Photochemistry of substituted cyclic enones. Part 2. 5-[Prop-2-enyl]cyclopentenones

The preparation of 5-(prop-2-enyl)-3,4,4-, and-2,4,4-trimethylcyclopentenones is described. Both isomers undergo an intramolecular Eaton–de Mayo photochemical reaction to give the appropriate trimethyltricyclo[3,2,1,03.6]octan-2-ones. The reaction appears to proceed via a singles excited state.The tricyclo-octanones can be oxidised under Baeyer–Villiger conditions to afford the expected lactones, and the 6,7,7-trimethyl isomer on treatment with acid gives 6-exo-7,7-trimethylbicyclo[3.2.1]oct-3-en-2-one. The 3,7,7-trimethyl ketone has been reduced to the corresponding alcohol. Quantitative europium-shift measurements have been used to determine the structure of these compounds.