Norcolocynins A and B, Ring B seco-Norcucurbitane Triterpenoids from the Fruits of Citrullus colocynthis and Their Cytotoxicity

Two unprecedented ring-B seco-norcucurbitane triterpenoids, norcolocynins A (1) and B (2), were isolated from Citrullus colocynthis fruits. The determination of their structures was based on the extensive analysis of spectroscopic data as well as X-ray crystallography. A biogenetic route for 1 involving a decarboxylation, followed by a rearrangement of 2 is proposed. Cytotoxicity assay revealed that 2 showed moderate activity against HCT116 cells (IC₅₀ = 27.92 ± 0.80 μM), comparable to that of positive control cisplatin.