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 mu g/mL. Methyl tetrahydrosarcoate (1) and diepoxynyalolide (5) exhibited LC50 values of 1.5 mu M in a brine shrimp bioassay, while desacetylnyalolide (4) was only mildly active.