Three benzyl 1-cycloalkenyl ketones, I–III, were irradiated in the presence of boron trifluoride-diethyl ether (0.05 mol/1) in benzene to give the products V–VII, formed by the bonding between the β-position of αβ-unsaturated ketone and the ortho position of the benzene ring. Benzyl 1-cyclooctenyl ketone (IV) was inactive to the photocyclization because of the preferential geometrical isomerization of the olefin moiety. Trifluoro-acetic acid promotes the reaction exceedingly, but the reaction takes place also in methanol. The reaction in methanol was deduced from quenching experiments to be a triplet process. The reaction takes both singlet and triplet processes when irradiation is carried out in the presence of boron trifluoride–diethyl ether. The participation of the complex between benzyl 1-cycloalkenyl ketone and boron trifluoride is proposed for the singlet process.
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