Synthesis, Olfactory Evaluation, and Determination of the Absolute Configuration of the 3,4‐Didehydroionone Stereoisomers

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 ).