Biosynthesis of the Irregular C 12 ‐Terpenoid Dehydrogeosmin in Flower Heads of Rebutia marsoneri W ERD. ( Cactaceae )

Farnesol ( 3a ) is the precursor to the irregular C 12 terpenoid (4 S , 4a S , 8a S )‐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 C 3 side chain from a C 15 eudesmane‐type intermediate ( Scheme 2 ). The bicyclic C‐framework is assembled by a formal addition of H 2 O 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.