Semisynthesis of rosmanol and its derivatives. Easy access to abietatriene diterpenes isolated from the genus Salvia with biological activities

The known diterpenes rosmanoi (3), rosmaquinone (4), 7-methoxyrosmanol (5), 7-ethoxyrosmanol (6), galdosol (7), and epirosmanol (8) have been obtained by partial synthesis from carnosol (2), an abundant natural product present in Salvia species. The physical and spectroscopic data of these semisynthetic diterpenes were identical to those of authentic natural samples and with data reported in the literature. These abietane diterpenes have very interesting biological activities and are present in the genus Salvia in low quantities; thus, the semisynthetic approach described here represents an efficient alternative method to obtain these compounds. Additionally, the known diterpene 16-hydroxyrosmanol (10) and a new aromatic diterpene 11 were obtained from 16-hydroxycarnosol (9) by reaction with Ph₃P/NBS in CH₂Cl₂. The structure of the new compound 11 was established from its spectroscopic data as 12,16-epoxycarnosol.