A potentially general method to control relative stereochemistry in enone–olefin 2+2-photocycloaddition reactions by using eniminium salt surrogates

A new strategy to control the stereochemistry of enone–olefin 2+2-photocycloaddition reactions, based on the use of eniminium salt surrogates, is experimentally tested. In contrast to enone–olefin photocycloadditions, which emanate from enone triplet excited states and follow non-concerted pathways, analogous reactions of conjugated eniminium salts can occur by singlet, concerted routes and, as a result, would deliver cyclobutane ring containing products with complete control of relative stereochemistry. This proposal was tested by using a series of eniminium salts, which contain electron rich and electron poor olefin tethers. The results show that intramolecular 2+2-photocycloadditions of these substrates display moderate to high degrees of stereospecificity.