Use of lipases for the kinetic resolution of lactic acid esters in heptane or in a solvent free system

tKinetic resolution of d,l-ethyl lactate (d,l-LA-Et) and d,l-butyl lactate (d,l-LA-Bu) was accomplished inthe presence of lipases. Transesterification of the lactate esters with alcohols was shown to be poorlyenantioselective, with a very low preference toward the l enantiomer. However, esterification of the freehydroxyl function of the lactate esters was much more enantioselective, with an opposite preferencetoward the d form. Among the 200 combinations screened (substrates and lipases), only two reactionsfulfilled the required criteria for an efficient resolution (fast reaction, enantioselectivity ratio higherthan 100). The best resolution was obtained by esterification of d,l-LA-Bu with butyric anhydride inthe presence of lipase B from Candida antarctica (CAL-B) in heptane (at 30◦C): LA-Bu was resolved in6 h with an initial esterification rate of d-LA-BU of 1.5 g L−1h−1g−1EZ, an enantioselectivity ratio higherthan 100, a l substrate consumption lower than 1%, and a final substrate enantiomeric excess superiorto 99%. After having demonstrated that neither LA-Bu nor BuAn were inhibitors of CAL-B, the reactionwas further improved by carrying it out in a solvent free system, at an elevated temperature (60◦C), andat a higher lipase concentration. LA-Bu was resolved in 6 h with an initial rate of 7.6 g L−1h−1g−1EZ, anenantioselectivity ratio higher than 100, and a final substrate enantiomeric excess of 95%.