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.