On-resin solid-phase synthesis of asparagine N-linked glycopeptides: Use of N-(2-acetoxy-4-methoxybenzyl) (AcHmb) aspartyl amide-bond protection to prevent unwanted aspartimide formation

The N-(2-acetoxy-4-methoxybenzyl) (AcHmb) backbone amide-protecting group has been applied in an on-resin solid-phase synthesis of asparagine N-linked glycopeptides. Backbone protection of the -Asp(OAllyl)-Ala- aspartyl amide bond suppressed the formation of aspartimide during both the initial chain assembly and the subsequent activation and glycosylation of the aspartyl beta-carboxy group. A 1.1-1.25 mole excess of glycosylamine coupled quantitatively, with minimal side reaction, to the activated aspartyl beta-carboxy group of the fully assembled backbone-protected peptide-resin. A parallel synthesis without backbone amide protection produced substantial aspartimide peptide, particularly during the initial chain assembly.