Photochemistry of Epoxyquinones. 6. Norrish Type II Photoreaction of 2,3-Dihydro-2,3-epoxy-1,4-naphthoquinone

Article abstract of DOI:10.1021/jo00137a019

Photochemical reactions of 2,3-dialkyl-substituted or 2-alkyl-substituted 2,3-dihydro-2,3-epoxy-1,4-naphthoquinones in which the carbonyl group has intramolecularly abstractable γ-hydrogen atoms have been studied.On irradiation, epoxynaphthoquinones 11a-h initially afforded cyclobutanols 12a-h and phthiocols 13a,b.The former are Norrish type II cyclization products, while the latter may be formed via an allene oxide intermediate which is a direct Norrish type II elimination product.The cyclobutanols 12a-h underwent secondary photorearrangement to β-diketones 14a-h and β-alkoxy enones 15a-d,g, but 14g and 14h were readily dehydrated on chromatographic separation over silica gel to give indenone derivatives 15g and 15h.Preference for the Norrish type II photoreaction over the generation of the carbonyl ylide or 1,3-diradical is discussed in terms of the rate constants for each process.A tentative mechanism for the novel photochemical reactions of the cyclobutanols is proposed.