Asymmetric Synthesis of Sterically Hindered 1-Substituted Tetrahydro-β-carbolines Enabled by Imine Reductase: Enzyme Discovery, Protein Engineering, and Reaction Development.

We report the discovery of a new imine reductase (IRED), named At IRED, by genome mining. Site-saturation mutagenesis on At IRED generated two single mutants M118'L and P120'G and the double mutant M118'L/P120'G with improved specific activity toward sterically hindered 1-substituted dihydro-β-carbolines. The synthetic potential of these engineered IREDs was showcased by the preparative-scale synthesis of nine chiral 1-substituted tetrahydro-β-carbolines (THβCs), including ( S )-1- t -butyl-THβC and ( S )-1- t -pentyl-THβC, in 30-87% isolated yields with excellent optical purities (98-99% ee).