Synthesis of Small Glycopeptides by Decarboxylative Condensation and Insight into the Reaction Mechanism

The chemical synthesis of homogeneous glycoproteins and glycopeptides facilitates progress toward understanding the functional role of carbohydrates attached to proteins and is important in the preparation of glycopeptide-based therapeutics. A series of protected and unprotected glycosyl dipeptides, glycopeptide 1, which contained the alpha-ketoacid moiety at the C-terminus, were synthesized and ligated with a series of O-tert-butyl-protected N-hydroxylamino acids to afford O-tert-butyl-protected glycosyl tripeptides, glycopeptide II. The reactions were carried out under both anhydrous and aqueous conditions at neutral pH to produce glycopeptide products in yields ranging from 15% to 86% depending on the amino acids present at the ligation junction. The best yields were obtained when both the alpha-ketoacid and the N-hydroxylamino acid contained medium-sized side chains. In addition to the expected tripeptide product, 2,5-substituted oxazoles were isolated when O-tert-butyl protected N-hydroxylamines of glycine were employed in the reaction. The formation of the oxazole is believed to result from an intramolecular cyclization of the O-tert-butyl ester on a nitrilium ion intermediate followed by aromatization. A decarboxylative condensation between O-18-labeled phenyl pyruvic acid and N-hydroxyphenethylamine oxalate salt resulted in amide products lacking the O-18-label, providing further support for the nitrilium ion in the reaction pathway.