Neo-5,10-seco-clerodane diterpenoids from Schnabelia terniflora

Three new neo-5,10-seco-clerodane diterpenoids (1–3), four previously undescribed ethoxy/methoxy acetal analogues (4–7), one new etherified labdane diterpenoid (8), and seven known diterpenoids (9–15) were isolated from the whole plant of Schnabelia terniflora. Their structures were established on the basis of extensive spectroscopic analysis, single-crystal X-ray diffraction data, calculated electronic circular dichroism (ECD), and Mo₂(OAc)₄-induced circular dichroism. Compounds 2 and 3 represent the first examples of neo-5,10-seco-clerodane diterpenoids containing a 1H-pyrrole-2,5-dione and a pyrrolidine-2,5-dione moiety, respectively. A plausible biosynthetic pathway for 1–3 is proposed. All diterpenoids were evaluated for their cytotoxic activity against non-small-cell lung cancer lines (A549 and H460) and gastric cancer lines (HGC27 and AGS). Among them, 2 and 14 showed moderate cytotoxicity against four cell lines.