Asymmetric dimeric ent-kauranoids from Croton tonkinensis and their cytotoxicity

Abstract Two asymmetric dimeric ent -kauranoids ( 1 – 2 ), together with three known ent -kauranoids ( 3 – 5 ), were isolated from the leaves of Croton tonkinensis . Their structures were determined on the basis of spectroscopic data. Diterpenoids with the 15-oxo-16-ene moiety showed significant inhibitory effects on the growth of eight human cancer cell lines. Additionally, the major diterpenoid 4 inhibited hypoxia-inducible factor-dependent transcription (HIF-1) in a luciferase reporter assay with an IC 50 of 2.5 μM. The biosynthesis of the two new dimers was also proposed.