Calvatianone, a Sterol Possessing a 6/5/6/5-Fused Ring System with a Contracted Tetrahydrofuran B-Ring, from the Fruiting Bodies of

is an extremely rare mushroom with a limited number of studies on its chemical components and biological activities published. Here we report the isolation of a novel sterol, calvatianone (), possessing a 6/5/6/5-fused ring system with a contracted tetrahydrofuran B-ring, and four known steroids (-) from the fruiting bodies of . The structure of calvatianone including its absolute configuration was determined by NMR spectroscopic analyses, HR-ESIMS, gauge-including atomic orbital NMR chemical shift calculations, and ECD calculations. Ergosterol peroxide () and cyathisterol () suppressed the cell viability increase induced by 17β-estradiol in MCF-7 breast cancer cell lines, suggesting a possible approach for these compounds to serve as ERα antagonists.