Catalytic Asymmetric Reduction of Difficult-to-Reduce Ketones: Triple-Code Saturation Mutagenesis of an Alcohol Dehydrogenase

Catalytic asymmetric reduction of prochiral ketones with the formation of enantio-pure secondary alcohols is of fundamental importance in organic chemistry, chiral man-made transition-metal catalysts, or organocatalysts and enzymes of the alcohol dehydrogenase (ADH) type. A distinct limitation is the traditional requirement that the α- and α′-moieties flanking the carbonyl function differ sterically and/or electronically. Difficult-to-reduce ketones such as tetrahydrofuran-3-one and tetrahydrothiofuran-3-one and related substrates are particularly challenging, irrespective of the catalyst type. The ADH from Thermoethanolicus brockii (TbSADH) is an attractive industrial biocatalyst, because of its high thermostability, but it also fails in the reduction of such ketones. We have successfully applied directed evolution using the previously developed concept of triple-code saturation mutagenesis at sites lining the TbSADH binding pocket with tetrahydrofuran-3-one serving as the model compound. Highly (R)- and (S)-selective variants were evolved (95%–99% ee) with minimal screening. These robust catalysts also proved to be effective in the asymmetric reduction of tetrahydrothiofuran-3-one and other challenging prochiral ketones as well. The chiral products, which are generally prepared by multistep routes, serve as synthons in the preparation of several important therapeutic drugs.