Cytotoxic sesterterpenes, 6-epi-ophiobolin G and 6-epi-ophiobolin N, from marine derived fungus Emericella variecolor GF10

Two new sesterterpenes, 6-epi-ophiobolin G () and 6-epi-ophiobolin N (), and six known ophiobolins were isolated from the extracts of the fungus, Emericella variecolor GF10, which was separated from marine sediment. The planar structures of the new compounds were deduced from analysis of the 2D NMR spectra, and the stereochemistry was determined by extensive examination of the NOESY spectrum. Additionally, the configuration of the C-6 proton in ophiobolin G () was revised from α to β, and the unsolved stereochemistry of ophiobolin H () was determined by its physicochemical evidence and the chemical correlation with ophiobolin K (). Ophiobolin K () showed cytotoxic activity against various tumor cell lines, including adriamycin-resistant mouse leukemia cells (P388), with IC50 of 0.27–0.65 μM.