Eight pairs of epimeric triterpenoids involving a characteristic spiro-E/F ring from Abies faxoniana.

Five pairs of new epimeric lanostane-type triterpenoids, abiespirones A-D (1-4) and G (7), two pairs of new epimeric cycloartane-type triterpenoids, abiespirones E and F (5, 6), and a pair of new epimeric 7(8→9)abeo-spirolanostane abiespirones H (8) with spiro-B/C and -E/F ring systems were isolated from Abies faxoniana as inseparable mixtures of C-23 epimers in a specific proportion. The HPLC plots showed that each purified isomer rapidly equilibrated with the C-23 epimer in solution. The structures of compounds 1-8 were elucidated by analysis of the NMR spectra and single-crystal X-ray diffraction. Compound 6 showed cytotoxicity against three hepatoma cell lines, namely, HepG2, Huh7, and SMMC7721, with IC50 values of 9.8, 7.5, and 10.7 μM, respectively, but exerted low cytotoxicity on normal QSG7701 hepatic cells, indicating its selective cytotoxicity for hepatoma cells. Compound 6 arrests the cell cycle at G2/M and induces cell apoptosis in Huh7 cells. In addition, the generation of reactive oxygen species (ROS) was detected in Huh7 cells when treated with compound 6, and a ROS scavenger partly blocked the effects of compound 6-induced Huh7 cell death, suggesting that compound 6-induced apoptosis is associated with elevated levels of ROS in Huh7 cells.