REMOVAL OF ACYL PROTECTIVE GROUPS FROM GLYCOPEPTIDES - BASE DOES NOT EPIMERIZE PEPTIDE STEREOCENTERS, AND BETA-ELIMINATION IS SLOW

Epimerization of glycopeptide stereocenters and beta-elimination have been considered as important potential side reactions on deacylation o f glycopeptides which have the carbohydrate moieties protected with O- acyl groups. Since no systematic investigation of these side reactions has been reported, a model acetylated, O-linked glycotripeptide and i ts three epimers at the alpha-carbon stereocenters were prepared. The model glycopeptide did not undergo any epimerization (<1%) or beta-eli mination, as determined by H-1 NMR spectroscopy, under various conditi ons which are in common use for deacetylation of glycopeptides. Under more severe conditions, which are required for removal of O-benzoyl gr oups, beta-elimination occurred slowly and was accompanied by slight ( <5%) epimerization. The surprisingly low tendency of glycopeptides to undergo base catalyzed epimerization and beta-elimination is most like ly due to protection of the alpha-carbon stereocenters by deprotonatio n of the adjacent amide groups.