Structure‐odor correlation, XXV. Synthesis and Olfactory Properties of the “Inverse” Megastigmatrienone and Its Analogs

The oxoisophorone monoacetal 2 is a convenient starting material for the synthesis of several megastigmatrienone analogs. Thus, the Wittig reaction of ketone 2 , followed by deprotection, led to the dienones 7–10 . The “inverse” megastigmatrienone 1 was synthesized from 2 via homologation (→ 11 → 12 ), Grignard reaction (→ 13 ) and hydrolysis. The mixture of the four stereoisomers of 1 is dominated by ( E , E )‐ 1 . In contrast, ( Z , Z )‐ 1 was obtained as the main product when starting from 2 and proceeding via the reaction sequence → 14 → 15 → 16 → 17 and then (Z)‐selective Wittig reaction of aldehyde 17 . Likewise, ketoaldehyde 17 is the starting material for the nor derivative 20 . The odor evaluation of 1 showed that the typical strong and pleasant tobacco odor of megastigmatrienone D was by no means observed, neither in strength nor in quality. On the other hand, the four stereoisomers of 1 all exhibit a similar weak to moderate odor of floral, sweet, fresh and coumarin‐like tonalities.