n-Pentenyl furanosides: synthesis and glycosidation reactions of some galacto derivatives

Galactofuranose antigens, a target for diagnosis of fungal infections in humans

The use of biomarkers for the detection of fungal infections is of interest to complement histopathological and culture methods. Since the production of antibodies in immunocompromised patients is scarce, detection of a specific antigen could be effective for early diagnosis. D-Galactofuranose (Galf) is the antigenic epitope in glycoconjugates of several pathogenic fungi. Since Galf is not biosynthesized by mammals, it is an attractive candidate for diagnosis of infection. A monoclonal antibody that recognizes Galf is commercialized for detection of aspergillosis. The linkage of Galf in the natural glycans and the chemical structures of the synthesized Galf-containing oligosaccharides are described in this paper. The oligosaccharides could be used for the synthesis of artificial carbohydrate-based antigens, not enough exploited for diagnosis.

D-Galactofuranose (Galf) is the unit in polysaccharides and glycoconjugates of several pathogenic fungi that is recognized by the immune system. Since Galf is not synthesized by mammals, it is an attractive candidate for diagnosis of infection. Since the production of antibodies in immunocompromised patients is scarce, detection of a specific antigen could be effective for early diagnosis. An antibody that recognizes Galf is commercialized for the detection of aspergillosis. Chemically synthesized Galf-containing oligosaccharides, reviewed in this paper, could therefore be used for the synthesis of artificial carbohydrate-based antigens and in diagnosis.

Recent Advances in Transition Metal-Catalyzed Glycosylation

Having access to mild and operationally simple techniques for attaining carbohydrate targets will be necessary to facilitate advancement in biological, medicinal, and pharmacological research. Even with the abundance of elegant reports for generating glycosidic linkages, stereoselective construction of α- and β-oligosaccharides and glycoconjugates is by no means trivial. In an era where expanded awareness of the impact we are having on the environment drives the state-of-the-art, synthetic chemists are tasked with developing cleaner and more efficient reactions for achieving their transformations. This movement imparts the value that prevention of waste is always superior to its treatment or cleanup. This review will highlight recent advancement in this regard by examining strategies that employ transition metal catalysis in the synthesis of oligosaccharides and glycoconjugates. These methods are mild and effective for constructing glycosidic bonds with reduced levels of waste through utilization of sub-stoichiometric amounts of transition metals to promote the glycosylation.

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.

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.

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 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

The synthesis of alpha-D-galactofuranosyl-(1-->2)-D-galactitol, which has been isolated by reductive beta-elimination from glycoproteins of Bacteroides cellulosolvens and Clostridium thermocellum, is described. The approach of selective glycosylation of an aldono-1,4-lactone by the trichloroacetimidate method was employed. The synthesis of alpha-D-Gal f-(1-->2)[beta-D-Gal f-(1-->3)]-D-Galol, that contains Gal f units in both anomeric configurations, is also reported. These are the first synthetic oligosaccharides with alpha-D-Gal f, previously found in natural products.

Stereoselective synthesis of a ketohexofuranose from an aldohexopyranose by a [6+1−1] strategy

Ozonolysis of 2-acetoxymethyl-1,5-anhydro-3,4,6-tri-O-benzyl-2-deoxy-D-arabino-hex-1-enitol gave 1-O-acetyl-3,4,6-tri-O-benzyl-4-O-formyl-D-arabino-hex-2-ulose (5). Subsequent hydrolysis and acetylation of 5 provided 1,2-di-O-acetyl-3,4,6-tri-O-benzyl-D-fructofuranose 6 in excellent yield. This methodology allows specific deuteration at C-1 of a protected D-fructofuranose derivative. This approach therefore could serve as [6+1-1] formulation for hexose series inter-conversion, that is, aldohexopyranose to ketohexofuranose.

Preparation, Properties, and Applications of n-Pentenyl Arabinofuranosyl Donors

[reaction: see text] The development of n-pentenyl furanosyl donors has been tested using arabinose as a model. The readily prepared ortho ester (NPOE) is converted into disarmed (NPG(AC)) and thence armed (NPG(ALK)) n-pentenyl arabinofuranosides. The reactivities of these furanosyl donors and pyranosyl counterparts have been assessed by allowing pairs of both to compete for an acceptor. For the NPOE and armed (NPG(ALK)) pairs, coupling products were obtained from donors, whereas for the disarmed (NPG(AC)) pair, only the arabinofurano coupled product was obtained. To probe their synthetic utility, the NPOE was shown to be the only precursor needed to prepare an alpha-1,5-linked arabinan segment of the complex lipoarabinomannan cell wall array of Mycobacterium tuberculosis.

Synthesis of galactofuranosyl-containing oligosaccharides corresponding to the glycosylinositolphospholipid of Trypanosoma cruzi

The oligosaccharide beta-D-Galf-(1-->3)-alpha-D-Manp-(1-->2)-[beta-D-Galf- (1-->3)]-alpha-D-Manp-(1-->2)-alpha-D-Manp corresponds to the terminal end of the glycosylinositolphospholipid oligosaccharide of the protozoan Trypanosoma cruzi, the causative agent of Chagas' disease. Syntheses of methyl or ethylthio glycosides of the terminal disaccharide, trisaccharide, tetrasaccharide, and pentasaccharide corresponding to this structure are described. These syntheses employ the selective activation of a phenyl 1-selenogalactofuranoside or a phenyl 1-selenomannopyranoside donor over ethyl 1-thioglycoside acceptors with NIS-TfOH.

Synthetic Approaches to Novel Archaeal Tetraether Glycolipid Analogues

Symmetrical and unsymmetrical archaeal tetraether glycolipid analogues have been prepared. The syntheses are based upon the elaboration of lipid cores from versatile chiral starting materials followed by simultaneous or sequential introduction of polar headgroups. Three pathways (A-C) were elaborated for the synthesis of stereochemically defined lipids 14-16 characterized by a straight bridging spacer and two dihydrocitronellyl chains attached to glycerol units at the sn-3 and sn-2 positions, respectively. Pathway C appeared to be particularly advantageous for the synthesis of tetraether 9, which possesses a cyclopentane unit as found in thermoacidophilic lipids. Diglycosylated lipids 4-6 were produced in 49-53% yields by reaction of diols 14-16 with beta-D-galactofuranosyl donor 31, whereas unsymmetrical lipids possessing either two different carbohydrate units 7 or a saccharidic moiety and a phosphate group 8 were efficiently prepared from monoprotected diol 35. These compounds represent the first examples of tetraether-type analogues containing a phosphate unit and/or glycosyl moieties.

Use of a phenyl 1-selenogalactofuranoside as a glycosyl donor for the synthesis of galactofuranosyl-containing disaccharides

The use of acetylated phenyl 1-seleno-beta-D-galactofuranoside as a glycosyl donor for the synthesis of protected D-Galf-beta-(1-->3)-alpha-D-Manp as its methyl or ethylthio glycoside has been demonstrated. Activation of the selenoglycoside over a thioglycoside acceptor by NIS/TfOH is extremely selective and gives the ethylthio disaccharide in 91% yield. The parent disaccharide is found as a terminal and branched unit in the lipopeptidophosphoglycan oligosaccharides of the protozoan Trypanosoma cruzi, the causative agent of Chagas' disease.