| Oil of vetiver, obtained from the roots of the grass Vetiveria zizanoides and widely used in the perfume industry, is purportedly repellent to insects. A sample of the oil of Javanese origin demonstrates potent topical irritant activity on flies and cockroaches; this activity is restricted largely to the carbonyl fraction, obtained by treatment of the oil with Girard's reagent T.;We now report the isolation and characterization of five new sesquiterpenes from this carbonyl fraction. The structures of three of these, E-isovalencenal (92), Z-isovalencenal (110), and noreudesmadienone (112) were confirmed by partial synthesis. Cadinenal (51) and dehydrocadinenal (50) were shown to have novel, cadinane-based structures with four contiguous asymmetric centers. An intramolecular Diels-Alder route is proposed as a possible entry to the cadinane stereochemistry. Octalone 16 was also isolated in the course of this work. This compound had been isolated previously from oil of vetiver; in the present study, we provide direct evidence for the trans stereochemistry of its two methyl groups by means of difference NOE proton NMR spectroscopy.;As a result of these studies, 80% of the carbonyl fraction of the essential oil from Vetiveria zizanoides, including all of the major and many of the minor constituents, has now been characterized. |
