Catalyst-free, visible-light-induced [2π + 2σ] cycloaddition towards azabicyclohexanes

Abstract

2-Azabicyclo[2.1.1]hexanes (aza-BCHs) are constrained pyrrolidine analogues with improved physicochemical characteristics in drug design. Here, we report a direct visible light-mediated photocycloaddition of 4-aza-coumarins with mono- or disubstituted bicyclo[1.1.0]butanes for synthesizing aza-BCHs without an external catalyst. The introduction of the ester group on 4-azacoumarin is critical for direct imine excitation and versatile synthetic utility. Preliminary mechanistic studies indicated that the reaction took place primarily at the triplet hypersurface.