Characterization, by two-dimensional NMR spectroscopy, of a complex tetrasaccharide glycoside isolated from Ipomoeastans

Abstract

A mixture of glycosidic acids, each of which consisted of a tetrasaccharide unit linked by an anomeric carbon to 11-hydroxypalmitic acid and containing short-chain fatty acids ester-linked to the sugar units, was isolated from Ipomoeastans. Basic hydrolysis allowed isolation of a single compound whose structure was determined by two-dimensional NMR methods to be (S)-11-hydroxypalmitic acid 11-O-β-quinovopyranosyl-(1-4)-α-rhamnopyranosyl-(1,2)-β-glycopyranosyl-(1-2)-β-quinovopyranoside. Further HPLC separation of the original mixture allowed separation of a compound that was shown, by 2D NMR, to have 2-mefhylbutanoic acid molecules linked to C-4 of the terminal quinovose and C-2 of rhamnose, 3-hydroxy-2-methyl butanoic acid linked to C-6 of glucose, and with the palmitic acid forming a lactone at C-3 of rhamnose. Closer inspection revealed that this compound was actually a mixture of diastereomers involving different ((2R,3R) and (2S,3S)) enantiomers of 3-hydroxy-2-methylbutanoic acid bonded to the chiral tetrasaccharide core. Keywords: tetrasaccharide, two-dimensional NMR.