Sila-[alpha]-galbanone and Analogues: Synthesis and Olfactory Characterization of Silicon-Containing Derivatives of the Galbanum Odorant [alpha]-Galbanone

Silicon compounds 1b-3b are sila-analogues of the galbanum odorants [alpha]-galbanone (1a), [alpha]-spirogalbanone (2a), and nor-[alpha]-galbanone (3a), respectively. Sila-[alpha]-galbanone (1b), sila-[alpha]-spirogalbanone (2b), and sila-nor-[alpha]-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 ngL-1 air (2a) and 3.8 ngL-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 ngL-1 air was measured, and the silicon analogue 3b with an odor threshold of 0.085 ngL-1 air is the sila-odorant with the lowest odor threshold measured so far.