Microwave assisted glycosylation for determining the absolute configuration of carbohydrates in plant glycosides

Glycosides are common natural products often isolated from plant sources. Rarely is a novel monosaccharide identified from a plant derived glycoside. However, numerous monosaccharides are found in natural products as their D or L enantiomers. A number of techniques have been developed to determine the absolute configuration of the monosaccharides found in glycosides. These procedures usually entail the derivatization of the free hydroxyl groups followed by an acid catalyzed methanolysis of the glycoside to give the individual O-methyl glycosides which are verified by applying GC-MS and comparison to known standards. Once the structural identity of the monosaccharide is determined the procedure is repeated with a chiral alcohol (e.g., R or S 2-butanol or 2-octanol) to produce a single enantiomer (or diasteromeric mixture) which is compared to independently synthesized standards. This presentation discusses our recent efforts to determine the absolute stereochemistry of monosaccharides using chiral 2-butanolysis of permethylated glycosides catalyzed by strong acid ion exchange resins with a microwave reactor. This procedure is run in a minimum amount of neat chiral alcohol with a small sample size and requires no reaction work-up. The experimental GC-MS results of both standard monosaccharides and intact glycosides will be presented.