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.