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.