An Engineered Biocatalyst for Enantioselective Hydrazone Reduction

Enantioselective reduction of hydrazones provides a convergent and versatile route to synthesize hydrazine‐containing motifs that are commonly found in pharmaceuticals and agrochemicals. However current methods require the use of precious metals, costly chiral ligands and/or forcing reaction conditions. Here, we report the development of a biocatalytic approach for enantioselective hydrazone reduction using engineered imine reductases. Following evaluation of an in‐house panel of >400 IRED sequences, we identified a single IR361 I127F L179V variant that promotes reduction of Cbz‐protected hydrazones. Introduction of an additional two mutations via directed evolution afforded HRED1.1 that is 20‐fold more active than the parent template and promotes reduction of a variety of protected hydrazones in high yields and selectivities (>99% e.e.), including in preparative scale biotransformations. Structural analysis of HRED1.1 provides insights into the origins of its unique hydrazone reductase activity. This study offers a powerful biocatalytic route to synthesize valuable chiral hydrazine products and further expands the impressive range of transformations accessible with engineered imine reductases.