- Selective synthesis of anomeric α-glycosyl acetamides via intramolecular Staudinger ligation of the α-azides
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α-Glycosyl azides can be transformed into the corresponding α-glycosyl acetamides with complete retention of configuration via reduction-acylation (Staudinger ligation) reactions using specifically functionalized phosphines. The α-acetamides of per-O-benz
- Bianchi, Aldo,Bernardi, Anna
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- Traceless Staudinger ligation of glycosyl azides with triaryl phosphines: Stereoselective synthesis of glycosyl amides
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α-Glycosyl amides can be synthesized from the corresponding O-benzyl-α-glycosyl azides using a traceless Staudinger ligation with diphenylphosphanyl-phenyl esters 4. All the phosphines employed and their phenol precursors are stable to air at 4 °C for months. Fast intramolecular trapping of the reduction intermediates results in the direct formation of the amide link, which, in turn, prevents epimerisation and allows retention of configuration at the anomeric carbon. Yields and α-selectivity are high when the reaction is performed in polar aprotic solvents. Removal of the benzyl ether protecting groups is achieved by catalytic hydrogenation. α-Glycosyl amides represent a class of virtually unexplored nonhydrolyzable monosaccharide derivatives that may find a useful application as sugar mimics. Conformational studies by NMR spectroscopy confirm that deprotected α-glycosyl amides in the gluco, galacto, and fuco series retain the normal pyranose conformation of the monosaccharide. The reaction of phosphines 4 with tetra-O-acetyl-glycosyl azides is nonstereoconservative, and β-glycosyl amides are obtained in good yields and with complete stereoselectivity starting from both α and β azides.
- Bianchi, Aldo,Bernardi, Anna
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p. 4565 - 4577
(2007/10/03)
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- Glycosyl iodides are highly efficient donors under neutral conditions
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Glycosyl iodides have been prepared and subjected to glycosylation under neutral conditions. The reactions are highly efficient, giving α glycosides even with sterically demanding glycosyl acceptors. Glucosyl iodides react with allyl alcohol slowest and require refluxing conditions. Galactosyl iodides are intermediate in reactivity, providing the allyl glycoside in 3 h at room temperature, whereas glycosylation of fucosyl iodides occurs in less than 1 h under similar conditions. The scope and limitations of the reactions were demonstrated with a variety of acceptors, including an anomeric hydroxyl group, to give trehalose analogs. β-Selective glycosylation of glucosyl iodides, in the absence of C-2 participation, could be achieved by simply changing the solvent from benzene to acetonitrile. Copyright (C) 1999 Elsevier Science Ltd.
- Hadd, Michael J.,Gervay, Jacquelyn
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