Terpenes and Terpene Derivatives, XXX. Isolation and Synthesis of Isohumbertiol, the First Naturally Occurring Sesquiterpene Alcohol with a Humbertiane Skeleton

From Vassoura oil (Baccharis dracunculifolia DC.) the four isomeric isohumbertiols A–D (7–10), the first naturally occurring sesquiterpene alcohols with a humbertiane skeleton could be isolated and assigned unambiguously by establishing a correlation with the known cabreuva oxides A–D (3–6). Reaction of the ketone (+)‐16 with vinylmagnesium bromide gave a mixture of the isohumbertiols B and D [(+)‐8 and ent‐10]. Treatment of 8/10 with SnCl4 furnished the tetraline derivative 18, whereas with hydrochloric acid the cabreuva oxides B and D (4 and 6) were obtained.