Highly Diastereo- and Enantioselective Imine Reductases Engineered for γ-Piperidinamines Bearing Contiguous...

Imine reductases (IREDs) represent a highly promising class of biocatalysts for the asymmetric synthesis of chiral amines bearing two stereogenic centers. Herein, we report an IRED-catalyzed asymmetric reductive amination strategy that converts racemic ketones into γ-piperidinamines with contiguous stereocenters and operates through a pathway consistent with a dynamic kinetic resolution-asymmetric reductive amination (DKR-ARA). The wild-type AcIRED afforded the 1-benzyl-N-cyclopropyl-3-methylpiperidin-4-amine (1a) in 60.8% conversion, but with only 78.5% enantiomeric excess (ee_cis) for (3S,4R)-1a and 23.5% ee_trans for (3R,4R)-1a, giving an 84:16 diastereomeric ratio (dr). To enhance stereoselectivity, we employed a "trans-drop, enantio-catch" strategy to reshape the pocket architecture of AcIRED. The resulting variant Mut-5 afforded 99.9% ee_cis for (3S,4R)-1a and > 99:1 dr, along with a 19-fold increase in specific activity and a 24.7°C elevation in $T_{50}^{15}$ . Remarkably, at a cumulative substrate loading of 700 mM (142.3 g·L^-1), Mut-5 furnished (3S,4R)-1a with 95% conversion, a space-time yield (STY) of 208.7 g·L^-1·d^-1. This strategy not only provided highly efficient imine reductases but also established a robust platform for the atom-economical synthesis of γ-piperidinamine building blocks bearing two stereogenic centers.