Synthesis and Olfactory Characterization of Novel Silicon-Containing Acyclic Dienone Musk Odorants

With an odor threshold value of 0.54 ng L–1 air, (3E,5E)‐7,7‐dimethyl‐5‐tert‐butylocta‐3,5‐dien‐2‐one (1a) constitutes the most potent member of a new family of acyclic musk odorants with ionone aspects. Replacement of the quaternary carbon atoms of 1a with silicon atoms leads to the (di)sila analogues 1b (replacement of the carbon atom C‐7), 1c (replacement of the quaternary carbon atom of the 5‐tert‐butyl group), and 1d (replacement of both quaternary carbon atoms). Compounds 1b–1d were prepared in multistep syntheses, and their olfactory properties were characterized. The disila analogue 1d turned out to be the weakest compound of the series (197 ng L–1 air), with only very faint musky aspects. The 7‐sila analogue 1b was very floral, of iononeand rose character, and also had a distinct musky note (1.63 ng L–1 air). The 5‐trimethylsilyl analogue 1c was the most musky sila odorant of the series (10.6 ng L–1 air), with rosy but not ionone‐like facets. Thus, exchange of one or both tert‐butyl groups of 1a for a trimethylsilyl substituent had quite dramatic effects on the odor, and steric bulk at the carbon atom C‐5 seems to be important for creating a musk odor impression. Compounds 1b–1d [(di)sila analogues of the acyclic dienone musk odorant 1a] were synthesized and characterized for their olfactory properties. C/Si exchange had quite a dramatic effect on the musky‐violet odor profile, and on the odor threshold. This indicates that the C‐5 substituent is critical for a musky impression, whereas the substituent on C‐6 has greater responsibility for floralcy.