4 : 7,10 : 13-Diepoxy[15]annulenones exchange their oxygen bridges on irradiation: 17O NMR spectroscopic and chemical verification of the exchanges

A O-17 NMR study shows that, on visible light irradiation, the unsubstituted 4:7,10:13-diepoxy[15]annulenone 1 undergoes a degenerate rearrangement, by which the carbonyl oxygen of 1a is interchangeable with the furan ring oxygens in 1b and in 1c; the 2,15-dimethyl analogue 2 undergoes similar oxygen-bridge exchange to give two positional isomers BC and DE in two different ways (i.e. via photo-irradiation and via a protonation-deprotonation sequence).