Xl. Cai et al., A potentially general method to control relative stereochemistry in enone-olefin 2+2-photocycloaddition reactions by using eniminium salt surrogates, TETRAHEDR L, 41(49), 2000, pp. 9445-9449
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.