SYNTHESIS OF ENANTIOMERICALLY PURE (R)-2-SULFANYLPROPANOIC AND (S)-2-SULFANYLPROPANOIC ACIDS (THIOLACTIC ACID) FROM ETHYL (S)-LACTATE USINGPIG-LIVER ESTERASE

The methanesulfonates of optically pure ethyl (S)-lactate or ethyl (R) -2-chloropropanoate 5, obtained with inversion of configuration from e thyl (S)-lactate on treatment with SOCl2, can be substituted by caesiu m thiolates with inversion of configuration to yield (R) and (S) ethyl 2-(acetylsulfanyl)propanoate, 2a and 2b, respectively. Hydrolysis of the carboxy ester of 2a, 2b to the acid under common acidic or basic c onditions always leads to substantial racemisation. However, the use o f the mild biocatalyst pig liver esterase (PLE) at neutral pH yields t he carboxylic acids 4a and 4b without racemisation. The acetylsulfanyl group remains unaffected during this process. Subsequent treatment of the acetylsulfanyl group of 4a or 4b with 2 mol dm(-3) aqueous ammoni a leads to the useful optically pure building block (R)- or (S)-2-sulf anylpropanoic acid ('thiolactic acid') 3a, 3b in an overall yield of 6 5% based on cheap ethyl (S)-lactate.