ent-kaurane and ent-pimarane diterpenoids from Siegesbeckia pubescens

Phytochemical investigation of the above-ground parts of Siegesbeckia pubescens yielded 25 Diterpenoids, including five new ent-pimarane Diterpenoids (1-3, 11, and 12) and four new ent-kaurane Diterpenoids (13-16). The structures and relative configurations of the new compounds were elucidated by comprehensive spectroscopic analysis and by comparison of their NMR data with those of related compounds. Single-crystal X-ray diffraction analysis confirmed the structures of 8, 13, and 14. Seven of these Diterpenoids were evaluated for cytotoxic activity against HSC-T6, HeLa, and B16 cell lines. Compounds 11 and 14 exhibited moderate cytotoxicity.