Asymmetric Synthesis of Chiral Benzoins Enabled by Engineered Ketoreductase-Catalyzed Stereoselective Reduction of Benzils

Chiral benzoins are versatile precursors to various valuable synthons such as chiral hydrobenzoins and chiral vicinal amino alcohols. In principle, ketoreductase (KRED)-catalyzed asymmetric reduction of benzils could be one of the most straightforward, green, and stereoselective methods for the synthesis of chiral benzoins. Nevertheless, the practical application potential of this appealing approach has been hampered by the low substrate loadings (usually <10 g/L) and unsatisfactory catalytic efficiency reported to date. In the present work, through enzyme screening, wild-type (WT) ketoreductase SSCR was revealed to possess a decent specific activity of 69.8 U/mg against the unsubstituted benzil molecule 1a. Aided by structure-guided semirational protein engineering including alanine-scanning and site-saturation mutagenesis, SSCRQ245G, a variant of SSCR, was evolved with 5.0-fold improvement in specific activity (350.5 U/mg) and 7.0-fold enhancement in catalytic efficiency (1131 s–1·mM–1), compared with the WT. By using SSCRQ245G as the reductase and employing glucose dehydrogenase (GDH) for the regeneration of nicotinamide adenine dinucleotide phosphate (NADPH), gratifyingly, as high as 100 g/L of 1a was completely reduced in a 5-gram-scale reaction, with the desired benzoin (S)-2a obtained in 96% isolated yield with 99% ee. Our developed biocatalytic synthesis also features a good space-time yield of 233 g·L–1·day–1 and a decent E-factor of 44 (including water). Moreover, a three-step transformation of such biosynthesized benzoin (S)-2a into a synthetically useful chiral vicinal amino alcohol ligand 5 was realized in 64% overall yield. Finally, the synthetic potential of SSCRQ245G was further underscored by the smooth reduction of 16 other symmetrical and unsymmetrical benzils, delivering the corresponding chiral benzoins in 57–93% isolated yields with mostly good-to-excellent stereoselectivities (up to >99% ee). The current study not only provides efficient and sustainable access to chiral benzoins but also highlights the synthetic versatility of engineered ketoreductase-catalyzed stereoselective reduction of prochiral ketones.