Chiral resolution of racemic p-methylsulfonylphenyl serine ethyl ester with lipases: the mechanism of side reaction and its suppression

The D-threo form of p-methylsulfonylphenyl serine ethyl ester (MPSE) is a key intermediate for the synthesis of florfenicol. In this study, chiral resolution of DL-threo-p-MPSE with lipases was investigated. Among a series of lipases, Novzyme 435 was the best to resolve DL-threo-p-MPSE with the conversion rate of 36.83% and ee value of 35.13%. To improve the conversion rate and ee value, a number of byproducts were identified and characterized using reverse-phase HPLC, normal-phase HPLC, (1)H NMR, and LC-MS when threo-p-MPSE was hydrolyzed by lipases in organic medium. Mechanisms of generating main byproducts are proposed, and a suppressing method is provided. The results showed that byproduct p-methylsulfonyl benzaldehyde serves as the key intermediate during the whole side reaction process. It was also observed that threo-p-MPSE with a proper hydrolytic velocity served as a driving force to generate p-methylsulfonyl benzaldehyde and accelerated the side reactions. Finally, a feasible approach to suppress side reactions in enzymatic catalysis is offered. The conversion rate and ee value were greatly improved by 69.29 and 46.26%, respectively, using Zn(2+) compared to those without Zn(2+).