Some of the photochemical reactions of γ, δ-epoxyenones of the ionone series which have been investigated by ETH-group during the years 1968-1984 are reviewed. In particular the influence of the various functional groups in the neghbourhood of the oxirane on the photochemical behavior is discussed.
On n, π*-excitation, in general, the γ, δ-epoxyenones undergo (E/Z) -isomerization and/or product formation via cleavage of the C (γ), O-bond of the oxirane. Selective π, π*-excitation, however, also causes reactions which include cleavage of the C (γ), C (δ) -bond of the oxirane leading to ylide and carbene intermediates which undergo various reactions with participation of neighbouring groups. The types and the distribution of the isolated products dramatically depends on the functional group placed in ε-position.
On vapor phase thermolysis, in general, the epoxyenones undergo a 1, 5-homosigmatropic H-shift with cleavage of the C (γ), C (δ) -bond of the oxirane leading in high yields to divinyl ethers. Furthermore, thermal rearrangements via ylide intermediates were also observed resulting in the same products as obtained on photolysis.