Isolation, Characterization, and Molecular Modeling Studies on Diterpenes From Marine Organisms Withing the Eunicea Genus

Mass spectrometry was used in conjunction with numerous 1-D and 2-D NMR techniques to determine the structures, devoid of stereochemistry, of five different compounds isolated from the extracts of Eunicea succinea, Eunicea tourneforti, and an unidentified species isolated from the Eunicea genus by Professor Meledath Govindan, of the University of the Virgin Islands. Three of the compounds were then identified as the known compounds eunicin, 12,13-bisepieupalmerin, and 7(S),8(S)-epoxy-1(S),11(R)-dolabella-3E,12(18) -dien-13-one by comparison of their spectroscopic data and optical rotations with those published in the literature. Optical rotations could not be measured accurately for the other two compounds because of small sample sizes; therefore, another method had to be found to elucidate the stereochemistry of these two structures.

To solve this problem, molecular modeling and NOESY were employed. Comparison of the NOESY interactions to the thermodynamically available conformations of several possible stereoisomers, calculated by molecular modeling, proved to be a useful technique. One of the remaining two structures was identified as the known stereoisomer euniolide. The stereochemistry of the one remaining structure could not be assigned because sample size was not large enough to obtain a clean NOESY spectra. Finally, based on published synthetic work by Corey and Kania, the absolute stereochemistry of the dolabellane was revised to 7(R),8(R)-epoxy-1(R),11(S)- dolabella-3E,12(18)-dien-13-one.