Phenyl 2-azido-2-deoxy-1-selenogalactosides: a single type of glycosyl donor for the highly stereoselective synthesis of α- and β-2-azido-2-deoxy-d-galactopyranosides

Thiourea-Cu(OTf)2/NIS-synergistically promoted stereoselective glycoside formation with 2-azidoselenoglycosides or thioglycosides as donors

A novel promoter system for glycosylation is described. A catalytic amount of thiourea and Cu(OTf)2 together with a slight excess of N-iodosuccinimide synergistically promotes glycosylation at room temperature. The combination of reagents applies to some 2-azidoselenoglycoside and thioglycoside donors. A wide range of alcoholic acceptors underwent smooth conversion to O-(2-azido)glycosides with good stereoselectivities. In addition, the value of this method has been highlighted by its convenient operation and outstanding functional group compatibility.

The Synthesis of Blood Group Antigenic A Trisaccharide and Its Biotinylated Derivative

Blood group antigenic A trisaccharide represents the terminal residue of all A blood group antigens and plays a key role in blood cell recognition and blood group compatibility. Herein, we describe the synthesis of the spacered A trisaccharide by means of an assembly scheme that employs in its most complex step the recently proposed glycosyl donor of the 2-azido-2-deoxy-selenogalactoside type, bearing stereocontrolling 3-O-benzoyl and 4,6-O-(di-tert-butylsilylene)-protecting groups. Its application provided efficient and stereoselective formation of the required α-glycosylation product, which was then deprotected and subjected to spacer biotinylation to give both target products, which are in demand for biochemical studies.

(Automated) Synthesis of Well-defined Staphylococcus Aureus Wall Teichoic Acid Fragments

Wall teichoic acids (WTAs) are important components of the cell wall of the opportunistic Gram‐positive bacterium Staphylococcus aureus. WTAs are composed of repeating ribitol phosphate (RboP) residues that are decorated with d‐alanine and N‐acetyl‐d‐glucosamine (GlcNAc) modifications, in a seemingly random manner. These WTA‐modifications play an important role in shaping the interactions of WTA with the host immune system. Due to the structural heterogeneity of WTAs, it is impossible to isolate pure and well‐defined WTA molecules from bacterial sources. Therefore, here synthetic chemistry to assemble a broad library of WTA‐fragments, incorporating all possible glycosylation modifications (α‐GlcNAc at the RboP C4; β‐GlcNAc at the RboP C4; β‐GlcNAc at the RboP C3) described for S. aureus WTAs, is reported. DNA‐type chemistry, employing ribitol phosphoramidite building blocks, protected with a dimethoxy trityl group, was used to efficiently generate a library of WTA‐hexamers. Automated solid phase syntheses were used to assemble a WTA‐dodecamer and glycosylated WTA‐hexamer. The synthetic fragments have been fully characterized and diagnostic signals were identified to discriminate the different glycosylation patterns. The different glycosylated WTA‐fragments were used to probe binding of monoclonal antibodies using WTA‐functionalized magnetic beads, revealing the binding specificity of these WTA‐specific antibodies and the importance of the specific location of the GlcNAc modifications on the WTA‐chains.

Glycoconjugate Vaccines for Prevention of Haemophilus influenzae Type b Diseases

This review summarizes the experience in laboratory- and industrial-scale syntheses of glycoconjugate vaccines used for prevention of infectious diseases caused by Haemophilus influenzae type b bacteria based on the linear capsular polysaccharide poly-3-β-D-ribosyl-(1→1)-D-ribitol-5-phosphate (PRP) or related synthetic oligosaccharide ligands. The methods for preparation of related oligosaccharide derivatives and results of the studies evaluating effect of their length on immunogenic properties of the conjugates with protein carriers are overviewed.

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 α- or β-anomers. Synthesis of N-acylated α-GlcN and α-GalN containing oligosaccharides is a special challenge due to the presence of a participating group at C2 which favors the formation of β- rather than α-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 α- or β-glycosides depending on the choice of the solvent. In acetonitrile, total β-stereocontrol was achieved, and the use of diethyl ether as a solvent favouring α-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 α-GalN derivatives. To date, homogenous azidophenylselenylation of glycals and glycosylation with phenyl 2-azido-2-deoxy-1-seleno-α-D-glycopyranosides can be regarded as most useful tool for introduction of 2-amino-2-deoxy-D-glycopyranoside residues into complex synthetic oligosaccharides.

Biotinylated Oligo-α-(1 → 4)-d-galactosamines and Their N-Acetylated Derivatives: α-Stereoselective Synthesis and Immunology Application

Using 3-O-benzoyl-4,6-O-di-tert-butylsilylidene-2-azido-2-deoxy-selenogalactoside, biotinylated oligo-α-(1 → 4)-d-galactosamines comprising from two to six GalN units were prepared for the first time together with their N-acetylated derivatives. The combination of blocking groups used herein provided stereocontrol for the α-stereospecific glycosylation, to show also high efficiency of phenyl 2-azido-2-deoxy-selenogalactosides as glycosyl donors. The obtained glycoconjugates are related to fragments of exopolysaccharide galactosaminogalactan (GG) found in Aspergillus fumigatus, which is the most important airborne human fungal pathogen in industrialized countries. The synthesized glycoconjugates were arrayed on streptavidin-coated plates and used to investigate the GG epitopes recognized by mouse monoclonal antibodies against GG and by human antibodies in the sera of patients with aspergillosis. The obtained data showed that the oligo-α-(1 → 4)-d-galactosamines and their N-acetylated derivatives allowed the first precise analysis of the specificity of the antibody responses to this extremely complex fungal polysaccharide.

Synthetic Oligosaccharides Mimicking Fungal Cell Wall Polysaccharides

The cell wall of pathogenic fungi is highly important for the development of fungal infections and is the first cellular component to interact with the host immune system. The fungal cell wall is mainly built up of different polysaccharides representing ligands for pattern recognition receptors (PRRs) on immune cells and antibodies. Purified fungal polysaccharides are not easily available; in addition, they are structurally heterogenic and have wide molecular weight distribution that limits the possibility to use natural polysaccharides to assess the structure of their active determinants. The synthetic oligosaccharides of definite structure representing distinct polysaccharide fragments are indispensable tools for a variety of biological investigations and represent an advantageous alternative to natural polysaccharides. The attachment of a spacer group to these oligosaccharides permits their efficient transformation into immunogenic glycoconjugates as well as their immobilization on plates or microbeads. Herein, we summarize current information on synthetic availability of the variety of oligosaccharides related to main types of fungal cell wall components: galactomannan, α- and β-mannan, α- and β-(1 → 3)-glucan, chitin, chitosan, and others. These data are supplemented with published results of biochemical and immunological applications of synthetic oligosaccharides as molecular probes especially as the components of thematic glycoarrays suitable for characterization of anti-polysaccharide antibodies and cellular lectins or PRRs.

Synthesis of E. faecium wall teichoic acid fragments

The first synthesis of different Enterococcus faecium wall teichoic acid (WTA) fragments is presented. The structure of these major cell wall components was elucidated recently and it was shown that these glycerolphosphate (GroP) based polymers are built up from -6-(GalNAc-α(1-3)-GalNAc-β(1-2)-GroP)- repeating units. We assembled WTA fragments up to three repeating units in length, in two series that differ in the stereochemistry of the glycerolphosphate moiety. The key GalNAc-GalNAc-GroP synthons, required for the synthesis, were generated from galactosazide building blocks that were employed in highly stereoselective glycosylation reactions to furnish both the α- and β-configured linkages. By comparing the NMR spectra of the synthesized fragments with the isolated material it appears that the hereto undefined stereochemistry of the glycerol phosphate moiety is sn-glycerol-3-phosphate. The generated fragments will be valuable tools to study their immunological activity at the molecular level.