Three Biscembranoids and their Monomeric Counterpart Cembranoid, a Biogenetic Diels–Alder Precursor, from the Soft Coral Sarcophyton elegans

Two new cembranoids, methyl tetrahydrosarcoate ( 1) and methyl tetrahydroisosarcoate ( 2), were isolated from the soft coral Sarcophyton elegans, collected at Kitangambwe Kenya, together with four biscembranoids, the known nyalolide ( 3) and the unprecedented desacetylnyalolide ( 4), diepoxynyalolide ( 5), and dioxanyalolide ( 6). The structures and relative stereochemistry of the compounds were elucidated by interpretation of MS, 1D NMR, COSY, HSQC, HMBC, and NOESY experiments. Compound 1 is most likely the dienophile affording, by a Diels-Alder reaction, the four biscembranoids. Dioxanyalolide ( 6) possesses antibacterial activity against Escherichia coli at a concentration of 1.25 microg/mL. Methyl tetrahydrosarcoate ( 1) and diepoxynyalolide ( 5) exhibited LC 50 values of 1.5 microM in a brine shrimp bioassay, while desacetylnyalolide ( 4) was only mildly active.