Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
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.
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