Stereoselective synthesis of a blood group A type glycopeptide present in human blood mucin

N,N-Dimethyl-O-(2-acetamido-2-deoxy-alpha-D-galactopyranosyl)-(1-->3)-O- [(alpha-L-fucopyranosyl)-(1-->2)]-O-(beta-D-galactopyranosyl)-(1-->3)-O- (2-acetamido-2-deoxy-alpha-D-galactopyranosyl)-(1-->3)-L-serine, a core I glycotetraosyl peptide structure and a predominant substructure in complex glycan-glycoproteins present in human blood group A ovarian mucin, was synthesized for the first time. The title compound was synthetically accomplished via the following key manoeuvres: regio- and stereo-controlled construction of the alpha-GalNAc-(1-->3)-Gal synthon, stereoselective glycosylation generating a alpha-GalN3-(1-->3)-Ser glycopeptide synthon and alpha-selective fucosylation towards an acceptor which was derived from glycosylation of the latter two synthons. An alternative route to that of the latter, to synthesize a fully protected equivalent of the title compound, involving the coupling of a alpha-GalNAc-(1-->3)-beta-Gal-(1-->3)-GalN3X synthon to an aglycon serine derivative, is described herein.