Photochemical [2+2] Cycloreversion Reactions of 1, 2, 2a, 8b-Tetrahydrocyclobuta [c] isoquinolin-4 (3H)-ones

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Irradiation (350nm) of 2-(3-butenyl)-and 2-(4-pentenyl) isoquinolin-1 (2H)-ones (1a and 1b) afforded the intramolecular [2+2] adducts (2a and 2b). The additions proceeded regioselectively to give only the parallel adducts. Though stable to irradiation at 350nm, these adducts (2a and 2b) gave o-vinylbenzamide derivatives (3a and 3b) on irradiation at 300nm. The intermolecular [2+2] adducts (7d and 7e) obtained by irradiation (≥300nm) of isoquinolin-1 (2H)-one (6) in the presence of a large excess of alkenes were also found to be unstable and reverted to the original components [isoquinolin-1 (2H)-one (6) and alkenes] on irradiation at 300nm in the absence of the alkenes.