Intramolecular photocycloaddition of olefin-tethered benzo[a]spiro[2,5]octa-1,4-dien-3-one derivatives

Photolysis of benzo[a]spiro[2,5]octa-1,4-dien-3-one derivative 8a afforded spiro-fused tetracycles 10 as a pair of diastereomers in 1.7:1 ratio. The photochemical process occurred by cycloaddition of the intermediate-sulfone stabilized biradical 9 to the tethered olefin. Photolysis of dienone 8b gave only spiro-fused tetracycle 11 without bridged adduct 12.