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-photocycl oaddition reactions, based on the use of eniminium salt surrogates, is expe rimentally tested. In contrast to enone-olefin photocycloadditions, which e manate from enone triplet excited states and follow non-concerted pathways, analogous reactions of conjugated eniminium salts can occur by singlet, co ncerted routes and, as a result, would deliver cyclobutane ring containing products with complete control of relative stereochemistry. This proposal w as 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 o f stereospecificity. (C) 2000 Elsevier Science Ltd. All rights reserved.