Synthesis of α-d-Galf-(1→2)-d-galactitol and α-d-Galf-(1→2)[β-d-Galf-(1→3)]-d-galactitol, oligosaccharide derivatives from Bacteroides cellulosolvens glycoproteins

Direct N-glycosylation of tosyl and nosyl carbamates with trichloroacetimidate donors

Acidic sulphonamide reactants act as both catalysts and nucleophiles to afford the desired N-glycofuranosyl sulfonamides stereoselectively.

Synthesis of the hexasaccharide from Trypanosoma cruzi mucins with the Galp(1 → 2)Galf unit constructed with a superarmed thiogalactopyranosyl donor

Hexasaccharide β-D-Galp-(1→ 2)-[β-D-Galp-(1 → 3)]-β-D-Galp-(1 → 6)-[β-D-Galp-(1 → 2)-β-D-Galf-(1 → 4)]-D-GlcNAc (1) was found O-linked in mucins of Trypanosoma cruzi epimastigotes and metacyclic trypomatigotes. Studies on the biological pathways and functionalities of the mucin oligosaccharides are prompted in order to understand the interactions of these molecules with the insect host. Trisaccharide constituent β-D-Galp-(1 → 2)-β-D-Galf-(1 → 4)-D-GlcNAc was constructed from the reducing to the non-reducing end. We discuss the difficulties to introduce a Galp unit at the O-2 position of a partially protected galactofuranosyl unit which were overcome using an anchimerically superarmed donor. By this route and employing a [3 + 3] nitrilium convergent approach hexasaccharide 1 was synthesized in moderate yield.

Convergent Synthesis of Oligosaccharide Fragments Corresponding to the Cell Wall O-Polysaccharide of Salmonella enterica O53

Conventional glycoconjugate vaccines are prepared with polysaccharides isolated from bacterial fermentation, an approach with some significant drawbacks such as handling of live bacterial strains, the presence of biological impurities, and inter-batch variations in oligosaccharide epitope structure. However, it has been shown in many cases that a synthetic fragment of appropriate structure conjugated to a protein can be an effective vaccine that circumvents the shortcomings of using full-length oligosaccharides. The development of synthetic strategies to prepare glycoconjugate derivatives against pathogenic bacterial strains is therefore of great interest. Oligosaccharide fragments corresponding to the repeat unit of the cell wall O-antigen of Salmonella enterica strain O53 were synthesized in good yield. Sequential and block glycosylation strategies were used for the synthesis of the target compounds. A number of recently developed reaction conditions were used in the synthetic strategy. A one-pot reaction scheme was also developed for the multiple glycosylation steps. The stereoselective outcomes of all glycosylation reactions were very good.

Identification of the S-layer glycoproteins and their covalently linked glycans in the halophilic archaeon Haloarcula hispanica

Haloarcula hispanica is one of members of the Halobacteriaceae, which displays particularly low restriction activity and is therefore important as one of the most tractable haloarchaea for archaeal genetic research. Although the Har. hispanica S-layer protein has been reported glycosylated, the S-layer glycoprotein and its glycosylation have not been investigated yet. In this study, the S-layer proteins of Har. hispanica were extracted and characterized. The S-layer was found containing two different glycoproteins which shared highly similar amino acid sequences. The genes coding for these two S-layer glycoproteins were found next to each other in the genome. Moreover, the N- and O-linked glycans were released from these two S-layer glycoproteins for structural determination. Based on the mass spectrometry and nuclear magnetic resonance, the N-glycan was determined as a branched trisaccharide containing a 225 Da residue corresponded to a 2-amino-6-sulfo-2, 6-dideoxy-quinovose, which was the first time that a naturally occurring form of sulfoquinovosamine was identified. Besides, the O-glycan was characterized as a Glcα-1,4-Gal disaccharide by mass spectrometry combined with monosaccharide composition analysis and glycosidase treatment. The determination of the N- and O-glycan structure will be helpful for studying the diverse protein glycosylation pathways in archaea utilizing H. hispanica as a new model.

Synthesis of D-galactofuranose-containing molecules: design of galactofuranosyl acceptors

D-Galactofuranose (D-Galf) is present in glycoconjugates of several pathogenic microorganisms but is absent in mammals, so it is a good target for the development of chemotherapeutic agents for the treatment of microbial infections. This fact has increased interest in the synthesis of D-Galf-containing molecules for corresponding glycobiological studies. The synthesis of oligosaccharides, glycoconjugates, and mimetics of D-Galf requires specific methods for the preparation of galactose derivatives in the furanosic configuration, the synthesis of appropriate acceptors, and efficient glycosylation methods for the construction of α- and β-D-Galf linkages. This review summarizes the different strategies developed for the preparation of partially protected derivatives of D-Galf, suitable as acceptors for the construction of (1→2), (1→3), (1→5), and (1→6) link- ages, and describes recent applications.

Synthesis of a derivative of α-D-Glcp(1->2)-D-Galf suitable for further glycosylation and of α-D-Glcp(1->2)-D-Gal, a disaccharide fragment obtained from varianose

The presence of galactofuranoyl units in infectious microorganisms has prompted the study of the metabolic pathways involved in their incorporation in glycans. Although much progress has been made with respect to the biosynthesis of β-D-Galf-containing glycoconjugates, the mechanisms by which α-D-Galf units are incorporated remain unclear. Penicillium varians is a non-pathogenic fungus that produces varianose, a polysaccharide containing both α- and β-D-Galf units, which can be used as a model for biosynthetic studies on α-D-Galf incorporation. Synthetic oligosaccharide fragments related to varianose are useful as potential substrates or standards for characterization of the α-galactofuranosyl transferases. In this paper we report a straightforward procedure for the synthesis of α-D-Glcp(1→2)-D-Gal (1) and the use of this compound to monitor the natural disaccharide released from varianose by mild acid degradation. The synthesis, performed by the glycosylaldonolactone approach, involved a glucosylgalactofuranose derivative, suitable for the synthesis of higher oligosaccharides with an internal D-Galf.

Glycosylation studies on conformationally restricted 3,5-O-(di-tert-butylsilylene)-D-galactofuranosyl trichloroacetimidate donors for 1,2-cis α-D-galactofuranosylation

Conformationally restricted 3,5-O-di-tert-butylsilylene-D-galactofuranosyl trichloroacetimidate donors were synthesized from allyl α-D-galactofuranoside for the construction of 1,2-cis α-D-galactofuranosyl linkages. Glycosylation reactions were performed with several acceptors, including D-galactono-1,4-lactone, D-rhamnopyranosyl, and D-mannopyranosyl derivatives. The influence of the temperature and the reaction solvents was evaluated, as well as the 6-O-substitution pattern of the donor. The higher α-selectivities were obtained at -78 °C in diethyl ether as solvent. 6-O-Acetyl substitution on constrained donor increased the α-selectivity compared to the 6-O-benzyl substitution. Almost no selectivities were observed in the non-participating solvent CH(2)Cl(2). In contrast, ethereal solvents enhanced the α-selectivity suggesting a participating effect in the reaction intermediate.

Influence of the solvent in low temperature glycosylations with O-(2,3,5,6-tetra-O-benzyl-β-D-galactofuranosyl) trichloroacetimidate for 1,2-cis α-D-galactofuranosylation

Glycosylation studies for the construction of 1,2-cis α-linkages with O-(2,3,5,6-tetra-O-benzyl-β-D-galactofuranosyl) trichloroacetimidate (1) and several acceptors, including D-mannosyl and l-rhamnosyl derivatives were performed. The reactions were conducted at low temperatures using CH(2)Cl(2), Et(2)O, and acetonitrile as solvents. A non-participating solvent such as CH(2)Cl(2) at -78°C, favored the α-D-configuration. In contrast, acetonitrile strongly favored the β-D-configuration, whereas no selectivities were observed with Et(2)O. The use of thiophene as an additive did not enhance the α-D-selectivity as in the pyranose counterpart. Although selectivities strongly depended on the acceptor, trichloroacetimidate 1 constitutes a valuable donor for the synthesis of α-D-Galf-(1→2)-l-Rha and α-D-Galf-(1→6)-D-Man. As these motifs are present in pathogenic microorganisms, these procedures described here are useful for the straightforward synthesis of natural oligosaccharides.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Synthesis of trisaccharides containing internal galactofuranose O-linked in Trypanosoma cruzi mucins

The trisaccharides beta-D-Galf-(1-->2)-beta-D-Galf-(1-->4)-D-GlcNAc (5) and beta-D-Galp-(1-->2)-beta-D-Galf-(1-->4)-D-GlcNAc (6) constitute novel structures isolated as alditols when released by reductive beta-elimination from mucins of Trypanosoma cruzi (Tulahuen strain). Trisaccharides 5 and 6 were synthesized employing the aldonolactone approach. Thus, a convenient D-galactono-1,4-lactone derivative was used for the introduction of the internal galactofuranose and the trichloroacetimidate method was employed for glycosylation reactions. Due to the lack of anchimeric assistance on O-2 of the galactofuranosyl precursor, glycosylation studies were performed under different conditions. The nature of the solvent strongly determined the stereochemical course of the glycosylation reactions when the galactofuranosyl donor was substituted either by 2-O-Galp or 2-O-Galf.

Chemistry and biology of galactofuranose-containing polysaccharides

The thermodynamically less stable form of galactose-galactofuranose (Galf)-is essential for the viability of several pathogenic species of bacteria and protozoa but absent in this form in mammals, so the biochemical pathways by which Galf-containing glycans are assembled and catabolysed are attractive sites for drug action. This potential has led to increasing interest in the synthesis of molecules containing Galf residues, their subsequent use in studies directed towards understanding the enzymes that process these residues and the identification of potential inhibitors of these pathways. Major achievements of the past several years have included an in-depth understanding of the mechanism of UDP-galactopyranose mutase (UGM), the enzyme that produces UDP-Galf, which is the donor species for galactofuranosyltransferases. A number of methods for the synthesis of galactofuranosides have also been developed, and practitioners in the field now have many options for the initiation of a synthesis of glycoconjugates containing either alpha- or beta-Galf residues. UDP-Galf has also been prepared by a number of approaches, and it appears that a chemoenzymatic approach is currently the most viable method for producing multi-milligram amounts of this important intermediate. Recent advances both in the understanding of the mechanism of UGM and in the synthesis of galactofuranose and its derivatives are highlighted in this review.

First O-Glycosylation from Unprotected 1-Thioimidoyl Hexofuranosides Assisted by Divalent Cations

The preparation of O-hexofuranosides was accomplished from unprotected 1-thioimidoyl furanosides as donors. The present methodology was first used for the synthesis of octyl galactofuranoside and further extended to D-galactofuranose-containing disaccharides. Within this study, we emphasized the need for additional complexing cations to maintain the furanose ring in its initial size. After experimentation, calcium ion was first used concomitantly with trimethylsilyl trifluoromethanesulfonate, the latter being able to activate the thioimidate and the former being likely to inhibit ring expansion. Moreover, an improvement was performed by using copper(II) trifluoromethanesulfonate which could then meet the requirements as both promoter and complexing agent.

2,3-Anhydrosugars in Glycoside Bond Synthesis. Application to α-d-Galactofuranosides

We report here the use of 2,3-anhydro-D-gulofuranosyl thioglycosides and glycosyl sulfoxides in the synthesis of alpha-D-galactofuranosidic bonds, which are present in a range of bacterial and fungal glycoconjugates. This two-step method involves a stereoselective glycosylation in which a 2,3-anhydro-alpha-D-gulofuranoside is obtained either as the sole or as the major product, followed by a regioselective opening of the epoxide ring using lithium benzylate in the presence of (-)-sparteine. In exploring the scope of the method, donors protected at O5 and O6 with an isopropylidene acetal, benzyl ethers, or benzoate esters were studied. Overall, the glycosyl sulfoxides provided the products in slightly higher yields and selectivity, with the best results being obtained with benzylated and benzoylated substrates. In the epoxide ring-opening reactions, the acetal- and ether-protected donors afforded poor to modest regioselectivity, whereas the benzoylated products gave good yields of the desired alpha-D-galactofuranosides. The benzoyl-protected species are, therefore, the donors of choice for these reactions. The utility of the approach was demonstrated through the synthesis of three alpha-D-galactofuranosyl-containing oligosaccharides.

Synthesis of a Pentasaccharide Fragment of Varianose, a Cell Wall Polysaccharide from Penicillium varians

The first synthesis of an oligosaccharide fragment of varianose, a polysaccharide produced by Penicillium varians, is reported. The target pentasaccharide features both alpha- and beta-galactofuranoside residues and the alpha-galactofuranoside residue is hindered, being substituted on adjacent oxygens (O1 and O2), both of which are cis to the two-carbon side chain at C4. Key features of the synthesis include a novel method for the selective protection of the C3 hydroxyl group of galactofuranosyl residues via an epoxide formation/opening sequence, the introduction of the alpha-d-galactofuranosyl residue using a 2,3-anhydrosugar donor, and the use of the 1-benzenesulfinylpiperidine/trifluoromethanesulfonic anhydride activation method for the addition of an alpha-D-glucopyranosyl residue to a hindered hydroxyl group in an advanced tetrasaccharide intermediate.