Photochemische Reaktionen. 106. Mitteilung. Zur Photochemie tetraalkylsubstituierter γ‐Keto‐olefine

The Photochemistry of Tetraalkyl Substituted γ‐Keto‐olefines The photochemistry of 7,8‐dihydro‐β‐ionone ( 1 ) in solution is shown to depend on temperature, polarity and viscosity of the solvent. UV. irradiation (λ ≥ 245 nm) in pentane at +25° converts 1 to the isomeric ethers 3 (16%), 5A (48%) and 5B (22%), whereas at −65° 7,8‐dihydro‐γ‐ionone ( 26 ) is obtained in 12% yield together with 13% of 3 , 12% of 5A and 9% of 5B . The 1 n ,π*‐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° (10% yield, cf . 40% at −15°). In alcoholic solvents, however, no formation of 26 is detected, but the hitherto unknown [2+2]‐photocycloaddition 1 → 11 can be observed (4% at −7°, 15% at −65S° 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 1 n ,π*‐excitation the δ,ϵ‐unsaturated bicyclic ketone 44 undergoes Norrish ‐Type‐II photofragmentation to the diene 45 or isomerizes to the γ, ϵ‐unsaturated ketone 17 . Competition between these two reactions is strongly temperature dependent: photolysis in pentane at −72° yields quantitatively 45 , whereas at + 35° only 30% of 45 and 68% 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 substrates show that the isomerization of type 28 → 29 is stereocontrolled.