Microascusteroids A and B, two 5,6-seco-9,10-seco steroids with a C-ring rearranged ergostane skeleton from the marine-derived fungus Microascus sp. SCSIO 41821

Two undescribed steroids, microascusteroids A and B (1 and 2), together with four known compounds (3-6) were isolated from the marine-derived fungal strain Microascus sp. SCSIO 41821. The structures of 1 and 2 were determined by extensive spectroscopic analysis, and their absolute configurations were established by quantum chemical calculations of electronic circular dichroism (ECD) spectra. Specially, compounds 1 and 2 were unusual 5,6-seco-9,10-seco steroids with a C-ring rearranged ergostane skeleton. In addition, 1 and 2 showed medium inhibition activity against lipopolysaccharide-induced nitric oxide production in RAW 264.7 cells and displayed mild enzyme inhibitory activity against PTP1B and cytotoxicity against human cancer cell lines H1975 and HepG2.