Photochemistry of some non-conjugated unsaturated 1,2-diketones

Abstract

A study of the photochemical reactions of some βγ-unsaturated and γδ-unsaturated 1,2-diketones is reported; it is concluded that the only efficient primary intramolecular processes involve the carbonyl group furthest removed from the alkyl chain, which may undergo cycloaddition via a five-membered transition state to give 5-oxabicyclo-[2,1,1]hexan-2-one derivatives, or γ-hydrogen abstraction leading to 2-hydroxy-3-vinylcyclobutanones. In the latter case a ready thermal isomerisation may then occur leading to an isomeric 1,2-diketone. 5,5-Dimethylhept-6-ene-2,3-dione, in which neither of the above processes can occur, undergoes a very inefficient intramolecular photocycloaddition whereas its saturated analogue enolises on irradiation.