Photochemische Reaktionen 94. Mitteilung. Vinyloge β-Spaltung bei Epoxy-enonen der Jonon-Reihe

Photoinduced Vinylogous β-Cleavage of Epoxy-enones of the Ionone Series The photochemistry of the α,β-unsaturated γ,δ-epoxy-enones 1–3 is determined by: (i) C(γ)-O-scission of the epoxide (vinylogous β-cleavage of Type A); (ii) C(γ)-C(δ)-cleavage of the oxirane (vinylogous β-cleavage of Type B); (iii) (E/Z)-isomerization of the enone chromophore. In contrast, 4 with tertiary C(β) shows no Type B cleavage. Type A cleavage is induced both by n,π*- and π,π*-excitation and arises probably from the T₁-state, but Type B cleavage is observed only on π,π*-excitation and represents presumably a S₂-reaction. On Type A cleavage 1–4 undergo 1,2-alkyl-shifts to 1,5-dicarbonyl compounds ( 15–18, 25–28, 34 and 35 ) or rearrange to dihydrofuranes ( 7 and 30 ). The isomerization 1→7 proceeds by a stereoselective 1,3]-sigmatropic shift. On Type B cleavage 1–3 isomerize to a bicyclic enol-ether ( 8, 29 ) or to a monocyclic enol-ether ( 9 ; product of a homosigmatropic 1,5]-shift) or undergo fragmentation to isomers such as allenes 10, 22 and 31 or cyclopropenes 11 and 21 . The non-isolated, unstable (Z)-epoxy-enones 14, 19, 24 and 38 isomerize by fragmentation to the furanes 12, 23, 33 and 39 respectively, on contact with traces of acid or by heating. However, for 19 and 4 , Type B cleavage may lead to the furanes 23 and 39 . On UV. irradiation of the epoxy-enone 4 the initially formed (E/Z)-isomers 34 and 35 yield on π,π*-excitation the enones 37 and 40 by a vinylogous β-fragmentation. In addition, on n,π*-excitation 34 isomerizes to 35 , which decarbonylates exclusively to the enone 37 . The reactions of 1–4 with BF₃ · O(C₂H₅)₂ were also studied (see appendix). The epoxy-enones 1 and 2 isomerize by an 1,2-alkyl shift in good yield to the 1,4-dicarbonyl compounds 79 and 81 , whereas 3 gives the 1,4-diketone 83 , and in small amounts the 1,5-diketone 84 . On the other hand, 4 is converted to the fluorohydroxy-enone 85 and to the 1,5-dicarbonyl product 34 , the only isomer in this series which is identical with one of the photoproducts.