Reaction of Singlet Oxygen with 4-Methyl-2,3-dihydro-γ-pyrans

4-Methyl-1,3-dihydro-γ-pyran (1) reacts with singlet molecular oxygen to yield both a dioxetane 2 and an allylic hydroperoxide 3, each of which is converted by heat into a single product stable under conditions of vapor chromatography. The product composition from 1 and from 1-4-d has been determined in benzene and in acetonitrile, the ratio 2/3 being 26 times greater in the latter solvent. The tritium kinetic isotope effects at the α, β, and γ positions indicate that for both products in both solvents the transition state has a weakened H-Cγ bond, a strengthened H-Cβ bond, and an H-Cα bond whose strength is not appreciably altered. Possible interpretations of these effects are discussed with respect to the detailed mechanism of attack of singlet oxygen on this enol ether, which is also compared with 4,4-dimethyl-l,3-dihydropyran (4).