Imine Reductase-Catalyzed Enantioselective Reduction of Bulky α,β-Unsaturated Imines en Route to a Pharmaceutically Important Morphinan Skeleton

The morphinan skeleton is an important sub-structure in many medicines such as dextromethorphan, and can be constructed from 1-benzyl-1,2,3,4,5,6,7,8-octahydroisoquinoline (1-benzyl-OHIQ) derivatives. 1-Benzyl-3,4,5,6,7,8-hexahydroisoquinolines (1-benzyl-HHIQs), the precursors of 1-benzyl-OHIQs, constitute a type of bulky α, β-unsaturated imines. Until now, the application of imine reductases (IREDs) to α, β-unsaturated imines has only rarely been reported. In this study, through evaluation of 48 IREDs, both enantiomers of 1-(4-methoxybenzyl)-1,2,3,4,5,6,7,8-octahydroisoquinoline (1-(4-methoxybenzyl)-OHIQ) were obtained in high yield and excellent optical purity. Among the enzymes, the most steric hindrance-tolerant IRED from Sandarearacinus amylolyticus (IR40) was able to convert various phenyl substituted 1-benzyl-HHIQ to the corresponding 1-benzyl-OHIQ derivatives with excellent enantiometric excess. These results provide an effective route to synthesize these important compounds via enantioselective reduction of bulky α, β-unsaturated imine precursors, which can be readily prepared from 2-(1-cyclohexenyl)ethylamine and corresponding aryl acetic acids.