Method of racemization of the R(−) isomer of the (2-chlorophenyl)-6,7-dihydrothieno[3,2-c]pyridine-5(4H)-acetic acid methyl ester
Racemization of the R(−) isomer of the (2-chlorophenyl)-6,7-dihydrothieno[3,2-c]pyridine-5(4H)-acetic acid methyl ester (Formula II) (also called R clopidogrel) is performed via conversion of a portion thereof to the S(+)isomer and it takes place in an organic solvent selected from alcohols, esters, ketones or ethers, or in their mixtures, in the presence of a base selected from substances of formula R1R2R3R4N+OH″ wherein R1, R2, R3 and R4 are identical or different substituents selected from C1-C5 alkyls or C5, C6 cycloalkyls or aryls, C7-C9 alkyl-cycloalkyls or alkyl-aryls, the molar ratio of the base to the starting substance being 1:1 to 1:10., OMe Cl I Cl<VOMe II.
1. A method of preparation of the S(+) isomer of the (2-chlorophenyl)-6,7-dihydro-thieno[3,2-c]pyridine-5(4H)-acetic acid methyl ester of formula I
wherein a mixture of R and S isomers of clopidogrel is dissolved in an ester of formula Ra(O)ORb, wherein Ra and Rb independently represent a C1-C5 aliphatic substituent or a C5-C8 aromatic substituent, and a solution of R-camphorsulfonic acid in a solvent of formula RaOH is added to the solution, from the mixed solution the camphorsulfonic salt of the substance of formula I is crystallized, which is converted to S(+) isomer of the (2-chlorophenyl)-6,7-dihydro-thieno[3,2-c]pyridine-5(4H)-acetic acid methyl ester of formula I in the next step; further a base of formula R 1 R 2 R 3 R 4 N + OH − wherein R 1 , R 2 , R 3 and R 4 are identical or different substituents selected from C1-C5 alkyls or C5, C6 cycloalkyls or aryls, C7-C9 alkyl-cycloalkyls or alkyl-aryls in a solution in a substance of formula RaOH is added to the mother liquors after crystallization, thus resulting in a mixture of R and S isomers of clopidogrel, that are used for preparation of S(+) isomer of the (2-chlorophenyl)-6,7-dihydro-thieno[3,2-c]pyridine-5(4H)-acetic acid methyl ester of formula I.
2. The method according to claim 1 wherein the substance of formula RaOH is methanol and the ester of formula Ra(O)ORb is isopropylacetate.