Calyculaglycosides D and E, Novel Cembrane Glycosides from the Caribbean Gorgonian Octocoral Eunicea Species and Structural Revision of the Aglycon of Calyculaglycosides A−C

A recent investigation of the constituents of Eunicea sp. from Colombia and Puerto Rico has provided two novel cembrane glycosides, calyculaglycosides D (6) and E (7), and a new cembranoid diterpene in free form, (+)-nephthenol (4). Metabolites 6 and 7 possess a glycosyl-fused cembrane skeleton with a 1S configuration. Biogenetic considerations have led to a revision of the previously assigned dilophol skeleton of calyculaglycosides A−C, which was confirmed by extensive 2D NMR investigation and a chemical degradation study. Consequently, the true structures for calyculaglycosides A−C are 8−10, respectively, not 1−3. This is the first report of the occurrence of marine diterpene glycosides having a cembrane aglycon.