Azidophenylselenylation of glycals towards 2-azido-2-deoxy-selenoglycosides and their application in oligosaccharide synthesis

2-Amino-2-deoxy-pyranosyl units are important structural components of cell-wall polymers in prokaryotes, fungi and mammals. With respect to the need for development of novel and efficient vaccines and tools for serodiagnosis of infectious diseases, of particular interest are the oligosaccharide cell-wall antigens of pathogenic bacteria and fungi, which comprise 2-amino-2-deoxy-D-glucopyranose and 2-amino-2-deoxy-D-galactopyranose units as alpha- or beta-anomers. Synthesis of N-acylated alpha-GlcN and alpha-GalN containing oligosaccharides is a special challenge due to the presence of a participating group at C2 which favors the formation of beta-rather than alpha-glycoside bond. Herein we overview the efficient two-step approach for preparation of 1,2-cis-glycosides of 2-amino-2-deoxy-D-glucopyranose and 2-amino-2-deoxy-D-galactopyranose, which was recently developed in our laboratory. In the first step, an efficient and straightforward azidophenylselenylation procedure of glycals gives phenyl 2-azido-2-deoxy-1-selenoglycosides as versatile glycosyl donors. In the second step, these donors can be efficiently transformed into alpha- or beta-glycosides depending on the choice of the solvent. In acetonitrile, total beta-stereocontrol was achieved, and the use of diethyl ether as a solvent favouring alpha-stereoselectivity of glycosylations with phenyl 2-azido-2-deoxy-1-selenoglycosides. Besides, it was shown, that low reactivity and nucleophilicity of glycosyl acceptors which are glycosylated with phenyl 2-azido-2-deoxy-1-selenogalactosides facilitated the formation of alpha-GalN derivatives. To date, homogenous azidophenylselenylation of glycals and glycosylation with phenyl 2-azido-2-deoxy-1-seleno-alpha-D-glycopyranosides can be regarded as most useful tool for introduction of 2-amino-2-deoxy-D-glycopyranoside residues into complex synthetic oligosaccharides.