Chemistry, Chemo-Ecology, and Biological Activities of Uncommon and Structurally Diverse Sesquiterpenoids from the Sanya Bay Nudibranch Hexabranchus sanguineus

A detailed chemical investigation of the Sanya Bay nudibranch Hexabranchus sanguineus yielded thirteen new sesquiterpenoids, namely sanyagunins A-H, sanyalides A-C, and sanyalactams A and B, along with eleven known related ones. Sanyalactams A and B feature an unprecedented hexahydrospiro[indene-2,3 '-pyrrolidine] core. The structures of new compounds were established by a combination of extensive spectroscopic data analysis, quantum mechanical-nuclear magnetic resonance methods, the modified Mosher's method, and X-ray diffraction analysis. Based on analysis of NOESY correlations and the modified Mosher's method, the stereochemistry of two known furodysinane-type sesquiterpenoids were revised. A plausible biogenetic relationship between these sesquiterpenoids wasproposed and discussed, and a chemo-ecological relationship of the title animal and its possible sponge preys has been analyzed. In bioassays, sanyagunin B showed moderate antibacterial activity, whereas 4 alpha-formamidogorgon-11-ene exhibited potent cytotoxicity with IC50 values ranging from 0.87 to 1.95 mu M.