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 4α‐formamidogorgon‐11‐ene exhibited potent cytotoxicity with IC50 values ranging from 0.87 to 1.95 μM. [ABSTRACT FROM AUTHOR]