Chiral Resolution of Racemic p-Methylsulfonylphenyl Serine Ethyl Ester with Lipases: The Mechanism of Side Reaction and Its Suppression

Abstract

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+).

Enantioselective high‐performance liquid chromatography of aryl‐substituted oxazolines as an efficient tool for determination of chiral purity of serine medicinal components

“…Similarly, the introduction of the fluorenylmethyloxycarbonyl group (Fmoc) might require additional protection of the serine hydroxyl group [13,14] which makes the preparation of serine analytes more complicated (Figure 1C). To our best knowledge, only a few studies dealing with enantiosepa-ration of serine esters were published [25][26][27][28][29], all focusing only on short alkyl (methyl or ethyl) esters. Moreover, chromatographic enantioseparations of serine esters were conducted under normal phase conditions requiring toxic non-polar solvents and often toxic trifluoroacetic acid as an additive, too.…”