Biocatalytic enantioconvergent separation of racemic mandelic acid

Abstract

Chiral mandelate was produced by biocatalytic enantioconvergent separation of racemic mandelic acid. One pot enantioconvergent synthesis of ( R)-mandelic acid ester from racemic mandelic acid was achieved by a novel aqueous/organic two-phase system with two-enzyme process consisting of (1) lipase-catalyzed enantioselective esterification of racemic mandelic acid in organic solvent and (2) in situ racemization of unreacted isomer, ( S)-mandelic acid in aqueous phase by recombinant mandelate racemase. Lipase was screened from commercially available sources and mandelate racemase was prepared by overexpression of cloned gene originated from Pseudomonas putida in E. coli Top 10. Organic solvent was selected based on its effect on the partition characteristics for substrate and product, and enzymatic esterification and racemization. The aqueous/organic two-phase dynamic kinetic resolution (DKR) was attempted in a hollow-fiber membrane bioreactor wherein Candida antarctica lipase (CHIRAZYME L-2, Roche)-catalyzed enantioselective esterification and mandelate racemase-mediated racemization of unreacted mandelic acid were performed in ethylene dichloride and aqueous buffer, respectively. Using the novel DKR process, ( R)-mandelic ethyl ester was obtained from racemic mandelic acid in 65% isolated yield and 98%ee as the sole product.