Photochemistry of dienones. Part 6. On the reacting excited states of retro-γ-ionones

Abstract

(E)- and (Z)-retro-γ-ionone (1a) and (2a) and (E)-retro-γ-ionol (1b) upon triplet photosensitization undergo only (Z)–(E) isomerization. The bicyclo-octene derivatives (3a and b) and the tricyclic oxetan (4) obtained upon direct irradiation of, respectively, (1a), (1b), and (2a) at λ 254 nm result from the excited singlet states. From the dependence of the photostationary state ratio on the triplet energy of the sensitizer it was concluded that the triplet energies of (Z)- and (E)-retro-γ-ionone and (Z)- and (E)-retro-γ-ionone and (Z)- and (E)-retro-α-ionone are 73 ± 2, 60 ± 2, 65 ± 1, and 60 ± 2 kcal mol^–1 respectively.