Effect of Protecting\nGroups and Activating Conditions\non 3‑Deoxy‑d-<i>glycero</i>-d-<i>galacto</i>-2-nonulosonic Acid (Kdn) Glycosylation:\nStereoselective Synthesis of α- and β‑Kdn Glycosides

Kdn is a common member of the sialic acid family. Carbohydrates\ncontaining Kdn residues are widely distributed in nature and embody\nimportant biological information. However, the methods for synthesizing\nKdn glycosides are limited, which restricts their biological study.\nIn this paper, we developed efficient α- and β-stereoselective\nKdn glycosylation methods by employing differentially protected Kdn\nthioglycoside donors under their respective activating protocols.\nThe 5,7-<i>O</i>-carbonate fused Kdn thioglycoside <b>1a</b> could be promoted with NIS/TfOH (cat.) in CH<sub>2</sub>Cl<sub>2</sub>/CH<sub>3</sub>CN (2:1) to afford Kdn glycosides with\nexcellent α-selectivity in high yields. Meanwhile, based on\nthe Ph<sub>2</sub>SO/Tf<sub>2</sub>O preactivation strategy, the nonfused\nKdn thioglycoside <b>1b</b> behaved as a high-yielding and β-selective\ndonor to couple with various carbohydrate alcohols, leading to formation\nof β-Kdn glycosides. The synthetic utility of these newly developed\nglycosyl donors has been demonstrated by the stereoselective and straightforward\nassembly of two natural Kdn-containing oligosaccharides.