Chemoenzymatic solution- and solid-phase synthesis of O-glycopeptides of the mucin domain of MAdCAM-1. A general route to O-LacNAc, O-sialyl-LacNAc, and O-sialyl-Lewis-X peptides

An efficient and general method for the solid-phase synthesis of glycopeptides containing an O-linked sialyl-Lewis-X (SLe(x)) tetrasaccharide is described. Using a combined chemoenzymatic approach, the first synthesis of an unnatural beta-O-linked SLe(x) attached to a partial sequence of the mucin domain of the L-selectin ligand MAdCAM-1, was demonstrated. A resin-bound O-glycoconjugate was synthesized from a new Fmoc-threonine building block which carries an O-unprotected beta-linked N-acetylglucoseamine (GlcNAc) moiety. The acid-and base-stable HYCRON-linker enabled the complete removal of all protecting groups on solid phase. Glycosyltransferases were employed to extend the glycan on supported and unsupported O-GlcNAc-octapeptide substrates. The acid-and base-sensitive O-glycopeptides were released under practically neutral conditions, taking advantage of the palladium(0)-catalyzed cleavage of the allylic linkage. Studies toward the selective O-deacetylation of ester-linked glycopeptides possessing O-acetyl-protected carbohydrates are also reported.