Spezifisch π→π*‐induzierte Reaktionen von γ‐Dimethoxymethylcyclohexen‐2‐onen: 1,3‐Umlagerung und Wasserstoffabstraktion durch das α‐Kohlenstoffatom

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Abstract
When α,β‐unsaturated γ‐dimethoxymethyl cyclohexenones are excited to the S2(π,π*) state, certain unimolecular reactions can be observed to compete with S2 → S1 internal conversion. These reactions do not occur from the S1(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8, 9 → 32 + 33), γ → α formylacetal migration (cf. 6 → 27, 8 → 30, 9 → 34, 12 → 37), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28, 8 → 31, 12 → 38, 20 → 40 + 41).The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ (40a + 41a) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*‐induced transformations is placed energetically higher than the S1(n,π*) state, and it is either identical with the thermally equilibrated S2(n,π*) state, or reached via this latter state.The linear dienone 14 undergoes a similar π→π*‐induced cyclization (→ 42) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross‐conjugated cyclohexadienones.