Combined Chemical and Enzymatic Synthesis of a Disialylated Tetrasaccharide Analogous to M and N Bloodgroup Determinants of Glycophorin a

Abstract Reaction of 2,3,4,6-tetra-O-acetyl-α-D-galactopyraaosyl bromide (1) with phenyl 2-acetamido-2-deoxy-4,6-O-(4-methoxy-benzylidene)-α-D-galactopyranoside (3) mediated by mercuric salts, followed by removal of the 4-methoxybenzylidene group and O-deacylation afforded phenyl 2-acetamido-2-deoxy-3-O-p-D-galactopyranosyl-α-D-galactopyranoside (6). Compound 6 was used as a substrate for the selective introduction of two neuraminic acid residues with partially purified sialyltrans-ferase preparations. First, disaccharide 6 was treated with CMP-[14c]-NeuAc as donor substrate and CMP-NeuAc: Gal-p(l-3)-GalNac-a(2-3)sialyltransferase from human placenta to afford trisaccharide 7 (yield 85X), sialylated at C-3 of the galactose residue. Treatment of 7 with CMP-[3H]-NeuAc and a micro-somal fraction from regenerating rat liver, containing the CMP-NeuAc: NeuAc-a(2-3)-Gal-p(l-3)GalNAc-α(2-6) sialyltrans-ferase activity, gave the disialylated tetrasaccharide 8 in 10X yield.