Over the last forty years, the isoIation and characterization of sesquiterpenes has become an increasingly straightforward task for the organic chemist. Early successes in establishing the structures of hydrocarbons such as caryophyllene and longifolene paved the way to the solution of more difficult structural problems such as patchouli alcohol and β-vetivone. But even today new and interesting sesquiterpenoid structures are still being established as evidenced by mint sulphide in peppermint residues and the pungent tamariscol from the liverwort Frullania tamarisci.
From the organic chemist’s point-of-view, one of the more significant developments has been the classification of these structures into 18 groups based on proven or suspected biogenetic relationships involving the cyclization of cis- or trans-farnesyl pyrophosphate. An interesting question is how the compounds in these groups are perceived and used by the creative perfumer.
Sesquiterpenes, by definition, contain 15 carbon atoms which places them among the less volatile perfumery ingredients. Nevertheless, there exists considerable diversity in physical properties between sesquiterpene hydrocarbons such as germacrene D and sesquiterpenoid alcohols, some of which are crystalline solids when isolated in a pure state. These mono-ols particuIarly play an important role in the base and fixative notes of the perfume; however, more polar materials with two or more functional groups are generally insufficiently volatile to be of perfumery interest.