Photochemical Reactions 151st Communication . Photo‐oxygenation of ( E )‐ and ( Z )‐7‐Methyl‐β‐ionone

Photo‐oxygenation of ( E )‐7‐methyl‐β‐ionone (( E )− 1 ) and ( E )‐8‐methyl‐β‐ionone (( E )− 2 ) gave rise to the formation of the hydroperoxy‐enones ( E )− 10 and ( E )− 15 , respectively, which, in part, underwent intramolecular epoxidation to the hydroxy‐epoxy‐ketones 11 and 16 , respectively, The product distribution of the photo‐oxidation of (Z)− 1 shows a marked influence of the skewed ground‐state conformation of the dienone chromophore. Thus, singlet oxygen ( 1 O 2 ) was added to C(γ) of the dienone chromophore leading to the spirocyclic peroxy‐hemiacetal 12 and to the endoperoxide 13 . In addition, the tricyclic peroxide 14 was formed as a new type of product via primary addition of 1 O 2 to C(γ) of the dienone chromophore. The structure of 14 was established by X‐ray crystal‐structure analysis of the hemiacetal 22 .