A novel pathway in the photooxygenation of cyclic allenes

[equation--see text] The photooxidation of cyclic allenes gives rise to cyclic 1,2, 3-trione hydrates. The formation of these compounds points to a novel photooxidation mechanism involving both singlet and triplet oxygen. Upon placement of a methyl group on the allene, the mechanism shifts to predominantly an "ene" reaction. The corresponding cycloadditions with 4-methyl-1,3,4-triazoline-3, 5-dione (MTAD) with cyclic allenes involve 2 equiv of MTAD. The dipolar intermediates are trapped with H(2)O to give alpha-urazole-substituted 2-cycloalkenones.