Galf-Specific Neolectins: Towards Promising Diagnostic Tools

In the absence of naturally available galactofuranose-specific lectin, we report herein the bioengineering of GalfNeoLect, from the first cloned wild-type galactofuranosidase (Streptomyces sp. strain JHA19), which recognises and binds a single monosaccharide that is only related to nonmammalian species, usually pathogenic microorganisms. We kinetically characterised the GalfNeoLect to confirm attenuation of hydrolytic activity and used competitive inhibition assay, with close structural analogues of Galf, to show that it conserved interaction with its original substrate. We synthetised the bovine serum albumin-based neoglycoprotein (GalfNGP), carrying the multivalent Galf units, as a suitable ligand and high-avidity system for the recognition of GalfNeoLect which we successfully tested directly with the galactomannan spores of Aspergillus brasiliensis (ATCC 16404). Altogether, our results indicate that GalfNeoLect has the necessary versatility and plasticity to be used in both research and diagnostic lectin-based applications.

Regioselective and Stereospecific β-Arabinofuranosylation by Boron-Mediated Aglycon Delivery

Regio- and stereoselective formation of the 1,2-cis-furanosidic linkage has been in great demand for efficient synthesis of biologically active natural glycosides. In this study, we developed a regioselective and β-stereospecific d-/l-arabinofuranosylation promoted by a boronic acid catalyst under mild conditions. The glycosylations proceeded smoothly for a variety of diols, triols, and unprotected sugar acceptors to give the corresponding β-arabinofuranosides (β-Arbf) in high yields with complete β-stereoselectivity and high regioselectivity. The regioselectivity was completely reversed depending on the optical isomerism of the donor used and was predictable a priori using predictive models. Mechanistic studies based on DFT calculations revealed that the present glycosylation occurs through a highly dissociative concerted SN i mechanism. The usefulness of the glycosylation method was demonstrated by the chemical synthesis of trisaccharide structures of arabinogalactan fragments.

Exploring the Glycosylation Reaction Inside the Resorcin[4]arene Capsule

In the past decade, there has been an increased interest in applying supramolecular capsule and cage catalysis to the current challenges in synthetic organic chemistry. In this context, we recently reported the resorcin[4]arene capsule-catalyzed conversion of α-glycosyl halides into β-glycosides with high selectivity. Interestingly, this methodology enabled the formation of a wide range of β-pyranosides as well as β-furanosides, although these two donor classes exhibit different reactivities and usually require different reaction conditions and catalysts. Evidence was provided that a proton wire plays a key role in this reaction by enabling dual activation of the glycosyl donor and acceptor. Here, we describe a detailed investigation of several aspects of this reactivity. Besides a mechanistic study, we elucidated the size limitation, the origin of catalytic turnover, and the electrophile scope of nonglycosylic halides. Moreover, a screening of the sensitivity to changes in the reaction conditions provides guidelines to facilitate reproducibility. Furthermore, we demonstrate the compatibility with environmentally benign solvent alternatives, including the renewable solvent limonene.

Structure and synthesis of a vaccine and diagnostic target for Enterocloster bolteae, an autism-associated gut pathogen – Part II

Enterocloster bolteae (formerly known as Clostridium bolteae) is a gastro-intestinal pathogenic bacterium often detected in the fecal microbiome of children in the autism spectrum. E. bolteae excretes metabolites that are thought to act as neurotoxins. This study is an update of our first E. bolteae investigation that discovered an immunogenic polysaccharide. Through a combination of chemical derivatizations/degradations, spectrometry and spectroscopy techniques, a polysaccharide composed of disaccharide repeating blocks comprised of 3-linked β-d-ribofuranose and 4-linked α-l-rhamnopyranose, [→3)-β-D-Ribf-(1 → 4)-α-L-Rhap-(1→]_n, was identified. To confirm the structure, and to provide material for subsequent investigations, the chemical synthesis of a corresponding linker-equipped tetrasaccharide, β-D-Ribf-(1 → 4)-α-L-Rhap-(1 → 3)-β-D-Ribf-(1 → 4)-α-L-Rhap-(1→O(CH₂)₈N₃, is also described. Research tools based on this immunogenic glycan structure can form the foundation for serotype classification, diagnostic/vaccine targets and clinical studies into the hypothesized role of E. bolteae in the onset/augmentation of autism related conditions in children.

Supramolecular Capsule-Catalyzed Highly β-Selective Furanosylation Independent of the SN1/SN2 Reaction Pathway

The resorcin[4]arene capsule was found to catalyze β-selective furanosylation reactions for a variety of different furanosyl donors: α-d- and α-l-arabinosyl-, α-l-fucosyl-, α-d-ribosyl-, α-d-xylosyl-, and even α-d-lyxosyl fluorides. The scope is only limited by the inherently finite volume inside the closed capsular catalyst. The catalyst is readily available on a multi-100 g scale and can be recycled for at least seven rounds without significant loss in activity, yield, and selectivity. The mechanistic investigations indicated that the furanosylation mechanism is shifted toward an SN1 reaction on the mechanistic continuum between the prototypical SN1 and SN2 substitution types, as compared to the pyranosylation reaction inside the same catalyst. This is especially true for the lyxosyl donor, as indicated by the nucleophile reaction order of 0.26, and supported by metadynamics calculations. The mechanistic shift toward SN1 is of high interest as it indicates that this catalyst not only enables β-selective furanosylations and pyranoslyations independently of the substrate configuration but in addition also independently of the operating mechanism. To our knowledge, there is no alternative catalyst available that displays such properties.

Recent advances in stereoselective 1,2-cis-O-glycosylations

For the stereoselective assembly of bioactive glycans with various functions, 1,2-cis-O-glycosylation is one of the most essential issues in synthetic carbohydrate chemistry. The cis-configured O-glycosidic linkages to the substituents at two positions of the non-reducing side residue of the glycosides such as α-glucopyranoside, α-galactopyranoside, β-mannopyranoside, β-arabinofuranoside, and other rather rare glycosides are found in natural glycans, including glycoconjugate (glycoproteins, glycolipids, proteoglycans, and microbial polysaccharides) and glycoside natural products. The way to 1,2-trans isomers is well sophisticated by using the effect of neighboring group participation from the most effective and kinetically favored C-2 substituent such as an acyl group, although high stereoselective synthesis of 1,2-cis glycosides without formation of 1,2-trans isomers is far less straightforward. Although the key factors that control the stereoselectivity of glycosylation are largely understood since chemical glycosylation was considered to be one of the useful methods to obtain glycosidic linkages as the alternative way of isolation from natural sources, strictly controlled formation of these 1,2-cis glycosides is generally difficult. This minireview introduces some of the recent advances in the development of 1,2-cis selective glycosylations, including the quite recent developments in glycosyl donor modification, reaction conditions, and methods for activation of intermolecular glycosylation, including the bimodal glycosylation strategy for 1,2-cis and 1,2-trans glycosides, as well as intramolecular glycosylations, including recent applications of NAP-ether-mediated intramolecular aglycon delivery.

Synthesis of naturally occurring β-l-arabinofuranosyl-l-arabinofuranoside structures towards the substrate specificity evaluation of β-l-arabinofuranosidase

Methyl β-l-arabinofuranosyl-(1 → 2)-, -(1 → 3)-, and -(1 → 5)-α-l-arabinofuranosides have been stereoselectively synthesized through 2-naphthylmethyl ether-mediated intramolecular aglycon delivery (NAP-IAD), whose β-linkages were confirmed by NMR analysis on the ³J_H1-H2 coupling constant and ¹³C chemical shift of C1. The NAP-IAD approach was simply extended for the synthesis of trisaccharide motifs possessing β-l-arabinofuranosyl-(1 → 5)-l-arabinofuranosyl non-reducing terminal structure with the branched β-l-arabinofuranosyl-(1 → 5)-[α-l-arabinofuranosyl-(1 → 3)]-α-l-arabinofuranosyl and the liner β-l-arabinofuranosyl-(1 → 5)-β-l-arabinofuranosyl-(1 → 5)-β-l-arabinofuranosyl structures in olive arabinan and dinoflagellate polyethers, respectively. The results on the substrate specificity of a bifidobacterial β-l-arabinofuranosidase HypBA1 using the regioisomers indicated that HypBA1 could hydrolyze all three linkages however behaved clearly less active to β-(1 → 5)-linked disaccharide than other two regioisomers including the proposed natural degradation product, β-(1 → 2)-linked one from plant extracellular matrix such as extensin. On the other hand, Xanthomonas XeHypBA1 was found to hydrolyze all three disaccharides as the substrate with higher specificity to β-(1 → 2)-linkage than bifidobacterial HypBA1.

Like Visiting an Old Friend: Fischer Glycosylation in the Twenty-First Century: Modern Methods and Techniques

Fischer glycosylation is typically the chemical reaction of a monosaccharide and an alcohol in presence of an acidic catalyst to afford glycosides in pyranosidic and furanosidic forms. This reaction is still applied today for the synthesis of specialized glycosides, and optimization and modification of the method have continued since its discovery by Emil Fischer in the 1890s. This review presents advancements in Fischer glycosylation described in literature of the past 15 years and its implementation in modern chemical methods.

Stereoselective 1,2-cis Furanosylations Catalyzed by Phenanthroline

Stereoselective formation of the 1,2-cis furanosidic linkage, a motif of many biologically relevant oligosaccharides and polysaccharides, remains an important synthetic challenge. We herein report a new stereoselective 1,2-cis furanosylation method promoted by phenanthroline catalysts under mild and operationally simple conditions. NMR experiments and density functional theory calculations support an associative mechanism in which the rate-determining step occurs from an inverted displacement of the faster-reacting phenanthrolinium ion intermediate with an alcohol nucleophile. The phenanthroline catalysis system is applicable to a number of furanosyl bromide donors to provide the challenging 1,2-cis substitution products in good yield with high anomeric selectivities. While arabinofuranosyl bromide provides β-1,2-cis products, xylo- and ribofuranosyl bromides favor α-1,2-cis products.

Synthesis of Tyrosol and Hydroxytyrosol Glycofuranosides and Their Biochemical and Biological Activities in Cell-Free and Cellular Assays

Tyrosol (T) and hydroxytyrosol (HOT) and their glycosides are promising candidates for applications in functional food products or in complementary therapy. A series of phenylethanoid glycofuranosides (PEGFs) were synthesized to compare some of their biochemical and biological activities with T and HOT. The optimization of glycosylation promoted by environmentally benign basic zinc carbonate was performed to prepare HOT α-L-arabino-, β-D-apio-, and β-D-ribofuranosides. T and HOT β-D-fructofuranosides, prepared by enzymatic transfructosylation of T and HOT, were also included in the comparative study. The antioxidant capacity and DNA-protective potential of T, HOT, and PEGFs on plasmid DNA were determined using cell-free assays. The DNA-damaging potential of the studied compounds for human hepatoma HepG2 cells and their DNA-protective potential on HepG2 cells against hydrogen peroxide were evaluated using the comet assay. Experiments revealed a spectrum of different activities of the studied compounds. HOT and HOT β-D-fructofuranoside appear to be the best-performing scavengers and protectants of plasmid DNA and HepG2 cells. T and T β-D-fructofuranoside display almost zero or low scavenging/antioxidant activity and protective effects on plasmid DNA or HepG2 cells. The results imply that especially HOT β-D-fructofuranoside and β-D-apiofuranoside could be considered as prospective molecules for the subsequent design of supplements with potential in food and health protection.

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.

An ultra-low thiourea catalyzed strain-release glycosylation and a multicatalytic diversification strategy

The utility of thiourea catalysis in selective glycosylation strategies has gained significant momentum lately due to its versatility in hydrogen bonding or anionic recognition activation modes. The use of these non-covalent interactions constitute a powerful means to construct glycosidic linkages as it mimics physiologically occurring glycosyltransferases. However, glycosyl donor activation through the currently employed catalysts is moderate such that, in general, catalyst loadings are rather high in these transformations. In addition, thiourea catalysis has not been well explored for the synthesis of furanosides. Herein, we demonstrate an ultra-low loadings stereoselective and stereospecific thiourea catalyzed strain-release furanosylation and pyranosylation strategy. Our ultra-low organocatalyzed furanosylation enables a multicatalytic strategy, which opens up a unique avenue towards rapid diversification of synthetic glycosides. In-situ NMR monitoring unravel insights into unknown reaction intermediates and initial rate kinetic studies reveal a plausible synergistic hydrogen bonding/Brønsted acid activation mode.

Non-covalent glycosyl donor activation often requires high organocatalyst loadings. Here, the authors demonstrate that strain-release glycosylations can take place at very low thiourea catalyst loadings. In addition, the authors developed a one-pot multicatalytic strategy that can diversify glycosides rapidly.

Stereocontrolled Synthesis of α-Xylofuranosides Using a Conformationally Restricted Donor

A number of biologically relevant glycoconjugates possess 1,2-cis-furanosidic linkages, a class of glycosidic bond that remains challenging to introduce with high stereoselectivity. In this paper, we report an approach to one family of such linkages, α-xylofuranosides, via the use of thioglycoside donors possessing a conformationally restricting xylylene protecting group. The method was shown to provide the desired targets in good to excellent yield and stereoselectivity. Computational investigations support the proposal that the protecting group locks the electrophilic intermediate in these reactions into a conformation that leads to the high selectivity. The power of the methodology was demonstrated through the synthesis of a complex hexasaccharide motif from lipoarabinomannan, an immunomodulatory polysaccharide from mycobacteria.

Regioselective 5-O-Opening of Conformationally Locked 3,5-O-Di-tert-butylsilylene-d-galactofuranosides. Synthesis of (1→5)-β-d-Galactofuranosyl Derivatives

The use of thiogalactofuranoside as donors for the construction of internal Galf containing oligosaccharide is limited, probably due to the difficulty to functionalize thiogalactofuranoside derivatives showing O-2, O-3, and O-5 with similar reactivity. An efficient method for complete regioselective 5-O-opening of conformationally restricted 3,5-O-di-tert-butylsilylene-d-galactofuranoside derivatives was developed. The use of a solution nBu₄NF (1.1 equiv) in CH₂Cl₂ on 6 gave the 5-OH free derivative 10 as the only product (90%). 3-O-Di-tert-butylhydroxysilyl derivative 10 was stable upon purification and glycosylation reaction. Preactivation of conformationally restricted thioglycoside 6 employing p-NO₂-benzensulfenyl chloride/AgOTf followed by condensation over the 5-OH thioglycoside acceptor 10 gave the corresponding disaccharide 12 without autocondensation byproduct. Regioselective 5-O-deprotection was also successfully performed over the (1→5)-β-d-galactofuranosyl di- and trisaccharide derivatives 12 and 13. This methodology allowed the differentiation between the secondary hydroxyl groups OH-3 and OH-5 of 1,2-cis or 1,2-trans d-galactofuranoside derivatives, and it still constitutes an innovative approach to access oligosaccharides of pharmacological importance.

Twenty Years of Mycobacterial Glycans: Furanosides and Beyond

The cell surface (or cell wall) of bacteria is coated with carbohydrate (or glycan) structures that play a number of important roles. These include providing structural integrity, serving as a permeability barrier to extracellular compounds (e.g., drugs) and modulating the immune system of the host. Of interest to this Account is the cell wall structure of mycobacteria. There are a host of different mycobacterial species, some of which cause human disease. The most well-known is Mycobacterium tuberculosis, the causative agent of tuberculosis. The mycobacterial cell wall is characterized by the presence of unusual carbohydrate structures that fulfill the roles described above. However, in many cases, a molecular-level understanding of how mycobacterial cell wall glycans mediate these processes is lacking.

Inspired by a seminar he heard as a postdoctoral fellow, the author began his independent research program with a focus on the chemical synthesis of mycobacterial glycans. The goals were not only to develop synthetic approaches to these unique structures but also to provide molecules that could be used to probe their biological function. Initial work addressed the preparation of fragments of two key polysaccharides, arabinogalactan and lipoarabinomannan, which contain large numbers of sugar residues in the furanose (five-membered) ring form. At the time these investigations began, there were few methods reported for the synthesis of oligosaccharides containing furanose rings. Thus, early in the program, a major area of interest was methodology development, particularly for the preparation of 1,2-cis-furanosides. To solve this challenge, a range of conformationally restricted donors have been developed, both in the author’s group and others, which provide 1,2-cis-furanosidic linkages with high stereoselectivity.

These investigations were followed by application of the developed methods to the synthesis of a range of target molecules containing arabinofuranose and galactofuranose residues. These molecules have now found application in biochemical, immunological, and structural biology investigations, which have shed light on their biosynthesis and how these motifs are recognized by both the innate and adaptive immune systems.

More recently, attention has been directed toward the synthesis of another class of immunologically active mycobacterial cell wall glycans, the extractable glycolipids. In this case, efforts have been primarily on phenolic glycolipids, and the compounds synthesized have been used to evaluate their ability to modulate cytokine release. Over the past 20 years, the use of chemical synthesis to provide increasingly complex glycan structures has provided significant benefit to the burgeoning field of mycobacterial glycobiology. Through the efforts of groups from around the globe, access to these compounds is now possible via relatively straightforward methods. As the pool of mycobacterial glycans continues to grow, so too will our understanding of their role in disease, which will undoubtedly lead to new strategies to prevent or treat mycobacterial infections.

Synthesis of Enantiomeric Polyhydroxyalkylpyrrolidines from 1,3-Dipolar Cycloadducts. Evaluation as Inhibitors of a β-Galactofuranosidase

Enantiomeric 2,3,4-tris(hydroxyalkyl)-5-phenylpyrrolidines have been synthesized from the major cycloadducts obtained by the 1,3-dipolar cycloaddition of sugar enones with azomethine ylides derived from natural amino acids. Reduction of the ketone carbonyl group of the cycloadducts, which possess a basic structure of bicyclic 6-(menthyloxy)hexahydropyrano[4,3-c]pyrrol-7(6H)one, afforded a number of pyrrolidine-based bicyclic systems. A sequence of reactions, which involved hydrolysis of the menthyloxy substituent, reduction, N-protection, and degradative oxidation, afforded varied pyrrolidine structures having diverse configurations and patterns of substitution; in particular, polyhydroxylated derivatives have been obtained. The unprotected products were isolated as pyrrolidinium trifluoroacetates. Because of the furanose-like nature of the target trihydroxyalkyl pyrrolidines, these molecules have been evaluated as inhibitors of the β-galactofuranosidase from Penicillium fellutanum. The compounds showed practically no inhibitory activity for concentration of pyrrolidines in the range of 0.1-1.6 mM.

The synthesis of heterosaccharides related to the fucoidan from Chordaria flagelliformis bearing an α-L-fucofuranosyl unit

Sulfated polysaccharides, fucoidans, from brown algae are built up mainly of α-L-fucopyranosyl units and form a group of natural biopolymers with a wide spectrum of biological activities. Systematic synthesis of oligosaccharides representing fucoidans' fragments gives molecular probes for detecting pharmacophores within fucoidan polysaccharide chains. Recently, it was discovered that the fucoidan from brown seaweed Chordaria flagelliformis contains not only α-L-fucopyranosyl units but also α-L-fucofuranosyl ones. To establish the influence of the unusual α-L-fucofuranose residue on the biological activity and conformational properties of fucoidans, the synthesis of selectively O-sulfated pentasaccharides, which represent the main repeating unit of the fucoidan from C. flagelliformis, was performed. The features of the synthesis were the use of the pyranoside-into-furanoside rearrangement to prepare the fucofuranoside precursor and remote stereocontrolling participation of O-acyl groups to manage stereoselective α-bond formation in glycosylation reactions.

An improved preparation of some aryl α-l-arabinofuranosides for use as chromogenic substrates for α-l-arabinofuranosidases

A short, robust, and expedient synthesis of various aryl α-l-arabinofuranosides using a trichloroacetimidate precursor is described. The procedure is compatible with a range of phenols with varying pKa values and may be amenable for preparing a wide range of other glycosides.

Convergent synthesis of isomeric heterosaccharides related to the fragments of galactomannan from Aspergillus fumigatus

Aspergillus fumigatus is a very common fungus with high pathogenic potential for immunosuppressed hospital patients. A. fumigatus galactomannan, being the part of its cell wall, is considered as a promising candidate for vaccine and diagnostic test-systems. In this article we report the convergent synthesis of pentasaccharide fragments of the galactomannan containing the β-(1→5)-linked galactofuranoside chain attached to O-3 or O-6 of a spacer-armed mannopyranoside residue. The synthesis of selectively protected galactofuranoside precursors has been performed using recently developed pyranoside-into-furanoside (PIF) rearrangement. For assembling the target galactomannan structures the [1 + 2 + 2]-scheme was applied. This strategy was shown to be highly efficient and can easily be extended to the synthesis of longer fragments of thegalactomannan.

Effect of conformational rigidity on the stereoselectivity of nucleophilic additions to five-membered ring bicyclic oxocarbenium ion intermediates

Nucleophilic substitution reactions of five-membered ring acetals bearing fused rings reveal that subtle changes in the structure of the fused ring can exert dramatic influences on selectivity. If the fused ring did not constrain the five-membered ring undergoing substitution, selectivity was comparable to what was observed for an unconstrained system (≥92% diastereoselectivity, favoring the product of inside attack on the oxocarbenium ion). If the ring were more constrained by including at least one oxygen atom in the ring, selectivity dropped considerably (to 60% diastereoselectivity in one case). Transition states of the nucleophilic addition of allyltrimethylsilane to selected oxocarbenium ions were calculated using DFT methods. These computational models reproduced the correlation between additional conformational rigidity and selectivity.

Synthesis of galactofuranosyl-(1 → 5)-thiodisaccharide glycomimetics as inhibitors of a β-d-galactofuranosidase

Description of the synthesis, molecular modeling and inhibitory properties of furanosyl thiodisaccharides that are mimetics of the motif β-d-Galf-(1 → 5)-d-Galf, found in glycoconjugates of pathogenic microorganisms.

Pyranoside-into-furanoside rearrangement: new reaction in carbohydrate chemistry and its application in oligosaccharide synthesis

Great interest in natural furanoside-containing compounds has challenged the development of preparative methods for their synthesis. Herein a novel reaction in carbohydrate chemistry, namely a pyranoside-into-furanoside (PIF) rearrangement permitting the transformation of selectively O-substituted pyranosides into the corresponding furanosides is reported. The discovered process includes acid-promoted sulfation accompanied by rearrangement of the pyranoside ring into a furanoside ring followed by solvolytic O-desulfation. This process, which has no analogy in organic chemistry, was shown to be a very useful tool for the synthesis of furanoside-containing complex oligosaccharides, which was demonstrated by synthesizing disaccharide derivatives α-D-Galp-(1→3)-β-D-Galf-OPr, 3-O-s-lactyl-β-D-Galf-(1→3)-β-D-Glcp-OPr, and α-L-Fucf-(1→4)-β-D-GlcpA-OPr related to polysaccharides from the bacteria Klebsiella pneumoniae and Enterococcus faecalis and the brown seaweed Chordaria flagelliformis.

Gold(III)-catalyzed glycosidations for 1,2-trans and 1,2-cis furanosides

Stereoselective synthesis of furanosides is still a daunting task, unlike the pyranosides, for which several methods exist. Herein, a unified stereoselective strategy for the synthesis of 1,2-trans and 1,2-cis furanosides is revealed for seven out of eight possible isomers of pentoses. The identified protocol gives access to diastereoselective synthesis of α- and β-araf, ribf, lyxf, and α-xylf furanosides. 1,2-trans glycosides were synthesized by the use of propargyl 1,2-orthoesters under gold-catalyzed glycosidation conditions, and subsequently, they are converted into 1,2-cis glycosides through oxidation-reduction as the key functional group transformation. All the reactions are found to be fully diastereoselective, mild, and high yielding.

Expedient synthesis of 1,6-anhydro-α-D-galactofuranose, a useful intermediate for glycobiological tools

A new and efficient three-step procedure for the synthesis of 1,6-anhydro-α-D-galactofuranose is described. The key step involves the formation of the galactofuranosyl iodide by treatment of per-O-TBS-D-Galf with TMSI, the selective 6-O-desilylation by an excess of TMSI, and the simultaneous nucleophilic attack of the 6-hydroxy group on the anomeric carbon, with the iodide as a good leaving group. This compound is a good precursor for building blocks for the construction of 1→6 linkages.

The use of O-trifluoroacetyl protection and profound influence of the nature of glycosyl acceptor in benzyl-free arabinofuranosylation

The influence of O-trifluoroacetyl (TFA) groups at different positions of thioglycoside glycosyl donors on stereoselectivity of α-arabinofuranosylation leading to corresponding disaccharides was studied. It was shown that TFA group in thioglycoside glycosyl donors, when combined with 2-O-(triisopropylsilyl) (TIPS) non-participating group, may be regarded as an electron-withdrawing protecting group that may enhance 1,2-cis-selectivity in arabinofuranosylation, the results strongly depending on the nature of glycosyl acceptor. The reactivities of the glycosyl donors were compared with those of a similar thioglycoside with O-pentafluoropropionyl groups and the known phenyl 3,5-O-(di-tert-butylsilylene)-1-thio-α-d-arabinofuranosides with 2-O-TIPS and 2-O-benzyl groups. The 'matching' in the donor-acceptor combination was found to be critical for achieving both high reactivity of glycosyl donor and β-stereoselectivity of arabinofuranosylation. The use of glycosyl donors with TFA and silyl protection may be useful in the realization of the benzyl-free approach to oligoarabinofuranosides with azido group in aglycon-convenient building blocks for the preparation of neoglycoconjugates.

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.

Preparation of p-nitrophenyl β-l-arabinofuranoside as a substrate of β-l-arabinofuranosidase

Synthesis of p-nitrophenyl β-l-arabinofuranoside 1 as the substrate for novel β-l-arabinofuranosidase has been achieved by using both our inter- and intra-molecular glycosylation methodologies. Although the intermolecular glycosylation with l-Araf donors 3 and 4 resulted in a mixture of both α- and β-isomers, NAP ether-mediated IAD with 3 and 6 afforded the desired β-l-arabinofuranoside stereospecifically which was confirmed by NMR analysis on the (3)JH1-H2 coupling constant and (13)C chemical shift of C1. As expected, 1 has been revealed to be an efficient substrate in the biological study of a novel β-arabinofuranosidase such as HypBA1 with higher apparent affinity compared with other reported substrates.

β-Arabinofuranosylation using 5-O-(2-quinolinecarbonyl) substituted ethyl thioglycoside donors

A new β-stereoselective D- and L-arabinofuranosylation method has been developed employing 5-O-(2-quinolinecarbonyl) substituted arabinosyl ethyl thioglycosides as glycosyl donors. The approach allows a wide range of acceptor substrates to be used; the β-selectivity is good-to-excellent. Stereoselective synthesis of a mannose-capped octasaccharide portion from a mycobacterial cell wall polysaccharide was then carried out to demonstrate the utility of this methodology.

Tuning effect of silyl protecting groups on the glycosylation reactivity of arabinofuranosyl thioglycosides

The tuning effect of silyl protecting groups on the glycosylation reactivity of arabinofuranosyl phenyl thioglycoside donors is presented. Silyl ethers on the 3-, 5-, and 3,5-positions of the arabinofuranose ring are found to have an arming effect on the donor reactivity, whereas the cyclic 3,5-acetal type protecting groups reduce the reactivity.

Facile synthesis of β- and α-arabinofuranosides and application to cell wall motifs of M. tuberculosis

Propargyl 1,2-orthoesters of arabinose are exploited for the synthesis of 1,2-trans furanosides; easily accessible 1,2-trans ribofuranosides are converted to challenging 1,2-cis-arabinofuranosides by oxidoreduction. Utility of these protocols was demonstrated by the successful synthesis of major structural motifs present in the cell surface of Mycobacterium tuberculosis. Key furanosylations were carried out under gold-catalyzed glycosidation conditions.

Synthetic tools for the characterization of galactofuranosyl transferases: glycosylations via acylated glycosyl iodides

With the aim of developing synthetic tools for the characterization of galactofuranosyltransferases, the synthesis of 9-decenyl glycosides of D-Manp, D-Galf, and β-D-Galf-(1→3)-D-Manp was targeted. The interest in the alkenyl aglycone arises via potential conjugation reactions, once the terminal double bond has been conveniently functionalized. The glycosylation of β-D-Galf-(1→3)-D-Manp was attempted by two different approaches: the trichloroacetimidate method and the glycosylation via the glycosyl iodide. The conditions for the latter were established on the basis of glycosylation assays of per-O-acetylmannose. On the other hand, the study of glycosylation reactions via per-O-benzoylated galactofuranosyl iodide confirms the versatility of glycosyl iodides as donors.

Design of chemical glycosyl donors: does changing ring conformation influence selectivity/reactivity?

This tutorial review focuses on the design of glycosyl donors, especially on attempts to control selectivity/reactivity by employing bulky substituents, cyclic protecting groups, or bridged structures. These structural modifications are performed to change the conformational distributions of pyranoside/furanoside rings. We also briefly discuss this issue with regard to studies on furanosides and enzymatic glycosylation reactions. Readers will find that some of the designed glycosyl donors have been used to achieve total syntheses of natural products.

Synthesis of the O-linked hexasaccharide containing β-D-Galf-(1→2)-β-D-Galf in Trypanosoma cruzi mucins

The hexasaccharide β-D-Galp-(1→2)-[β-D-Galp-(1→3)]-β-D-Galp-(1→6)-[β-D-Galf(1→2)-β-D-Galf(1→4)]-D-GlcNAc (1) is the largest carbohydrate structure released as alditol by reductive β-elimination from mucins of some strains of T. cruzi. The terminal β-D-Galp units are sites of sialylation by trans-sialidase which transfers sialic acid from the host to the parasite. Hexasaccharide 1 was synthesized by a [3 + 3]-convergent strategy based on a nitrile assisted glycosylation, using the trichloroacetimidate method. The β-D-Galf-(1→2)-β-D-Galf-D-GlcNAc synthon was sequentially constructed from the reducing end to the non-reducing end employing benzyl α-D-galactofuranoside as starting material for the internal Galf unit. The choice of this novel precursor, obtained in one-reaction step from galactose, allowed the introduction of an orthogonal and participating levulinoyl group at O-2. Thus, the diastereoselective construction of the Galf-β(1→4)-GlcNAc linkage by the trichloroacetimidate method of glycosylation was achieved. The (1)H NMR spectrum of alditol 2 was identical to the product released by β-elimination from the parasite mucin.

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.