Photochemische Reaktionen. 110. Mitteilung [1]. Zur Photochemie γ,δ‐Methano‐α‐enone

Photochemistry of γ,δ‐Methano‐α‐enones Direct excitation (λ = 254 or ≥ 347 nm) converts the γ,δ‐methano‐α‐enone ( E )‐ 10 into the isomeric ether 23 and the isomeric diene‐ketone 24 . Furthermore, on 1 π,π*‐excitation (λ = 254 nm) ( E )‐ 10 undergoes an 1,3‐homosigmatropic rearrangement yielding the enone ( E )‐ 25 . In addition ( E → Z )‐isomerization of ( E )‐ 10 and conversion of 10 to the isomeric furan 28 is observed. The isomerization ( E )‐ 10 → 23 , 24 and ( E )‐ 25 proceeds by photocleavage of the C(γ), C(δ)‐bond, whereas the formation of 28 occurs by photocleavage the C(γ), C(δ)‐bond together with that of the C(γ), C(δ′)‐bond of 10 . On direct excitation the bicyclic diene‐ether 23 yields the methano‐enone 10 , the dieneketone 24 and the tricyclic ether 29 . Evidence is given, that the conversion 23 → 10 is a singulet process. On the other hand, the isomerization 23 → 24 and the intramolecular [2 + 2]‐photocycloaddition 23 → 29 are shown to be triplet reactions. Irradiation (λ = 254 nm) of the homoconjugated ketone 24 yields the isomeric ketone 27 by an 1,3‐acyl shift. The excitation of the ( E )‐enone 25 induces ( E → Z )‐isomerization and photoenolization to give the homoconjugated ketone 26 .