Cytotoxic abietane and kaurane diterpenoids from Celastrus orbiculatus

Celastrus orbiculatus is a medicinal plant belonging to the Celastraceae family. In this survey on the secondary metabolites of plants for obtaining antitumor substances, the chemical constituents of the stems of C. orbiculatus were investigated. Nortriptonoterpene (1), a new C₁₉-norabietane diterpenoid, together with six other known abietane-type diterpenoids (2-7) and five known kaurane-type diterpenoids (8-12) were isolated and identified from the EtOAc extract of C. orbiculatus. Their structures were elucidated on the basis of extensive spectroscopic methods, including UV, IR, HR-ESI-MS, ECD, and NMR experiments, and by comparison with literature data. Compound 1 is a new C₁₉-norabietane diterpenoid with 19 carbons. All compounds except for 10 and 11 were isolated from C. orbiculatus for the first time. The NMR data of 9 were reported for the first time. Compounds 1, 7 and 11 showed cytotoxicities against SGC-7901 with IC₅₀ values of 63.2, 80.9 and 56.7 μM, respectively.