On addition of either BF3· Et2O or H2SO4 the valence photoisomerization of 2- methoxytropone (1) in CH3CN is diverted from 1-methoxy-(2) to 3-methoxybicyclo [3.2. 0] hepta-3, 6-dien-2-one (3) as products whereas with tropone there is a change from photodimerization to valence isomerization to give cleanly bicyclo [3.2. 0] hepta-3, 6-dien-2- one (5).