Guanacastane Diterpenoids from the Plant Endophytic Fungus Cercospora sp.

Cercosporenes A-F (1-6, respectively), six new guanacastane diterpenes, including a homodimer (5) and a heterodimer (6), were isolated from the crude extract of the fungus Cercospora sp., endophytic to the medicinal plant Fallopia japonica. The structures of 1-6 were elucidated by nuclear magnetic resonance experiments, and 4 and 5 were further confirmed by X-ray crystallography. The absolute configuration of 1 and 3 was assigned by electronic circular dichroism calculations, whereas that of 6 was deduced by the application of the circular dichroism exciton chirality method. In addition to its cytotoxicity against a panel of five human tumor cell lines, HeLa, A549, MCF-7, HCT116, and T24, heterodimer 6 also induced autophagy in HCT116 cells.