Enantioselective Lewis Acid Catalysis in Intramolecular 2+2 Photocycloaddition Reactions of Coumarins

Photochemistry goes acidic! In the presence of the chiral Lewis acid catalyst 2 the intramolecular 2+2 photocycloaddition of 4-(alk-4-enyl)coumarins (1) provides the corresponding products 3 with up to four stereogenic centers in a highly chemo- (84–89 % yield) and stereoselective fashion. For R=H, enantioselectivities up to 82 % ee are possible with 50 mol % catalyst 2 and up to 78 % ee with only 20 mol % of catalyst 2. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.