Stereoselective Synthesis of α- and β-Glycofuranosyl Amides by Traceless Ligation of Glycofuranosyl Azides

A highly stereoselective synthesis of a- or b-glycofuranosyl amides based on the traceless Staudinger ligation of glycofuranosyl azides of the galacto, ribo, and arabino series with 2-diphenylphosphanyl- phenyl esters has been developed. Both a- and b-isomers can be obtained with excellent selectivity from a common, easily available precursor. The process does not depend on the anomeric configuration of the starting azide but appears to be controlled by the C2 configuration and by the protection/deprotection state of the substrates. A mechanistic interpretation of the results, supported by 31P NMR experiments, is offered and merged with our previous mechanistic analysis of pyranosyl azide ligation reactions.