Site-selective and stereoselective intramolecular (2 pi+2 pi) photocycloaddition of arylalkenes to pyrene and its photocycloreversion

[GRAPHICS] Irradiation of a benzene solution containing methyl p(1-pyrenylmethoxymethy l)cinnamate (1a) with a high-pressure Hg lamp through Pyrex filter stereose lectively gave an intramolecular (2 pi + 2 pi photocycloadduct (2a) in an 8 3% yield in a site-selective manner at the 4,5-position of the pyrene ring. Similar irradiation of an ortho-substituted derivative (3) afforded the co rresponding (2 pi + 2 pi) cycloadduct (4) as a sole product at the 9,10-pos ition of pyrene. The site-selective photocycloaddition can be reasonably ex plained by the intramolecular sandwich-type singlet exciplexes between the pyrene and phenyl rings.