Difficulties encountered during glycopeptide syntheses.

Methods for the efficient solid-phase synthesis of glycopeptides have developed rapidly over the past two decades. Incorporation of both O- and N-linked glycosides, as well as branched carbohydrates, into peptides is readily achieved. Synthetic glycoproteins of modest size have also been constructed. As glycopeptide synthesis protocols have progressed, so has the recognition of distinct categories of synthetic difficulties. Such categories include (a) unstable glycosidic linkages, (b) multifunctional amino acids not easily glycosylated and incorporated into peptides, and (c) glycosylated peptide sequences that are subject to side reactions. In the present overview,we describe specific examples for each category of problematic glycopeptide syntheses, as well as solutions to these problems.