| Summary: | <p> N-acetyl neuraminic acid is most common member of the sialic acid family. This unique monosaccharide is displayed at the terminal position of oligosaccharides in glycoproteins and glycolipids on outer surface of a cell membrane. </p><p> The biological features of N-acetyl neuraminic acid involve cell-cell interactions in immunogenesis, as well as pathogens attacks, and overexpression in cancer cells. </p><p> The synthesis of sialic acid containing glycoconjugates is essential for a better understating of their biological function, as well as for the design of therapeutics. In addition, to the limitations of the enzymatic approach, chemical synthesis of these glycosidic linkages offer the opportunity to large scale isolation of common as well as uncommon oligosaccharides. </p><p> However, the stereocontrolled synthesis of sialic acid containing glycoconjugates is undoubtedly one of the most challenging research goals in the carbohydrate synthetic field. In particular, little is known on the effect of O-protecting groups in sialylation reactions. Recently we proved that a picoloyl group at C-4 can indeed favor an alpha sialylation if in the presence of an excess of triflic acid. Excellent stereoselectivities and yields were obtained with a wide range of galactosyl acceptors. </p><p> As a natural evolution of this finding, we decided to investigate the effect of picoloyl at other positions, as well stereoselectivities and yield in sialylations when di-picoloylated sialic acid donors are used. </p><p> Herein we report a systematic investigation of the regioselective introduction of picoloyl groups as well as the synthesis of a 7,8 di-picoloylated donor. The latter, when tested in sialylation reactions, gave exceptionally high yields and stereoselectivities.</p><p>
|
|---|
