Spiroseoflosterol, a Rearranged Ergostane-Steroid from the Fruiting Bodies of <i>Butyriboletus roseoflavus</i>

Spiroseoflosterol (1), a 7(8→9)-abeo-ergostane steroid with a unique spiro[4.5]­decan-6-one system, was isolated from the fruiting bodies of Butyriboletus roseoflavus. Its structure was determined by interpretation of comprehensive spectroscopic, X-ray, and computational data. A plausible biogenetic pathway for spiroseoflosterol (1) was postulated based on a key semipinacol rearrangement. Compound 1 was cytotoxic to HepG2 and Huh7/S (sorafenib-resistant Huh7) with IC50 values of 9.1 and 6.2 μM, respectively.