| An unidenitified tunicate from Pohnpei, Micronesia, yielded sagittamides A, B and D---compounds comprised of a long-chain C-26 dicarboxylic acid that acylates terminal L-valine and L-ornithine groups. Minor homologs sagittamides C, E, and F were also isolated. The structures, which contain an unprecedented internal O-hexacetyl-1,2,3,4,5,6-hexaol moiety, were determined by combined spectroscopic analysis including mass spectrometry, 1D and 2D NMR, and chemical degradation. The relative and absolute stereochemistry of the new compounds was assigned using a progressive-convergent approach that integrates four powerful regimens for stereochemical analysis of acyclic natural products: Marfey's analysis, J-based analysis, 13C NMR universal database comparisons and exciton coupling circular dichroism. Two series of model compounds totaling 14 analogs were synthesized from the pentoses D-(+)-xylose and D(-)-ribose.; Eleutherobia sp., a soft-coral from Pohnpei, Micronesia, yielded a series of two novel cholic acid derivatives and three known steroids along with known briareins minabein-1, minabein-4, minabein-6, minabein-8, and minabein-9. X-ray crystallographic structure determination of the known compound minabein-4 allowed assignment of its correct relative and absolute stereochemistry. Consequently, the structure of nui-inoalide-D, another name proposed for the same compound, is incorrect.; Trididemnun solidum, a deep-water tunicate from The Bahamas, yielded the known cytotoxic didemin B and methyleno-didemnin A. |
