STRUCTURE ELUCIDATION AND PROTON AND C-13 ASSIGNMENT OF A PENTASACCHARIDE GLYCOSIDE FROM AGAVE AMERICANA

A glycoside extracted from the rhizomes of Agave americana was identif ied as 3-O-{{beta-D-xylosyl(1 --> 3)}{beta-D-xylosyl(1 --> 3)-beta-D- glucopyranosyl(1 --> 2)}-beta-D-glucopyranosyl(1 --> 4)}-beta-D-galact opyranosyl-(25R)-5 alpha-spirostan-3 beta-ol by concerted use of prot on and carbon-13 2D NMR methods. Complete assignments for the sugar re sonances were obtained with standard methods, but the presence of chem ical shift degeneracies prevented an unequivocal determination of two of the glycosidic linkage points by the HMBC experiment. Further two-d imensional experiments failed to resolve the ambiguity, but a clear id entification of the linkages was obtained using a selective reverse IN EPT experiment with homonuclear double resonance.