Synthesis of C-terminal glycopeptides from resin-bound glycosyl azides viaa modified Staudinger reaction

The solid-phase synthesis of glycopeptides containing the sugar at the C-te rminus is reported. The method is demonstrated on a model, the endogenous a ntinociceptive peptide Leu-enkephalin. 2,3,4- Tri-O-acetyl-1-azido-1-deoxy- beta-D-glucopyranuronic acid was synthesized and immobilized onto a variety of derivatized resins. Conjugation of the first amino acid was accomplishe d by reaction of the resin-bound glycosyl azide with an activated amino aci d, in one step, via a modified Staudinger reaction. Standard solid-phase pe ptide synthesis then resulted in the desired amide-linked glycopeptide. Rea ction conditions and reagents for the glycosylation were varied to optimize the yield and purity of the product. The optimum conditions were found to be the use of a 4-fold molar excess of activated amino acid and 3-fold exce ss of tri-n-butylphosphine in tetrahydrofuran. This methodology is generall y applicable to most peptide sequences and is compatible with both Bocand F moc- synthetic strategies on a variety of resins.