Synthesis of N-linked glycopeptides via solid-phase aspartylation

An efficient strategy for the preparation of N-linked glycopeptides is described. The method relies on the use of side chain protecting groups on aspartic acid residues, namely the allyl and Dmab esters, which are orthogonal to those utilised in Fmoc-strategy SPPS. After peptide assembly these protecting groups were selectively removed and the resulting free side chains derivatised with a glycosylamine to afford a resin bound glycopeptide bearing a native N-linkage. Initially, N-linked glycopeptides were successfully synthesised according to this strategy, however, yields varied substantially depending on the nature of the amino acid residue situated adjacent (C-terminal) to the putative glycosylation site. This was due to generation of substantial quantities of aspartimide by-products. Aspartimide formation was overcome by incorporation of a 2,4-dimethoxybenzyl (Dmb) backbone amide protecting group on the residue adjacent to an allyl- or Dmab-protected aspartic acid residue. N-linked glycopeptides were prepared in excellent yield after the solid-phase aspartylation reactions. The utility and orthogonality of the allyl and Dmab ester solid-phase approaches were exploited in the preparation of an N-linked glycodecapeptide bearing two different carbohydrate moieties. This exemplified the efficiency of the solid-phase methodology for the preparation of glycopeptides bearing various combinations of N-linked glycans.