Mechanistic photochemical studies on the 1,3‐acyl shift and oxa‐di‐π‐methane rearrangement of two β,γ·γ′,δ′‐dienones

The photochemical rearrangement of the two 2‐(2‐alkenyl)‐2‐(1‐cyclopentenyl)cyclopentanones (E)‐1aand (E)‐2ahave been studied under conditions of both direct (λ 300 nm) and tripletsensitized irradiation in order to determine the applicability of 2‐alkenyl moieties as intramolecular quenchers of the lowest excited triplet states of the β,γ‐enone chromophore. The β,γ·γ′,δ′‐dienone (E)‐1a, upon direct irradiation, exhibits a reversible 1,3‐acyl shift (1,3‐AS) leading to an eventual photostationary state {[(E)‐1a]/[(E)‐1b]}pss= 4.0 ± 0.8, whereas triplet photosensitization of (E)‐1agives the ODPM isomer (E)‐1c. Both processes occur without (E)‐(Z) isomerization of the 2‐butenyl side‐chain. Direct irradiation of (E)‐2ayields, albeit very inefficiently, the 1,3‐AS compound (E)‐2b(Φ < 0.001) as the primary product which, upon electronic excitation, forms, in competing reactions, (E)‐2a(Φ = 0.18) and (Z)‐2b(Φ = 0.035). Upon triplet photosensitization of both (E)‐2aand (E)‐2b, only (E)‐(Z) isomerization of the styryl side‐chain is observed. The eventual photostationary state in the direct irradiation equals {[(E)‐2a]/[(Z)‐2a]/[(E)‐2b]/[(Z)‐2b]}pss=(33± 1)/(33 ± 1)/(17 ± 1)/(17 ± 1). The differences in photochemical behaviour of the β,γ·γ′,δ′‐dienone substrates and their 1,3‐AS products are discussed with special emphasis being placed upon the triplet energies of the 2‐alkenyl chromophores relative to those of the β,γ‐enone chromophores and the occurrence of CT interaction which enhances the internal conversion of theS1(nπ*) states of the β,γ‐enone chromophores.