Microscale structure determination of oligosaccharides by the exciton chirality method

Abstract

Application of the circular dichroic exciton chirality method, a powerful chiroptical tool for the determination of absolute configuration at stereocenters in organic molecules, to oligosaccharide structure elucidation is described. The general strategy is to exploit the highly sensitive CD spectra of sugars derivatized with exciton chromophores to differentiate the hydroxyl groups that are free from those that are involved in glycosidic linkages, without the need for authentic standards. One derivatization scheme involves the phenylbenzyl chromophore, which withstands methanolysis conditions, and can be oxidized to the more sensitive phenylbenzoates (PhBn/PhBz scheme). Alternatively, the free hydroxyl groups can be tagged with p-bromobenzoate, followed by cleavage of the peracylated oligosaccharide with 48% HBr, trifluoroacetic acid anhydride and oxalyl bromide (HBr scheme, monochromophoric). More highly characteristic spectra can be obtained if the newly liberated hydroxyl groups—indicating the site of glycosidic linkage—are derivatized with a second chromophore, p-methoxycinnamate (HBr scheme, bichromophoric). In all these approaches, the resulting sugar derivatives are separated by HPLC, followed by CD analysis and comparison to published reference spectra to unambigously determine the identity of the sugars, linkage patterns and absolute configurations. © 1989 IUPAC.