Highly enantioselective synthesis of (R)-1,3-butanediol via deracemization of the corresponding racemate by a whole-cell stereoinverting cascade system.

BACKGROUND Deracemization, the transformation of the racemate into a single stereoisomeric product in 100% theoretical yield, is an appealing but challenging option for the asymmetric synthesis of optically pure chiral compounds as important pharmaceutical intermediates. To enhance the synthesis of (R)-1,3-butanediol from the corresponding low-cost racemate with minimal substrate waste, we designed a stereoinverting cascade deracemization route and constructed the cascade reaction for the total conversion of racemic 1,3-butanediol into its (R)-enantiomer. This cascade reaction consisted of the absolutely enantioselective oxidation of (S)-1,3-butanediol by Candida parapsilosis QC-76 and the subsequent asymmetric reduction of the intermediate 4-hydroxy-2-butanone to (R)-1,3-butanediol by Pichia kudriavzevii QC-1. RESULTS The key reaction conditions including choice of cosubstrate, pH, temperature, and rotation speed were optimized systematically and determined as follows: adding acetone as the cosubstrate at pH 8.0, a temperature of 30 °C, and rotation speed of 250 rpm for the first oxidation process; in the next reduction process, the optimal conditions were: adding glucose as the cosubstrate at pH 8.0, a temperature of 35 °C, and rotation speed of 200 rpm. By investigating the feasibility of the step-by-step method with one-pot experiment as a natural extension for performing the oxidation-reduction cascade, the step-by-step approach exhibited high efficiency for this cascade process from racemate to (R)-1,3-butanediol. Under optimal conditions, 20 g/L of the racemate transformed into 16.67 g/L of (R)-1,3-butanediol with 99.5% enantiomeric excess by the oxidation-reduction cascade system in a 200-mL bioreactor. CONCLUSIONS The step-by-step cascade reaction efficiently produced (R)-1,3-butanediol from the racemate by biosynthesis and shows promising application prospects.