Ophiobolin O and 6-Epi-Ophiobolin O, Two New Cytotoxic Sesterterpenes from the Marine Derived Fungus Aspergillus Sp

Two new sesterterpenes, ophiobolin O (1) and 6-epi-ophiobolin O (2), together with the known ophiobolins G (3), H (4), and K (5), and 6-epi-ophiobolin K (6) were isolated from the marine derived fungus Aspergillus sp. The structures of these compounds were elucidated based on chemical and physicochemical evidence, including MS, UV, IR and NMR spectra. The stereochemistry of 1 was further confirmed by catalytic reaction of 5 with p-TsOH as a catalyst. Compounds 1 to 6 showed cytotoxicity against mouse leukemia cell line P388, with IC50 values of 4.7, 9.3, 24.6, 105.7, 13.3 and 24.9 μM, respectively.