Constrained glycopeptide ligands for MPRs. Limitations of unprotected phosphorylated building blocks

A new methodology for the synthesis of cyclic and phosphorylated glycopeptide templates was developed. First, fully protected building blocks containing mannose and mannose disaccharides with bis-trichloroethyl phosphate on Fmoc-Thr-OPfp were synthesized. These were used in solid-phase assembly through side chain anchoring of glycosylated hexa- and octa-peptides protected at the C-terminal carboxylate as the allyl ester. Selective allyl ester cleavage and head-to-tail cyclization under pseudodilution conditions gave a high yield of pure cyclic peptide templates. Unprotected phosphate in the building block was evaluated as an alternative to the problematic trichloroethyl group. It was found that one unprotected phosphate is readily incorporated, whereas the second unprotected phosphorylated building block react very slowly due to electrostatic repulsion in the solid-phase synthesis. For comparison with previous binding studies modified glycopeptide templates containing only phosphorylated mannose monosaccharides or templates modified in the peptide part were synthesized. All the structures were tested for their binding to the mannose 6-phosphate receptor, and it was found that although mannose disaccharides are required for optimal interaction, the detailed structure of the peptide template has a strong influence on binding to the receptor. The restricted conformations of the cyclic peptides decreased the binding considerably.