Enantiocomplementary Bioreduction of Flexible Ring N-(3-Oxobutyl)Heterocycles Providing Enantiopure Chiral Fragments for Drug Discovery

In this study, the bioreduction of prochiral N-(3-oxobutyl)heterocycles comprising various (partially) saturated, flexible rings is explored using microbial whole-cell ketoreductases such as wild-type yeast strains including baker's yeast (Saccharomyces cerevisiae) and Escherichia coli cells expressing two enantiocomplementary recombinant alcohol dehydrogenases. Initially, four wild-type yeast strains are screened for ketoreductase activity on a series of nine flexible N-heterocycles with prochiral carbonyl group in the N-(3-oxobutyl) sidechain. The yeast strains resulted in the corresponding (S)-alcohols with a low to moderate conversions. Using recombinant alcohol dehydrogenase whole-cell preparations as biocatalysts ((S)-selective ADH from Rhodococcus aetherivorans (RaADH) and (R)-selective ADH from Lactobacillus kefir (LkADH)) resulted in higher conversions in most cases, while maintaining the full enantiotopic selectivity. Usually, the preparative-scale bioreductions showed comparable or even higher conversions than those observed in the small-scale screening reactions, resulting in virtually enantiopure (S)- and (R)-alcohols (ee > 99%), which are promising chiral fragments with a high degree of drug-likeness. Docking studies confirmed the absolute configuration of the forming (S)- and (R)-alcohols.