Eleven undescribed sesquiterpenoids with multiple skeleton types and cytotoxicity from the South China Sea soft coral Lemnalia sp.

Marine organisms have great potential for medical and bioproducts applications, with corals having been in the spotlight as an important research target for the discovery of new bioactive marine natural products. Eleven undescribed sesquiterpenoids, named lemnalinoids A-K (1-11), and one known sesquiterpenoid (12) were isolated from the South China Sea soft coral Lemnalia sp. The structures and absolute configurations of new compounds were elucidated by extensive analysis of spectroscopic data, single crystal X-ray diffraction, quantum chemical calculations and TDDFT-ECD calculations. Compounds 1-3 are unusual sesquiterpenoids with spironolactone structure, of which compound 1 is an uncommon brominated sesquiterpenoid of soft coral origin. Compounds 4-6 and 12 are four rare sesquiterpenoid alkaloids from soft coral, and compound 7 is an unusual sesquiterpenoid with a 6/5/6 tricyclic skeleton system, besides, compounds 8-11 are undescribed nardosinane and nornardosinane sesquiterpenoids. In the bioactivity assay, compound 2 exhibited cytotoxicity against NCI-H446 cell lines with an IC value of 19.32 μM, and the molecular docking results indicated that the mechanism of action may be related to its inhibitory effect on the target proteins VEGFR2, CD56, and PARP.