Disialyl groups

Occasionally, the branchings are incomplete and the formation of the N-acetyl-lactosamine residues is only started in outline, as in the glycans of ovotransferrin (see Fig. 15), ovalbumin (see Fig. 16), and ovomucoid (see Fig. 18). In other instances, glycan structures such as have just been described are enriched with supplementary monosaccharide residues for example, the occurrence of disialyl groups [a-NeuAc-(2—>8)-a-NeuAc], and of /3-Gal-(l- 3) residues linked to the terminal galactosyl residues, has been demonstrated in different tissues and cell membranes,114,115 and in calf-thymocyte membranes,118 respectively. 

A total synthesis of ganglioside DSLe , a(2-3)/a(2-6)-disialyl Lewis A, which was isolated from human colonic adenocarcinoma [80], was recently reported by Kiso s group as illustrated in Fig. 3 and Scheme 7 [81,82]. DSLe is characterized by a sterically hindered tri-anten-nary structure at 0-3, 0-4, and 0-6 of glucosamine residue. The cmcial points of the synthesis... 

Like lacNAc termini, type-1 structures may be fucosylated by a2-FucT and a4-FucT (Lewis enzyme, FucT III) to yield the H-type-1 and the Lewis structures, respectively [24, 81]. In turn the H-type-1 structure can be converted into a Lewis, or a blood group A or B active compound, by the action of a4-FucT, and blood group A- (a3-GalNAcT) or B-enzyme (a3-GalT), respectively [24, 81]. Alternatively, type-1 structures can be disialylated by the concerted action of a a3-NeuAcT and a a6-NeuAcT to yield the terminal NeuAc(a2-3)Gal(pi-3)[NeuAc(o2-6)]-GlcNAc structure [111, 112],... 

Application of strategy B also made accessible nucleotide monophosphosialic acid derivatives with the cytidine replaced by other pyrimidine and purine bases, although in modest overall yields only [55]. By following pathway B a CMP- disialyl derivative could be synthesized. This compound has a second sialic acid unit a-linked to the 8-OH group of the CMP-activated sialic acid [56]. 

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