Reconsideration of the Structures of Stemara-13(14)-en-18-ol and Related Diterpene Natural Products: Vinylic Hydrogen Chemical Shifts Are Key.

The reported synthesis of stemara-13(14)-en-18-ol, which revealed that the structure of this natural product was misassigned, prompted an investigation using density functional theory methods into the structural reassignment of this natural product and related diterpenoids extracted from Calceolaria plants. 1 H and 13 C chemical shift predictions led to the reassignment of relative configuration, and in one case the carbon skeleton, of several diterpenoids from Calceolaria . In many of these cases, the chemical shift of the vinylic hydrogen was found to be diagnostic.