Antitrypanosomal cycloartane glycosides from Astragalus baibutensis

Baibutoside (5), a new cycloartane-type triterpene glycoside, has been isolated from the roots of Astragalus baibutensis along with four known glycosides, acetylastragaloside 1 (1), and astragalosides 1, 11, and IV (2-4, resp.). The structure elucidation of the compounds were achieved by a combination of one- and two-dimensional NMR techniques (DQF-COSY, HSQC, HMBC, and ROESY), and mass spectrometry (ESI-MS), where all the compounds were shown to have cycloastragenol (=(20R,24S)-3 beta,6 alpha,16 beta,25-tetrahydroxy-20,24-epoxy-9,19-cyclolanostane) as aglycone. All compounds were tested for in vitro antiprotozoal activity. Compounds 1-4 displayed notable activity vs. Trypanosoma brucei rhodesiense, with acetylastragaloside 1 (1) being the most potent (IC50 9.5 mu g/ml). Acetylastragaloside I (1) was also lethal to T cruzi (IC50 5.0 mu g/ml), and it is the first cycloartane-type triterpene with remarkable trypanocidal activity against both T brucei rhodesiense and T cruzi. However, it exhibits some cytotoxicity on mammalian cells.