New norclerodane diterpenoids from the tubers of Dioscorea bulbifera.

Phytochemical investigations of the tubers of Dioscorea bulbifera L. resulted in the isolation of nine norclerodane diterpenoids, including two new compounds, diosbulbins N (1) and P (3), a new naturally occurring compound, diosbulbin O (2), and six known ones, diosbulbins A-D, F and G (4-9). Their structures were established by spectroscopic and chemical methods. The absolute stereochemistry of 1 was determined by a modified Mosher's method, and the absolute configuration of 2 was determined by a single-crystal X-ray diffraction analysis with CuKα irradiation. Compounds 1-3 were evaluated for in vitro cytotoxicity against five human cancer cell lines.