15. Photochemische Reaktionen. Zur Photochemie tetraalkylsubstituierter γ-Keto-olefine

Article abstract of DOI:10.1002/hlca.19800630116

The phtochemistry of 7,8-dihydro-β-ionone (1) in solution is shown to depend on temperature, polarity and viscosity of the solvent.UV irradiation (λ>=245nm) in pentane at +25 deg converts 1 to the isomeric ethers 3 (16percent), 5A (48percent) and 5B (22percent), whereas at -65 deg 7,8-dihydro-γ-ionone (26) is obtained in 12percent yield together with 13percent of 3, 12percent of 5A and 9percent of 5B.The ¹n, ?^*-excitation of 1 in acetonitrile gives similar results.In the more viscous 1,2,3-triacetoxypropane the photoisomerization 1 - 26 takes place even at +60 deg (10percent yield, cf. 40percent at -15 deg).In alcoholic solvents, however, no formation of 26 is detected, but the hitherto unknown <2+2>-photocycloaddition 1 - 11 can be observed (4percent at -7 deg, 15percent at -65 deg in 2-propanol).An intermediate e may be involved (Scheme 14).In addition to the photoreactions 1 - 3, 5A, 5B and 11 the isomerization of 1 to the novel spirocyclic ketone 28 takes place in alcoholic solvents only.Photoisomerization 1 - 3 is presumably a photo-ene process involving a stereoselective intramolecular H-transfer.This type of photoisomerization is restricted to cyclic γ-keto-olefines.The tetraalkylated acyclic γ-keto-olefines 14 and 15 photoisomerize exclusively by <2+2>-cycloaddition, independent of the solvent.On ¹n, ?^*-excitation the δ, ε-unsaturated bicyclic ketone 44 undergoes Norrish-Type-II photofragmentation to the diene 45 or isomerizes to the γ, δ-unsaturated ketone 17.Competition between these two reactions is strongly temperature dependent: photolysis in pentane at -72 deg yields quantitatively 45, whereas at +35 deg only 30percent of 45 and 68percent of 17 are obtained.UV irradiation of the novel spirocyclic ketone 28 gives as primary photoproduct the isomeric aldehyde 29, and in a secondary photoreaction the isomeric oxetanes 30A and 30B.Experiments with deuteriated substartes show that the isomerization of type 28 - 29 is stereocontrolled.