Synthesis of disaccharide Neu5Gc alpha(2-6)GalNAc alpha as a spacer glycoside

The first synthesis of the Neu5Gc analogue of SiaT(n) disaccharide, which c an be detected in breast tumors by immunochemical methods, is reported. The regioselective sialylation of (3-trifluoroacetamidopropyl)-2-azido-2-deoxy -alpha-D-galactopyranoside with peracetate of the methyl ester of N-acetoxy acetylneuraminic acid beta-ethylthioglycoside in the presence of N-iodosucc inimide and trifluoromethanesulfonic acid (or its trimethylsilyl ester) res ulted in the derivatives of alpha- and beta-sialyl(2 --> 6)galactosaminide in 39 and 32% yields, respectively. The catalytic hydrogenolysis of the azi do group and subsequent N- and O-acetylation of the alpha-anomer gave the t rifluoroacetamidopropyl glycoside peracetate. Removal of the protective gro ups led to glycoside Neu5Gc alpha(2 --> 6)GalNAc alpha-O(CH2)(3)NH2. Using the Neu5Gc derivative with acetoxyacetyl groups at positions O9 and O4 as a donor increases the alpha-selectivity of sialylation to afford the alpha- and beta-anomers in 69 and 8% yields, respectively.