Photochemische Reaktionen. 103. Mitteilung [1]. Zur Photochemie konjugierter Epoxy‐en‐carbonylverbindungen der Jonon‐Reihe: UV.‐Bestrahlung α,β‐ungesättigter ε‐Oxo‐γ, δ‐epoxy‐carbonylverbindungen und Untersuchung des Mechanismus der Epoxy‐enon/Furan‐Isomerisierung

Abstract

The Photochemistry of Conjugated γ,δ‐Epoxy‐ene‐carbonyl Compounds of the Ionone Series: UV.‐Irradiation of α,β‐Unsaturated ε‐Oxo‐γ,δ‐epoxy Compounds and Investigation of the Mechanism of the Isomerization of Epoxy‐enones to FuranesOn 1n, π*‐excitation (λ ≥ 347 nm; pentane) of the enonechromophore of 3, three different reactions are induced: (E/Z)‐isomerization to give 13 (7%), isomerization by cleavage of the C(γ)–C(δ) bond to yield the bicyclic ether 14 (36%) and isomerization by cleavage of the C(γ)O bond to give the cyclopentanones 15 (13%) and 16 (11%; s. Scheme 2). On 1π, π*‐excitation (λ = 254 nm; acetonitrile) 13 (14%), 15 (6%), and 16 (6%) are formed, but no 14 is detected. In contrast, isomerization by cleavage of the C(δ)O bond to give the cyclopentanone 17 (23%) is observed. The reaction 3→17 appears to be the consequence of an energy transfer from the excited enone chromophore to the cyclohexanone chromophore, which then undergoes β‐cleavage. Irradiation of 4 with light of λ = 254 nm (pentane) yields the analogous products 20 (18%), 21 (9%), 22 (7%), and 24 (7%; s. Scheme 2). Selective 1n, π*‐excitation (λ ≥ 280 nm) of the cyclohexanone chromophore of 4 induces isomerization by cleavage of the C(δ)O bond to give the cyclopentanones 23 (9%) and 24 (44%). Triplet‐sensitization of 4 by excited acetophenone induces (E/Z)‐isomerization to provide 20 (12%) and isomerization by cleavage of the C(δ)O bond to yield 21 (26%) and 22 (20%), but no isomerization via cleavage of the C(δ)O bond.It has been shown, that the presence of the ε;‐keto group facilitates C(γ)C(δ) bond cleavage to give a bicyclic ether 14, but hinders the epoxy‐en‐carbonyl compounds 3 and 4 from undergoing cycloeliminations.The activation parameters of the valence isomerization 13 → 18, a thermal process, have been determined in polar and non‐polar solvents by analysing the 1H‐NMR. signal intensities. The rearrangement proceeds faster in polar solvents, where the entropy of activation is about −20 e.u. Opening of the epoxide ring and formation fo the furan ring are probably concerted.