Cameroonane, Prenopsane and Nopsane, Three New Tricyclic Sesquiterpene Skeletons

Abstract

The strong patchouli-like and woody smelling essential oil of the rhizomes of Echinops giganteus var. lelyi C. D. Adams (Compositae) contains only sesquiterpenes, which are mainly triquinanes. Besides the known tricyclic compounds, silphiperfol-5- (1, 3) and -6-ene (4), modhephen-2-ene (5), α- (6) and β-isocomene (7), silphiperfolan-7β-ol (12), presilphiperfolan-8-ol (13), silphiperfol-6-en-5-one (14) and 7-epi-silphiperfolan-6β-ol (20), the following compounds, three of which (15, 17, 18) have new skeletons, were found, for the first time, occurring naturally: presilphiperfol-7-ene (2), cameroonan-7-ol (15), an 11(7→8)-abeo-presilphiperfolan-7-ol, prenopsan-8-ol (17), a 1(8→7)-abeo-cameroonan-8-ol, and nopsan-4-ol (18), a 3(4→8)-abeo-prenopsan-4-ol, three diastereomers of silphiperfolan-6-ol (19, 21, 22), modheph-2-en-8-ol (23) and silphiperfola-4,7(14)-diene (24). All structures were elucidated by NMR spectroscopy. A biogenetic pathway from a presilphiperfolane cation C to the cameroonane K, prenopsane L and nopsane M cations is shown. Cameroonanol (15) and prenopsanol (17) are the main contributors to the fragrance of the total oil.