Zur Photochemie des Iso‐methyl‐α, (E)‐jonons

UV.‐irradiation of iso‐methyl‐α, (E)‐ionone (4) in neutral solvents yields iso‐methyl‐α, (Z)‐ionone (5), the bicyclic ether 6 and the epoxide 7 by a sequence of successive photoisomerizations. The steps leading to des‐methyl homologues of 6 and 7 do not occur on irradiation of α, (E)‐ionone (1) [10]. The reversible isomerization 4 ⇄ 5 is followed by the irreversible photoprocess 5 → 6 and the final transformation 6 → 7. Irradiation of iso‐methyl‐α, (E)‐ionone (4) in acidic or basic solvents leads to a deep change in the type of products and gives the isomeric ketones 9 and 10 in high yields. A tentative mechanism for the photoisomerization steps 5 → 6, 6 → 7 and 5 → 9 + 10 is proposed.