UVA self-photosensitized oxygenation of beta-ionone

The steady-state UVA (350 nm) photolysis of (E)-beta-ionone (1) in aerated toluene solutions was studied by (1)H NMR spectroscopy. The formation of the 1,2,4-trioxane (2) and 5,8-endoperoxide (5) derivatives in the ratio of 4:1 was observed. Time-resolved laser induced experiments at 355 nm, such as laser-flash photolysis, photoacoustic and singlet oxygen (1)O(2) phosphorescence detection, confirmed the formation of the excited triplet state of 1 with a quantum yield Phi(T) = 0.50 as the precursor for the generation of singlet oxygen (1)O(2) (Phi(Delta) = 0.16) and the isomeric alpha-pyran derivative (3), which was a reaction intermediate detected by NMR. In turn, the reaction of (1)O(2) with 1 and 3 occurred with rate constants of 1.0 x 10(6) and 2.5 x 10(8) M(-1)s(-1) to yield the oxygenated products 5 and 2, respectively, indicating the relevance of the fixed s-cis configuration in the alpha-pyran ring in the concerted [2+4] cycloaddition of (1)O(2).