Abstract
Sialic acid, which is terminal components of many glycoproteins and glycolipids, is involved in manifold cell functions. The terminally exposed position allows sialic acid-containing glycoconjugates to be exploited as receptors for many viruses and bacteria, in addition to governing a wide variety of biological processes. On the other hand, the synthesis of glycosides containing sialic acids remains a challenge for carbohydrate chemists. Rencently, a notable discovery of cyclic 5-N,4-O-oxazolidinone-protected thiosialoside donors has improved this awkward situation. In herein study, we developed a new chemical agent based on 5-N,4-O-oxazolidinone-protection, but equipped with a di-n-butyl phosphate as a reactive leaving group. We demonstrated that the conversion of sulfide-based leaving groups to phosphates has positive impact to the □-selectivity. Meanwhile, the donor showed the capability to stereoselective synthesize the five most common sialic acid linkages, NeuAc□(2→6)Gal, NeuAc□(2→3)Gal, NeuAc□(2→6)GalNAc, NeuAc□(2→8)NeuAc, and NeuAc□(2→9)NeuAc. To our knowledge, this is the first phosphate-based sialic acid donor that can be used for selective formation of □-glycosidic bond of sialic acid. Methodological improvements such as one-pot glycosylation and solid-phase oligosaccharide synthesis (SPOS) have been widely pursued to expditate processes for oligosaccharide syntheses. Although one-pot protocols available for sialoside syntheses are still rare, this 4,5-oxazolidinone-protected sialyl phosphate donors are applicable to two chief one-pot strategies, including (a) reativity-based programmable one-pot, and (b) orthogonal one-pot, with several sialosides were synthesized in high yields and high □-selectivity. he infection of influenza virus is initiated by attachment of the virus to cell-surface sialoside receptors via influenza surface glycoproteins, hemagglutinin (HA). Human-adapted virus HA preferentially bind to Neu5Ac□(2→6)Gal moieties on epithelial cell of the upper respiratory tract and avian-adapted virus HA preferentially bind to Neu5Ac□(2→3)Gal moieties of intestinal epithelial cells. Glycan microarrays have recently emerged as a powerful tool for studying receptor specificities of influenza HAs. In herein study, a sialoside array was created in which some glycans were prepared by using the new sialylation reaction. This array was employed to study the the role of influenza H5 HA glycosylation in the interactions between HA and its binding oligosaccharides. The glycan microarray analyses showed that trunction of the N-glycan structures on H5HA increased sialoside ligand binding affinities while decreasing specificity toward disparate ligands. We also developed a direct Cy3-labeling methodology of influenza HA, which allowed the calculation of surface dissociation constants by using glycan microarray Because human influenza viruses preferentially bind long oligosaccharide chain, we next synthesized N-acetyllactosamine dimer, trimer and their sialylated versions to complement our sialoside array. We found that the sulfoxide approach is efficient for the polymerization of N-acetyllactosamine chain. The octasaccharide synthesized via block synthesis can further undergo oxidation and subsequent glycosylation to give a nonasaccharide in good yield. These N-acetyllactosamine saccharides were included into the previous array, which was applied for studying the binding specificity of various HA subtypes including the 2009 swin-originated H1N1. In addition, we showed that the terminal sialylation inhibited the binding of human Galectin-3 to lactosamine oligosaccharide chain□□where□□(2→3) sialylation is more significant than □(2→6).