Chlorosilphanol A and silphanepoxol, labdane diterpenes from Silphium perfoliatum

Two carterochaetols, chlorosilphanol A and silphanepoxol, isolated from the leaves of Silphium perfoliatum were assigned structures 1 and 3, respectively, from spectral studies and with absolute stereochemistry established by X-ray crystallography of 1. Complete 1H- and 13C-nmr assignments for the parent compounds and their acetates 2 and 4 were made at high-field using 1D and 2D methods, and chlorosilphanol A was chemically converted via its acetate to silphanepoxol. Revision of structure for related carterochaetols in the literature is required, in particular with respect to the stereochemistry at C-12 and that of the side-chain.