PROCESS FOR PREPARING (2R, 3S) 2-BENZYLOXY-3-TERT-BUTOXY CARBONYL AMINO-3-PHENYL PROPIONIC ACID

The following specification particularly describes the invention and the manner in which it is to be performed.

INTRODUCTION

Particular aspects of the present application encompasses the process for preparation of (2R, 3S) 2-Benzyloxy-3-tert-Butoxy Carbonyl Amino-3-Phenyl Propionic Acid and is represented by the formula (I)

PICTURE
(2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I)

which is often used as one of the useful and economically viable side chain in the process for preparing Docetaxel and Cabazitaxel having following structures-
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1,7 β,10β-trihydroxy-9-oxo-5β,20-epoxytax-1 l-ene-2a,4,13a-triyl 4-acetate 2-benzoate 13-{(2R,3S)-3-[(tert-butoxycarbonyI)amino]-2-hydroxy-3-phenylpropanoate OR DOCETAXEL or TAXOTERE
PICTURE

1-hydroxy, 7,10 dimethoxy-9-oxo-5 β,20-epoxytax-l l-ene-2a,4,13a-triyl 4-acetate 2-benzoate 13-{(2R,3S)-3-[(tert-butoxycarbonyl)amino]-2-hydroxy-3-phenylpropanoateOR CABAZITAXEL OR JEVTANA

Docetaxel and Cabazitaxel are well known and approved antineoplastic agent of taxoid family. These taxane compounds found to exhibit significant anti-tumor activity and are currently marketed under the trade name TAXOTERE® and JEVTANA ® respectively.

Docetaxel (known from US 4,814,470) as well as Cabazitaxel (known from US 5,847, 170) are among outstanding cancer chemotherapeutic substances, contains a (2R, 3S)- phenylisoserine side chain i.e (2 R, 3 S) - 2 - Benzyloxy - 3- tert -butoxy carbonyl amino - 3 - phenylpropionic acid.

Denis et in US5726346 (filed on Oct 04, 1993) and its continuation application issued as US 6114550 discloses a stereoselective process for preparation of a derivative of β - phenylisoserine of formula (I)
PICTURE

by the reaction of an N-carbonyl-benzylimine of formula (II) with an optically active amide of a protected hydroxyacetic acid of formula (III), followed by hydrolysis of the product (IV) obtained based on the following synthetic scheme-
PICTURE

L(+)-2,10-camphorsultam

Kanazawa et al in J.Org. Chem 1994, 59, 1238-1240 reported another stereo controlled synthesis method for protected isoserine side chain for Taxeotere, which involved the use of resolving agent like camphor sulphonic acid amide while reacting with pure imine compound based on the following synthetic scheme-
PICTURE

Shibasaki and co-workers in J . Am Chem. Soc. (2004) 126, 8777 reported a four-step synthesis of the Taxotere side-chain based on a direct zinc organometallic complex-catalyzed Mannich reaction between an N-Boc imine and 2-hydroxy-2'-methoxyacetophenone.
Pawel Dziedzic and co-workers in Tetrahedron Letters 49 (2008) 6631-6634 also reported a synthesis of the Taxotere benzylated side-chain based on the following synthetic scheme-
PICTURE

Though there are different processes known in the art for making (2R, 3S) 2-Benzyloxy-3-tert- butoxy carbonyl amino-3-phenyl propionic acid (I), however, despite the aforementioned various disclosures mentioning various processes, there still remains a need for new economically viable and commercially feasible process, which is convenient and amenable to industrial scale up during the need of higher scale productions.

Hence, the present specification is aimed to provide an improved process for the synthesis of (2R, 3S) 2-Benzyloxy-3-tert-Butoxy Carbonyl Amino-3-Phenyl Propionic Acid (I) in better yields and purity, which also involves the use of inexpensive, non-hazardous and easily available reagents.

SUMMARY OF INVENTION

Particular aspects of the present specification relate to the process for the preparation of (2R, 3S) 2-benzyloxy-3-tert-butoxy carbonyl amino-3-phenyl propionic acid of formula (I)
PICTIRE

(2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I)

In an aspect, the present invention provides a process of preparation of (2R, 3S) 2-Benzyloxy- 3-tert-butoxy carbonyl amino -3- phenyl propionic acid of formula (I), comprising the steps of-

a) . Esterifying the (2R, 3S)-3-phenylisoserine or its salt with an alcohol (C1-C4 alkyl alcohol) and an acid to form (2R, 3S)-3-amino-2-hydroxy-3-phenyl propionic acid alkyl (C1-C4) ester (DC-01);
PICTURE

b) . Reacting (2R, 3S)-3-amino-2-hydroxy-3-phenyl propionic acid alkyl (C1-C4) ester (DC-01) with t-Boc anhydride in an Organic solvent and a Base to form (2R, 3S)-2- hydroxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid alkyl (C1-C4) ester (DC- 02);
PICTURE

wherein R= alkyl (C1-C4)

c) Selectively benzylating the (2R, 3S)-2-hydroxy-3-tert-butoxycarbonylamino-3-phenyl propionic acid alkyl (C1-C4) ester (DC-02) in an organic solvent and sodium hydride to form (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid alkyl (C1-C4) ester (DC-03)
PICTURE

d) De-esterifying the (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid alkyl (C1-C4) ester (DC-03) using aqueous base solution;
PICTURE

e) . Isolating the (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid (DC-04)

In another aspects, the present invention provides process of purification and isolation of (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid (I) comprising the steps of-

a. providing a solution of (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I) with a solvent or taking direct solution from the reaction step resulting (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I)

b. Recover Solvent from the solution of step a)

c. Solvent extraction with non polar solvent

d. Drying the extracted organic layer using anhydrous Na2SO4 or similar desiccants

e. Concentrating the dried organic layer partly or completely

f. adding an anti-solvent selected from hydrocarbon solvent (C6 to C8) to get the isolated product

g. Filter and optionally dry the product

The purification and isolated product- (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I) may be directly used for condensation to provide the desired anti¬tumor taxane compounds like docetaxel and cabazitaxel after desired steps.
DETAILED DESCRIPTION

As set forth herein, aspects of the present invention relate to the process for preparation of (2R, 3S) 2-Benzyloxy-3-tert-butoxy carbonyl amino -3- phenyl propionic acid of formula
(I).

In one embodiment of the present application, it provides a process of preparation of (2R, 3S) 2-Benzyloxy-3-tert-butoxy carbonyl amino -3- phenyl propionic acid of formula (I) comprising the steps of-

a). Esterifying the (2R, 3S)-3-phenylisoserine or its salt with an alcohol (C1-C4 alkyl alcohol) and an acid to form (2R, 3 S)-3-amino-2-hydroxy-3-phenyl propionic acid alkyl (C1-C4) ester (DC-01);
PICTURE

In one of the preferred embodiment of the present invention of the process of esterifying involved the use of (2R, 3S)-3-phenylisoserine salt as HX , which is selected from HC1 or HBr or HI.

The alcohol- R-OH utilized in the step can be selected from CI to C4 alkyl alcohol. The esterification reaction is carried out at temperature ranging between 35-60°C. Preferably, it may be carried out at 40 to 55°C in the presence of a mineral acid like sulphuric acid.

b). Reacting (2R, 3 S)-3-amino-2-hydroxy-3-phenyl propionic acid alkyl (C1-C4) ester (DC-01) with t-Boc anhydride in an Organic solvent and a Base to form (2R, 3S)-2- hydroxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid alkyl (C1-C4) ester (DC- 02);
PICTURE

This step of reacting ester so formed in the step a) preferably utilizes Organic solvent selected from halohydrocarbon and a Base selected from NaOH or Na2CO3 or NaHCO3 or K2CO3. In one of the preferred embodiments, dichloromethane as Halohydrocarbon solvent is utilized in the reaction. The reaction is carried out at temperature ranging between 20-40°C. Preferably, it may be carried out at 25 to 30°C in the presence of a base selected from
NaOH or Na2CO3 or NaHCO3 or K2CO3.

c) Selectively benzylating the (2R, 3S)-2-hydroxy-3-tert-butoxycarbonylamino-3-phenyl propionic acid alkyl (C1-C4) ester (DC-02) in an organic solvent and sodium hydride to form (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid alkyl (C1-C4) ester (DC-03)
PICTURE

wherein R= alkyl (C1-C4) Bn = Benzyl

Selective benzylation of DC-02 using a halobenzyl compound involves utilization of an organic solvent selected from C1 to C4 alcohol or dimethylformamide or THF or a ketone.

The benzylation reaction is carried out at temperature ranging between 0-35°C.

d) . De-esterifying the (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid alkyl (C1-C4) ester (DC-03) using aqueous base solution;
PICTURE

The de-esterification step utilizes an aqueous base solution comprising the bases selected from LiOH, NaOH, KOH or Na2CO3 or K2CO3.

The process utilizes an organic solvent selected from an alcohol selected from C1 to C4 or a ketone solvent selected from C3 to C8.

The de-esterification reaction is carried out at temperature ranging between 0-35°C.In one of the preferred embodiment, it is carried out at room temperature.

e). Isolating the (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid (DC-04). This step of isolation involves optional purification followed by isolation of (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid (DC-04) by conventional methods, which includes recovery of the solvent, extraction, followed by filtering and optional drying.

The details of the process are delineated in the scheme-1 and their specifics demonstrated in the example may not be construed to limit the scope of the present invention.
PICTURE

Scheme-I: Preparation of (2R, 3S) 2-Benzyloxv-3-tert-butoxy carbonyl amino -3- phenyl propionic acid as per the present invention

In another embodiment, the present invention provides process of purification and isolation of (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid (I) comprising the
steps of -

a. providing a solution of (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I) with an organic solvent or taking direct solution from the reaction step resulting (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I). For purification purpose, the solution of (2R, 3S)-2- benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid (I) in an organic solvent is prepared by combining together, however, any direct solution from the process reaction step resulting (2R, 3S)-2-benzyloxy-3-tert- butoxycarbonylamino-3-phenylpropionic acid (I) may also be taken up for the purification.

b. Recover Solvent from the solution of step a)
Solvent recovery is carried out under vacuum and well below the boiling point temperature of solvent. In one of the preferred embodiment, the recovery temperature was about 10 °C less than the boiling point of the solvent taken.

c. Solvent extraction with non polar solvent
Non polar solvent utilized for the extraction can be selected from organic solvents, but are not halohydrocarbon solvent. In one of the preferred embodiment of the present invention, the non polar solvent utilized for the extraction was dichloromethane.

d. Drying the extracted organic layer using anhydrous Na2SO4 or similar desiccants

e. Concentrating the dried organic layer partly or completely

f. adding an anti-solvent selected from hydrocarbon solvent (C6 to C8) to get the isolated product

g. Filter and optionally dry the product

The purification and isolated product- (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I) may be directly used for condensation to provide the desired anti¬tumor taxane compounds like docetaxel and cabazitaxel after desired steps. The side chain may be modified with the incorporation of benzoyl (Ph-CO-) in place of (t-Boc) to prepared a similar Benzyl protected side chain, utilization in preparing Paclitaxel, which is another important and naturally occurring anti-tumor taxane compound.

Certain specific aspects and embodiments of the present application will be explained in more detail with reference to the following examples, which are provided by way of illustration only and should not be construed as limiting the scope of the invention in any manner.

EXAMPLE

The process of preparation of (2R, 3S) 2-Benzyloxy-3-tert-butoxy carbonyl amino -3- phenyl propionic acid comprises of following four stages, namely 1 to 4. Individual stages are provided separately herein below-

Stage 1. Preparation of (2R, 3S) 3-amino-2- hydroxyl-3phenvl propionic acid ethyl ester (DC-01):
PICTURE

Charge absolute ethanol to a clean and dry RB flask. Add slowly 56.0gm sulphuric acid under stirring. Add 250 gm (2R, 3S)-3- Phenylisoserine hydrochloride. Heat the reaction mixture to 45-55°C and maintain this temperature for 7-8 hours. Monitor the reaction by HLPC: un reacted (2R, 3S)-3- Phenylisoserine hydrochloride should not be more than 0.5% Note: If limit is not achieved, send the sample every two hour till achieved the limit. Recover the ethanol completely at 50-60°C under vacuum up to dryness. Add 3 litre Dichloromethane and 500ml DM water to residue and adjust the pH by addition of 105gm solid sodium bicarbonate (pH should be in the range of 8-9). Again extract the aqueous layer with 31itre dichloromethane (pH of aqueous layer should be in the range of 8-9). Combined the dichloromethane layer and washed with 500gm saturated sodium chloride solution. Make the dichloromethane layer moisture free by addition of 500gm sodium sulfate. Concentrate the organic layer at 50-60°C under 50-350-mbar pressures until get solid product. Scratch the material, break the lumps (if any) and finally dry at 0 mbar pressure and 60°C for 6 hours. Dry weight: 195.0 gm

Water content: 0.17%w/w

Stage 2. Preparation of (2R, 3S) 2- hydroxy-3 tert- butoxycarbonvlamino-3-phenyl propionic acid ethyl ester (DC-02);
PICTURE

To a clean RB flask charge 1 litre of Dichloromethane and 221gm of DC-01 under stirring and stir for 30 minutes at room temperature. Add 226 gm Di-tert-butylpyroearbonate mixed with 100 ml dichloromethane and stir for 15 minutes. Charge 110 gm of sodium bicarbonate slowly within 30 minutes and further stir the reaction mixture for 5-6 hours. Monitor the reaction by HLPC: unreacted DC-01 should not be more than 0.50%. Note: If limit is not achieved, send the sample every two hour till achieved the limit. Filter the reaction mass over Buckner funnel and wash the solid cake with 2.5 litre Dichloromethane and collect the filtrate. Wash the dichloride methane layer with 1.0 litre of DM water. Make the layer moisture free by addition of 500 gm sodium sulfate. Recover the Dichloromethane layer at 40-45°C temp under vacuum up to dryness up to product started solidifying. Purify the DC-02 with n-Hexane to get the pure product. Dry the product at 60°C under full vacuum. Dry weight: 270 gm Water content: 0.31%w/w

Stage 3. Preparation of (2R, 3S)-2-Benzvloxv-3-tert-butoxycarbonvlamino-3- phenylpropionic acid ethyl ester (DC-03):
PICTURE

To a clean and dry flask charge 0.6 litre of N,N-Dimethylformamide DMF() at RT under stirringCool the reaction mixture to 0°C and slowly added sodium hydride under nitrogen. Add DC-02 (310.0g DC-02 dissolve in 1.2 litre of DMF) under nitrogen atmosphere (Reaction temperature increases to 25-30°C ). Again cool the reaction mixture to 10°C. Mix 179.6gm benzyl bromide to 200mL N, N-Dimethylformamide and add to the reaction mass. The temperature will increase to 25-35°C.Continue cooling and stirring for one hour. Monitor the reaction by HLPC: unreacted DC-02 should not be more than 0.5%.Note: If limit is not achieved, send the sample every two hour till achieved the limit. Add 1.0 litre DM water to the reaction mass and 1.0 litre ethyl acetate stirred for one hour. Separate the upper organic layer. Again extract the aqueous layer with ethyl acetate (2x1.0L).Combined the ethyl acetate layer and give water wash (llitre). Dissolve 100 gm ammonium chloride in 1 litre DM water, add to ethyl acetate layer, stir for two hours and hold for separation for one hour. Make ethyl acetate layer moisture free by addition of anhydrous sodium sulphate. Recover the ethyl acetate layer on rotatory evaporator at 40-60°C under full vacuum to get a viscous semisolid material. Purify the crude product with column chromatography over silica gel (60-120) mesh using n-Hexane and ethyl acetate as an eluent. Combined the appropriate fraction and recover the ethyl acetate at 40-60°C under vacuum up to dryness. Add llitreNa2CO3 n-Hexane to the residue and cool to 100°C and maintain this temperature for one hour under stirring. Filter the solid product, wash with 500ml n-Hexane. Dry at 60°C under vacuum for 4-5 hour.

Dry weight: 171.0gm Water content: 0.3%w/w

Stage 4. Preparation of (2R, 3S)-2-Benzyloxv-3-tert-butoxycarbonylamino-3- phenylpropionic acid (DC-04):
PICTURE

To a clean and dry RB flask charge absolute ethanol under stirring. Add 165gm of DC-03 at RT under stirring and stir for 1-2 hour to dissolve the DC-03. Add lithium hydroxide solution (20.9g lithium hydroxide solution dissolved in DM Water 495ml). Stir the reaction mixture for 8 hours at room temperature. Monitor the reaction by HLPC: unreacted DC-03 should not be more than 0.50%.

Note: If limit is not achieved, send the sample every two hour till achieved the limit.

Recover the solvent at 50-60°C under vacuum up to dryness to get the viscous liquid. Add 1.0 litre DM water , 2.0 litre dichloromethane and 0.4 litre 5% HC1 solution (pH should be in the range of 2-4) and stir for 30 minute then separate the dichloromethane layer. Again extract the aqueous layer with 2x1.0 litre dichloromethane. Combined the dichloromethane layer and wash the layer with 1.0 litre DM water. Make moisture free by addition of 450gm anhydrous sodium sulfate. Recover the DCM layer at 40-50°C under vacuum to get the viscous liquid. Add 2.5 litre of n- Hexane to the viscous liquid and stir for 4-5 hours at room temperature. Filter and wash with 0.5 litre of n-Hexane. Dry at 60°C under vacuum for 5-8. Dry weight: 133.0gm Water content=0.48%w/w Chromatographic purity(By HPLC)=97.5-98.5%

Claims:
1) A process of preparation of (2R, 3S) 2-Benzyloxy-3-tert-butoxy carbonyl amino -3-phenyl propionic acid of formula (I)
PICTURE
(I)

comprising the steps of-

a). Esterifying the (2R, 3S)-3-phenylisoserine or its salt with an alcohol (C1-C4 alkyl alcohol) and an acid to form (2R, 3S)-3-amino-2-hydroxy-3-phenyl propionic acid alkyl (C1-C4) ester (DC-01);
DC-01
(2R,3S)-3-Phenylisoserine.HCI wherein R=alky, (C1.C4)
wherein x=halogen

b). Reacting (2R, 3S)-3-amino-2-hydroxy-3-phenyl propionic acid alkyl (C1-C4) ester (DC-01) with t-Boc anhydride in an Organic solvent and a Base to form (2R, 3S)-2- hydroxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid alkyl (C1-C4) ester (DC- 02);
PICTURE

wherein R= alkyl (C1-C4)

c) Selectively benzylating the (2R, 3S)-2-hydroxy-3-tert-butoxycarbonylamino-3-phenyl propionic acid alkyl (C1-C4) ester (DC-02) in an organic solvent and sodium hydride to form (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid alkyl (C1-C4) ester (DC-03)
PICTURE

DC-03
wherein R= alkyl (C1-C4) Bn = Benzyl

d). De-esterifying the (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid alkyl (C1-C4) ester (DC-03) using aqueous base solution;
PICTURE

e) . Isolating the (2R, 3S)-2-Benzyloxy-3-tert-butoxycarbonylamino-3-phenylpropionic acid (DC-04)

2. The process according to claim 1, wherein the step a) involves the use of (2R, 3S)-3- phenylisoserine salt as HX, wherein HX is selected from HCl or HBr or HI.

3. The process according to claim 1, wherein the step a) utilizes an alcohol selected from C1 to C4.

4. The process according to claim 1, wherein the step b) utilizes Organic solvent selected from halohydrocarbon and a Base selected from NaOH or Na2CO3 or NaHCO3 or K2CO3.

5. The process according to claim 1, wherein the benzylation of step c) utilizes an organic solvent selected from CI to C4 alcohol or dimethylformamide or THF or a ketone (C3-C7).

6. The process according to claim 1, wherein the benzylation of step c) is carried out at a temperature ranging between 0 to 35°C.

7. The process according to claim 1, wherein the de-esterification in step d) utilizes an aqueous base solution comprising the bases selected from LiOH, NaOH, KOH or Na2CO3 or K2CO3.

8. The process according to claim 1, wherein the step d) utilizes an organic solvent selected from an alcohol selected from CI to C4 or a ketone solvent selected from C3 to C8.

9. A process of purification and isolation of (2R, 3S)-2-benzyloxy-3-tert-butoxy carbonyl amino-3-phenylpropionic acid (I) comprising the steps of –

a. providing a solution of (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I) with a solvent or taking direct solution from the reaction step resulting (2R, 3S)-2-benzyloxy-3-tert-butoxycarbonylamino-3- phenylpropionic acid (I)

b. Recover Solvent from the solution of step a)

c. Solvent extraction with non polar solvent

d. Drying the extracted organic layer using anhydrous Na2SO4 or similar desiccants

e. Concentrating the dried organic layer partly or completely

f. adding an anti-solvent selected from hydrocarbon solvent (C6 to C8) to get the isolated product

g. Filter and optionally dry the product

10. The process according to claim 9, wherein the anti-solvent utilized in step e) is selected from hydrocarbon solvent (C6 to C8).