Synthesis and Olfactory Evaluation of (+)- and (-)- gamma-Ionone.

The synthesis of enantiometically pure (+)- and (-)-gamma -ionone 3 is reported. The first step in the synthesis is the diastereoisomeric enrichment of 4-nitrobenzoate derivatives or racemic gamma -ionol 12. The enantioselective lipase-mediated kinetic acetylation of gamma -ionol 13b afforded the acetate 14 and the alcohol 15, which are suitable precursors of the desired products (-)- and (+)-3, respectively. The olfactory evaluation of the gamma -ionone isomers shows a great difference between the two enantiomers both in fragrance response and in detection threshold. The selective reduction of (-)-3 and (+)-3 to the gamma -dihydroionones (-)-(R)-16 and (+)-(S)-17, respectively, allowed us to assign unambiguously the absolute configuration of the gamma -ionones.