2-Hydroxycyclopentenone derivatives from photocyclisation of αβ-unsaturated 1,2-diketones: a suggested triplet biradical intermediate

Abstract

The photocyclisation of 5-methylhex-4-ene-2,3-dione and of 1-(o-alkylphenyl)propane-1,2-diones to hydroxycyclopentenone derivatives is examined. In view of the observed stereochemical preference of the reaction it is suggested that initial H abstraction leads to a biradical which photocyclises as an excited triplet and does not lead to the photoenol as an intermediate. The reported photoenolisation of certain o-alkylaceto- and benzo-phenones is discussed in the light of the present work and is rationalised in terms of a thermal opening of a benzocyclobutenol intermediate.