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

Abstract

A hexapeptide containing a β- d-Gal p-(1 → 3)-α- d-Gal p 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.