Comparison of the singlet oxygen ene reactions of cyclic versus acyclic β,γ-unsaturated ketones: an experimental and computational study

The photooxygenation of two β,γ-unsaturated ketones was studied by experimental and computational methods: 5-methyl-hex-4-en-2-one (1) and cyclohex-3-en-1-one (6) as model compounds for acyclic versus cyclic deconjugated enones. The open-chain substrate delivered a 1:1 mixture of regioisomers 2a,b following the established cis-selectivity model whereas the cyclic substrate reacts with 1O2 to give preferentially the conjugated product 7. This effect is in agreement with the mechanistic two-stage no-intermediate model and on a computational level corresponds to a regioselectivity control following the steepest decent pathway from the corresponding transition stages in a valley ridge potential energy surface region.