New dihydroxycucurbitacin D's from the Namib desert endemic plant Acanthosicyos horridus (!nara)

Abstract

Acanthosicyos horridus Welw. ex Hook.f. (!nara) is a leafless, thorny, melon-producing plant endemic to the hyper-arid Namib Desert. The methanol crude extract prepared from the ripe fruits of !nara afforded the known dihydroxycucurbitacin 7β-hydroxy-23,24-dihydrocucurbitacin D (1), along with four new congeners 7β,15β-dihydroxy-23,24-dihydrocucurbitacin D (2), 25-O-β-glucopyranosyl-7β-hydroxy-23,24-dihydrocucurbitacin D (3), 25-O-β-glucopyranosyl-7β-hydroxy-23,24-dihydroisocucurbitacin D (4) and 25-O-β-glucopyranosyl-7β-hydroxy-23,24-dihydro-3-epi-isocucurbitacin D (5). These compounds were isolated through a combination of preparative normal phase thin-layer chromatography (TLC) and semi-preparative reversed phase high performance liquid chromatography (HPLC). Their structures were established by comprehensive analysis of HR-ESI-MS data, 1D and 2D NMR spectroscopic data and by comparison with literature values of similar cucurbitacins. The five isolated compounds exhibited poor cytotoxic activity against the MDA-MB-231 breast cancer cell line. To the best of our knowledge, this is the first report of glycosylated cucurbitacins in Acanthosicyos horridus.

Keywords: Acanthosicyos horridus (!nara); Namib desert; cucurbitacin-glycosides; cucurbitacins; cytotoxicity; dihydrocucurbitacin D.