Photochemistry of 4H,7h-Benzo[1,2-c : 4,3-C ']Dipyran-4,7-dione, A Twofold Isocoumarin

[GRAPHICS]The title compound 1 was synthesized starting from 1,2-bis(2-hydroxyethyl)benzene (2), Irradiation (350 nm) of 1 in CH3CN affords the all-cis-dimer 6 in >90% yield. Irradiation (>390 nm) of 1 in the presence of tetrachloroethene leads selectively to the monocyclobuta adduct 8, which on subsequent irradiation is converted to a 4:1 mixture of biscyclobuta derivatives 9 and 10.