A Lewis Acid Catalyzed Asymmetric Photocycloaddition

Significance

Enantioselective photochemical ­reactions are inherently difficult due to the high ­energy of the excited intermediates. Over two decades ago, the [2+2] photocycloaddition of coumarin was found to be enhanced by the addition of a Lewis acid (F. D. Lewis, S. V. Barancyk J. Am. Chem. Soc. 1989, 111, 8653). In the present communication, the authors disclose an intramolecular, enantioselective variant of this reaction, ­revealing a new concept for asymmetric induction of photochemical reactions.