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 Me 2 SiCl 2 , (CH 2 =CH) 2 SiCl 2 , and Me 2 (CH 2 =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.