Retrobisabolane A, a Novel Bisabolane-Derived Sesquiterpenoid Isolated from Deep-Sea-Derived Fungus Retroconis fusiformis MCCC 3A00792.

One novel bisabolane-derived sesquiterpenoid retrobisabolane A (1), featuring a methyl group location at the C-4 position instead of C-3 in the bisabolanes, and a known ester-substituted eremophilane-type sesquiterpenoid cryptosphaerolide (2), along with three known indole alkaloids (3‒5) were discovered from the fermented cultures of a deep-sea-derived fungus Retroconis fusiformis MCCC 3A00792. The planar structure of new compound 1 was determined by extensive analysis of the NMR and HRESIMS spectra. The relative and absolute configurations of 1 were resolved by the coupling constant (J), calculation of ECD and NMR spectra, and the DP4+ probability analysis of the 1H and 13C NMR data. Interestingly, retrobisabolane A was the new subclass of bisabolanes bearing a methyl group linkage at C-4 instead of C-3 position. Three human cancer cell lines (Hela, AGS, and BIU-87) were subjected to evaluate the cytotoxic activities of compounds 1‒5. As a result, compound 2 exhibited significant inhibitory activities against three cell lines with IC50 values ranging from 9.95 to 18.77 μM.