Photochemistry of non-conjugated dienones. Part IV. Photolysis and thermolysis of some retro-α-ionones and retro-α-ionols

Abstract

The isomerizations of (E)-retro-α-ionone and a series of methylated and demethyl analogues and some corresponding ionols on direct photolysis and on triplet sensitization have been studied. The direct photolysis of the retro-α-ionones with λ 300 nm leads to 1,3- and 1,5-acyl migration; with λ 254 nm in addition substantial (Z)–(E)-isomerization is observed. On prolonged irradiation the 1,5-acyl shift product is in some cases eventually converted into the 1,3-acyl shift product. Triplet sensitized reactions lead exclusively to (Z)–(E)-isomerization, no oxa-di-π-methane-type rearrangement products are found. The different types of photochemical isomerizations of the retro-α- and retro-γ-ionones on direct photolysis can be correlated with the stereochemical configuration of their hepta-4,6-dien-2-one system. The photolysis of the retro-α-ionols leads to (Z)–(E)-isomerization. Thermolysis of (E)-retro-α-ionone in decalin at 150° leads to (Z)-α-ionone and (E)-β-ionone as primary products. The former product slowly isomerizes to (E)-α-ionone which is thermally stable.