Reprogramming Imine Reductases for Enantioselective Reduction of Electron-Rich Enamides

Imine reductases (IREDs) are powerful biocatalysts to afford valuable enantioenriched amines via the stereoselective reduction of imines. While significant progress has been made in expanding their catalytic capabilities, direct reduction of electron-rich alkenes by IREDs remains a formidable challenge and is unprecedented in biology. Here, we report the reprogramming of an IRED from <i>Sinorhizobium</i> (<i>Sin</i>IRED) to catalyze the enantioselective reduction of enamides. Through directed evolution, the final optimized <i>Sin</i>IRED-V5 variant is able to reduce a variety of enamides and deliver diverse protected chiral amines in up to 97% yield and >99:1 enantiomeric ratio (e.r.). Mechanistic studies suggest a novel synergistic pathway involving enamine-imine tautomerization followed by NADPH-mediated hydride transfer process. This work expands the catalytic versatility of IREDs and provides the first known example of IRED-catalyzed reduction of electron-rich enamides.