbon and a hydrocarbon solvent to obtain crude tau-fluvalinate having purity less than about 90%; and
b) treating the crude tau-fluvalinate with a hydrocarbon solvent system comprising a hydrocarbon solvent and an adsorbent.

13. The process as claimed in claim 12, wherein the tau-fluvalinate comprises diastereomeric pair of (RR) and (SR) in a ratio of about 50(±2):50 (±2).