Microascones, Decahydrofluorene-Class Alkaloids from the Marine-Derived Fungus <i>Microascus</i> sp. SCSIO 41821

Eight new decahydrofluorene-class alkaloids, microascones A and B (1 and 2), 2,3-epoxyphomapyrrolidone C (3), 14,16-epiascomylactam B (4), 24-hydroxyphomapyrrolidone A (5), and microascones C–E (6–8), along with five known analogs (9–13) were isolated from the marine-derived fungus Microascus sp. SCSIO 41821. Compounds 1 and 2 have an unprecedented complex macrocyclic alkaloid skeleton with a 6/5/6/5/6/5/13 polycyclic system. Their structures and absolute configurations were determined by spectroscopic analysis, quantum chemical calculations of ECD spectra, and 13C NMR chemical shifts. Compounds 10–13 showed selective enzyme inhibitory activity against PTPSig, PTP1B, and CDC25B, and 4, 9, and 10 exhibited strong antibacterial activity against seven tested pathogens. Their structure–bioactivity relationship was discussed, and a plausible biosynthetic pathway for 1–8 was also proposed.