Abstract
The synthesis of 3,4‐didehydroionone isomers 4, (+)‐6, and (−)‐6 and of 3,4‐didehydro‐7,8‐dihydroionone isomers 5, (+)‐7, and (−)‐7 was accomplished starting from commercially available racemic α‐ionone (1). Their preparation of the racemic forms 4–7 was first achieved by mean of a number of chemo‐ and regioselective reactions (Schemes 1 and 2). The enantio‐ and diastereoselective lipase‐mediated kinetic acetylation of 4‐hydroxy‐γ‐ionone (10a/10b) provided 4‐hydroxy‐γ‐ionone (+)‐10a/(±)‐10b and (+)‐4‐(acetyloxy)‐γ‐ionone ((+)12b) (Scheme 3). The latter compounds were used as starting materials to prepare the 3,4‐didehydro‐γ‐ionones (+)‐ and (−)‐6 and the 3,4‐didehydro‐7,8‐dihydro‐γ‐ionones (+)‐ and (−)‐7 in enantiomer‐enriched form. The absolute configuration of (+)‐12b was determine by chemical correlation with (+)‐(6__S__)‐γ‐ionone ((+)‐3) and with (−)‐(6__S__)‐α‐ionone ((−)‐1) therefore allowing to assign the (S)‐configuration to (+)‐6 and (+)‐7. Olfactory evaluation of the above described 3,4‐didehydroionone isomers shows a significant difference between the enantiomers and regioisomers both in fragrance feature and in detection threshold (Table).