Streamlined enzymatic process for (R)-α-lipoic acid intermediate production enabled by a high organic-solvent-tolerant carbonyl reductase

(R)-Ethyl-8-chloro-6-hydroxyoctanoate [(R)-ECHO] is a crucial intermediate in the synthesis of (R)-α-lipoic acid [(R)-α-LA]. Currently, the asymmetric synthesis of (R)-ECHO mediated by carbonyl reductases is limited by the enzymes' poor tolerance to the solvents present in chemically prepared ethyl 8-chloro-6-oxooctanoate (ECOO). This limitation necessitates a solvent-removal step prior to enzymatic reduction, which compromises compatibility with the chemical process. To address this issue, we began with the organic solvent-tolerant carbonyl reductase AxSDR and, based on computer-aided structural analysis, remodeled its substrate-binding pocket. We successfully expanded the substrate-binding pocket and reversed the relative sizes of its two cavities, converting the anti-Prelog AxSDR into a Prelog 6-point mutant, Mu6. By coupling this with an appropriate coenzyme regeneration system, we obtained Mu6-BtGDH whole cells exhibiting optimal catalytic performance. Under optimal conditions, Mu6-BtGDH whole cells converted a 50% ECOO stock solution (with a final ECOO concentration of 100 g/L) within 10 h to produce (R)-ECHO with an enantiomeric excess (ee) greater than 99% and a space-time yield of 10 g·L^-1·h^-1. The Mu6-BtGDH catalyst and catalytic process developed in this study eliminate the need for solvent removal of the ECOO stock solution prepared by the chemical method, thereby seamlessly integrating the upstream chemical step and downstream enzymatic process for producing (R)-ECHO. Additionally, this approach enhances the atomic economy of (R)-α-LA chiral precursor, which will ultimately reduce the overall synthesis cost of (R)-α-LA.