Sila‐α‐galbanone and Analogues: Synthesis and Olfactory Characterization of Silicon‐Containing Derivatives of the Galbanum Odorant α‐Galbanone

Silicon compounds 1b–3b are sila-analogues of the galbanum odorants α-galbanone (1a), α-spirogalbanone (2a), and nor-α-galbanone (3a), respectively. Sila-α-galbanone (1b), sila-α-spirogalbanone (2b), and sila-nor-α-galbanone (3b) were synthesized in multistep syntheses in isomerically pure form, starting from Me2SiCl2, (CH2=CH)2SiCl2, and Me2(CH2=CH)SiCl, respectively. Hydroformylation of vinylsilanes, followed by either ring-closing aldol condensation or ring-closing alkene metathesis, were the key steps in these syntheses. The C/Si pairs 1a/1b, 2a/2b, and 3a/3b were studied for their olfactory properties. All compounds possess green-fruity galbanum-type odors with pineapple aspects and thus are olfactorily related. However, sila-analogues 1b–3b were found to be weaker than the corresponding parent carbon compounds 1a–3a. This effect is most pronounced for the C/Si pair 2a/2b as indicated by the odor thresholds of 0.023 ng L–1 air (2a) and 3.8 ng L–1 air (2b). However, due to their higher molecular mass, the silicon compounds are less volatile and thus more substantive in functional applications. In contrast to the stable 5-silacyclohex-1-enes 1b and 2b, the 4-silacyclopent-1-ene 3b shows a limited chemical stability. For compound 3a an extremely low odor threshold of 0.0087 ng L–1 air was measured, and the silicon analogue 3b with an odor threshold of 0.085 ng L–1 air is the sila-odorant with the lowest odor threshold measured so far.