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 of 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 its three epimers at the alpha-carbon stereocenters were prepared. The model glycopeptide did not undergo any epimerization (<1%) or beta-elimination, as determined by (1)H NMR spectroscopy, under various conditions which are in common use for deacetylation of glycopeptides. Under more severe conditions, which are required for removal of O-benzoyl groups, 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 likely due to protection of the alpha-carbon stereocenters by deprotonation of the adjacent amide groups.