Solid-phase synthesis of a glycosylated hexapeptide of human sialophorin, using the trichloroacetimidate method

A hexapeptide containing a β-d-Galp-(1 → 3)-α-d-Galp NAc-(1 → O)-l-threonine unit was synthesized using glycosylated pentafluorophenyl esters in an Fmoc-based strategy. In all of the glycosylation reactions, trichloroacetimidates were successfully employed. The disaccharide moiety was prepared from tetra-O-acetyl-α-d-galactopyranosyl trichloroacetimidate and tert-butyldimethylsilyl 2-azido-6-O- benzoyl-2-deoxy-β-e-galactopyranoside with boron trifluoride etherate as a catalyst. The glycosylated active esters were obtained in the reaction of α and β 2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl- (1 → 3)-4-O-acetyl-2-azido-6-O-benzoyl-2-deoxy-d-galactopyranosyl trichloroacetimidates with Fmoc-protected pentafluorophenyl esters of l-serine and l-threonine in the presence of trimethylsilyl trifluoromethanesulfonate as Lewis acid. The glycosylated pentafluorophenyl ester of l-threonine was transformed into glycopeptides via a solid-phase synthesis. Azide reduction and N-acetylation were performed on the solid phase with a thioacetic acid- pyridine mixture. The glycopeptide was then cleaved from the resin with strong acid, also removing the acid-labile protecting groups of the peptide chain. Finally, the acyl groups used for sugar protection were cleaved with sodium methoxide, affording the completely deprotected N-acetyl-l-leucyl- l-glutamyl-O-[β-d-galactopyranosyl-(1 → 3)-2-acetamido- 2-deoxy-α-d-galactopyranosyl]-l-threonyl-l-seryl- l-threonyl-glycinamide (1) in high purity.