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.