Fusidane-Type Antibiotics from the Marine-Derived Fungus <i>Simplicillium</i> sp. SCSIO 41513

Simplifusidic acids A–K (1–11), 11 new fusidane-type nortriterpenoids, were isolated from the marine-derived fungus Simplicillium sp. SCSIO 41513. Compound 1 possessed an unprecedented fusidane triterpene skeleton with a 6/6/7/5/5 polycyclic system. Their structures were elucidated by spectroscopic methods, and their absolute configurations were further determined by quantum chemical calculations of ECD spectra, comparison of experimental ECD spectra, and single-crystal X-ray diffraction analysis. Compound 9 showed strong antibacterial activity toward Staphylococcus aureus with an MIC value of 0.078 μg/mL. Their structure–bioactivity relationship was also discussed.