Euphiquorins A-E, five new structurally diverse diterpenoids from Euphorbia antiquorum L

Phytochemical investigation of the whole plant of Euphorbia antiquorum L. led to the isolation of sixteen diterpenoids, including five new compounds, euphiquorins A-E (1, 5-8), together with eleven known analogues (2-4, 9-16). The structures of the new compounds were elucidated by comprehensive spectroscopic analyses and quantum chemical calculations. Compound 1 possesses an unprecedented 5/6/6/5 fused tetracyclic ent-abietane-derived framework featuring oxidative A-ring cleavage, carbon contraction, and intramolecular cyclization. Compounds 5-8 represent new ingol-type diterpenoids with diverse oxygenated substitution patterns. The known compounds were identified as three ent-abietane diterpenoids (2-4), six ingol diterpenoids (9-14), and two ent-atisane diterpenoids (15-16) by comparison of their spectroscopic data with those reported in the literature. A plausible biogenetic pathway for compounds 1-4 was proposed. Selected compounds were evaluated for their cytotoxic activities against MCF-7 and MCF-7/DOX cells, and none of the tested compounds exhibited significant cytotoxicity at a concentration of 30 μM.