Biosynthesis of the Irregular C12-Terpenoid Dehydrogeosmin in Flower Heads of Rebutia marsoneri WERD. (Cactaceae)

Farnesol (3a) is the precursor to the irregular C12 terpenoid (4S, 4aS, 8aS)‐1,2,3,4,4a,5,8,8a‐octahydro‐4,8a‐dimethylnaphthalen‐4a‐ol (= dehydrogeosmin; 1). The irregular C‐backbone originates from the oxidative removal of a C3 side chain from a C15 eudesmane‐type intermediate (Scheme 2). The bicyclic C‐framework is assembled by a formal addition of H2O across the endocyclic double bonds of a monocyclic germacradiene‐type precursor. The biosynthetic pathway follows from administration of the deuteriated farnesols 3b–e to flower heads of the cactaceae Rebutia marsoneri Werd. and mass‐spectroscopic analysis of the collected volatiles.