Herpetosin A, a nor-cucurbitane triterpenoid with a 5/6/6/5 ring system from Herpetospermum pedunculosum

Two previously undescribed triterpenoids, herpetosin A (1) and herpetosin B (2), together with two known compounds, cucurbitacin B (3) and dihydrocucurbitacin E (4), were isolated from the seeds of Herpetospermum pedunculosum. Their structures were elucidated by analysis of the NMR, HR-ESI-MS, X-ray data and quantum chemical calculation. Compound 1 featured an unprecedented nor-cucurbitane triterpenoid skeleton with a 5/6/6/5 ring system. Among all the isolates, compound 4 exhibited anti-proliferative effect with IC₅₀ values of 4.5 ± 0.1 μM against A549 cells, 8.2 ± 0.5 μM against MDA-MB-231 cells, and 2.4 ± 0.3 μM against HepG2 cells, respectively.