Cephalotanins A-D, Four Norditerpenoids Represent Three Highly Rigid Carbon Skeletons from Cephalotaxus sinensis

Four polycyclic norditerpenoids, cephalotanins A–D (1–4) representing three unprecedented carbon skeletons with highly rigid ring systems, were isolated from Cephalotaxus sinensis and structurally characterized by a combination of various methods. Compounds 1 and 2 are new skeletal norditerpenoid trilactones, while 3 and 4 are two norditerpenoids featuring different new carbon skeletons. Biosynthetic pathways for 1–4 were proposed by involving diverse and very fascinating chemical events with the coexisting cephalotane troponoids as the precursors. Compound 1 exhibited good NF‐κB inhibition with an IC50 value of 4.12±0.61 μΜ. Natural products: Cephalotanins A–D (1–4) representing three highly rigid new carbon skeletons were isolated from C. sinensis. Compounds 1 and 2 are norditerpenoid trilactones, while 3 and 4 are two norditerpenoids featuring different carbon skeletons. Biosynthetic pathways for all four cephalotanins have been proposed.