Norcolocynthenins A and B, two cucurbitane 3-nor-Triterpenoids from Citrullus colocynthis and their cytotoxicity

Two novel cucurbitane 3-nor-triterpenoids, named norcolocynthenins A (1) and B (2), were isolated from the fruits of Citrullus colocynthis. The structures including their absolute configurations were determined by extensive spectroscopic analyses and theoretical calculations. Compound 1 features an unprecedented 5/6/6/5-fused ring system while compound 2 possesses a rare lactone moiety at modified ring A. Compounds 1 and 2 showed significant cytotoxic activity against human cancer cell lines of HL-60 (IC50 = 8.32, 6.49 mu M) and PC-3 (IC50 = 31.26, 13.42 mu M). The plausible biosynthetic pathway of compounds 1 and 2 via a key enzymatic Baeyer-Villiger reaction is proposed.