Photochemical oxygenation of phenols by pyrimido[5,4-g]pteridne N-oxide. Comparative studies with pyridazine and isoalloxazine N-oxides

1,3,7,9-Tetrabutylpyrimido[5,4-g]pteridirie-2,4,6,8(1H,3H,7H,9H)-tetraone 5-oxide 1 transfers its N-oxide oxygen to phenols, i.e., phenol 5, p-cresol 6, L-tyrosine methyl ester 7, and p-hydroxyacetanilide (acetaminophen)8, under photochemical conditions to give the corresponding dihydric phenols as major products without any accompanying photochemical intramolecular rearrangements of the N-oxide group taking place. This oxygenation is reasonably explained in terms of a photo-induced single-electron transfer (SET) followed by oxygen-atom transfer (the SET mechanism) which occurs via the initial formation of a charge-transfer complex between compound 1 and the phenols employed. Comparative experiments with 3,10-dibutylisoalloxazine 5-oxide 3 and 3-methylpyridazine 2-oxide 4 well demonstrate the simplicity and the mechanistic characteristics of the photochemistry of compound 1.