Characterization of endoperoxide and hydroperoxide intermediates in the reaction of pyridoxine with singlet oxygen

The photosensitized oxidation of vitamin B6, pyridoxine, is investigated by product and kinetic analysis. Singlet oxygen quenching rates, measured by time-resolved laser flash generation of singlet oxygen followed by monitoring singlet oxygen phosphorescence decay, confirm previous observations that pyridoxine is a moderate quencher. The quenching rate for 3-methoxypyridine is 100 times slower than that for 3-hydroxypyridine, indicating the hydroxy moiety is required for efficient quenching. The chemical quenching rate constant, kr, was estimated by comparison with a known singlet oxygen reaction. Results indicate that the chemical quenching rate of pyridoxine dominates the total quenching. The major reaction product in methanol was isolated and characterized by NMR and MS. The data are consistent with a solvent adduct of the substituted 2,5-pyridinedione. At low temperature, two semistable intermediates were characterized by NMR. The data are consistent with a hydroperoxide and endoperoxide. These intermediates suggest initial attack of singlet oxygen para to the hydroxy group followed by either proton transfer to form the hydroperoxide or addition of the peroxide to the imine to form the endoperoxide. In the presence of protic solvents, the solvent adducts to the imine and elimination of water yield the observed 2,5-pyridinedione product.