Enzymatic Amination for Stereocontrolled Functionalization of Cyclohexanones

Abstract

Functionalizing the symmetric carbonyl carbon of cyclohexanone to achieve stereocontrol, resulting in saturated cyclohexanes with either cis/trans or axial stereochemistry, poses significant challenges in chemical synthesis, and existing methodologies are limited. In this study, we present an enzymatic reductive amination strategy to attain this objective. By engineering the enzyme pocket of the sole imine reductase (IRED) M5, we successfully synthesized over 80 cis/trans and axially chiral 4-substituted cyclohexylamines in a stereo-complementary fashion, adhering to industrial standards, via the reductive amination of 4-substituted cyclohexanones. Mechanistically, the reshaping of the enzyme pocket allows the optimized variants to distinguish between different imine precursors and selectively bind their specific configurations with favorable binding energies, thereby facilitating the generation of stereochemically distinct products. We propose that this stereocontrolled-functionalization strategy could be extended to a broader range of cyclohexylamines with diverse substituents.