Novel solvent hydrogen-bonding effects in the singlet oxygen ene reaction: a comparison of α,β-unsaturated esters and acids

The photooxidations of tiglic acid, angelic acid, 2,3-dimethyl-2-butenoic acid and their corresponding methyl esters using singlet oxygen were investigated in methanol and methanol/water solvent mixtures and compared to non-hydrogen-bonding solvents with different dielectric constants. The major and minor ene allylic hydroperoxide products were quantified and this ratio for the acids was found to be dependent on a hydrogen-bonding interaction between the solvent and substrate.