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

Acid-Catalyzed Transformation of Pyranosides into Furanosides as a Tool for Preparation of Furanoside Synthetic Blocks

The importance of natural glycoconjugates containing furanoside residues causes a continued demand for the development of efficient methods for the synthesis of corresponding oligosaccharide derivatives to be used as molecular probes in glycobiological studies. Currently, the chemical synthesis of furanose-containing oligosaccharides often represents a significant challenge because of the lack of short, efficient, and reliable methods for the preparation of selectively substituted furanoside blocks. Herein, we report an easy protocol toward galactofuranose-containing molecules based on the unusual equilibrium between pyranoside and furanoside forms observed for a series of substituted galactosides. The method's utility is illustrated by the syntheses of furanoside-containing oligosaccharides related to the antigenic polysaccharides of Aspergillus fumigatus and Klebsiella pneumoniae O2ac.

Synthesis of methylphosphorylated oligomannosides structurally related to lipopolysaccharide O-antigens of Klebsiella pneumoniae serotype O3 and their application for detection of specific antibodies in rabbit and human sera

Methylphosphorylated mono-, di- and trimannosides structurally related to the lipopolysaccharide (LPS) O-antigens of Klebsiella pneumoniae of serotype O3 were synthesized and conjugated with a biotin tag. The stereo- and regioselective assembly of target carbohydrate chains was conducted using uniform monosaccharide synthetic blocks. After that, a methylphosphate group was introduced by coupling with a methyl-H-phosphonate reagent followed by oxidation and deprotection to give the target oligosaccharides. The 1H and 13C NMR spectra of the obtained compounds showed a good fit with the spectrum of the corresponding natural polysaccharide. The newly prepared biotinylated oligosaccharides along with the previously reported biotinylated glycoconjugates related to galactan I and galactan II of K. pneumoniae LPS were used for the ELISA detection of antibodies in anti-K. pneumoniae rabbit sera. Anti-O3 serum antibodies specifically recognized the synthesized oligosaccharide ligands with terminal methylphosphomannosyl residues, whereas anti-O1 serum antibodies recognized the oligosaccharide related to K. pneumoniae galactan II. The analysis of human sera from patients with confirmed Klebsiella infection also revealed the presence of antibodies against the synthesized oligosaccharides in clinical cases. Thus, the described compounds together with other Klebsiella related antigenic oligosaccharides could be potentially used as molecular probes for K. pneumoniae serological diagnostics development and strain serotyping.

Progress and challenges in the synthesis of sequence controlled polysaccharides

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure-properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

Boronic Acid-Catalyzed Regioselective Koenigs-Knorr-Type Glycosylation

Boronic acid-catalyzed regioselective Koenigs-Knorr-type glycosylation is presented. The reaction of an unprotected or partially protected glycosyl acceptor with a glycosyl halide donor in the presence of silver oxide and a low catalytic amount of imidazole-containing boronic acid was found to proceed smoothly, which enables construction of a 1,2-trans glycosidic linkage with high regioselectivities. This is the first example of the use of a boronic acid catalyst to initiate regioselective glycosylation via the activation of cis-vicinal diols in glycosyl acceptors.

Synthetic Analogs of Streptococcus pneumoniae Capsular Polysaccharides and Immunogenic Activities of Glycoconjugates

Streptococcus pneumoniae is a Gram-positive bacterium (pneumococcus) that causes severe diseases in adults and children. It was established that some capsular polysaccharides of the clinically significant serotypes of S. pneumoniae in the composition of commercial pneumococcal polysaccharide or conjugate vaccines exhibit low immunogenicity. The review considers production methods and structural features of the synthetic oligosaccharides from the problematic pneumococcal serotypes that are characterized with low immunogenicity due to destruction or detrimental modification occurring in the process of their preparation and purification. Bacterial serotypes that cause severe pneumococcal diseases as well as serotypes not included in the composition of the pneumococcal conjugate vaccines are also discussed. It is demonstrated that the synthetic oligosaccharides corresponding to protective glycotopes of the capsular polysaccharides of various pneumococcal serotypes are capable of inducing formation of the protective opsonizing antibodies and immunological memory. Optimal constructs of oligosaccharides from the epidemiologically significant pneumococcal serotypes are presented that can be used for designing synthetic pneumococcal vaccines, as well as test systems for diagnosis of S. pneumoniae infections and monitoring of vaccination efficiency .

Synthetic Oligomers Mimicking Capsular Polysaccharide Diheteroglycan are Potential Vaccine Candidates against Encapsulated Enterococcal Infections

Infections caused by Enterococcus spp. are a major concern in the clinical setting. In Enterococcus faecalis, the capsular polysaccharide diheteroglycan (DHG), composed of ß-d-galactofuranose-(1 → 3)-ß-d-glucopyranose repeats, has been described as an important virulence factor and as a potential vaccine candidate against encapsulated strains. Synthetic structures emulating immunogenic polysaccharides present many advantages over native polysaccharides for vaccine development. In this work, we described the synthesis of a library of DHG oligomers, differing in length and order of the monosaccharide constituents. Using suitably protected thioglycoside building blocks, oligosaccharides up to 8-mer in length built up from either Galf-Glcp or Glcp-Galf dimers were generated, and we evaluated their immunoreactivity with antibodies raised against DHG. After the screening, we selected two octasaccharides, having either a galactofuranose or glucopyranose terminus, which were conjugated to a carrier protein for the production of polyclonal antibodies. The resulting antibodies were specific toward the synthetic structures and mediated in vitro opsonophagocytic killing of different encapsulated E. feacalis strains. The evaluated oligosaccharides are the first synthetic structures described to elicit antibodies that target encapsulated E. faecalis strains and are, therefore, promising candidates for the development of a well-defined enterococcal glycoconjugate vaccine.

Conformational changes in common monosaccharides caused by per-O-sulfation

Polysulfated carbohydrates play an important role in many biological processes because of their ability to bind to various protein receptors such as different growth factors, blood coagulation factors, adhesion lectins etc. Precise information about spatial organization of sulfated derivatives is of high demand for molecular modelling of such interactions as well as for understanding of the mechanism of pyranoside-into-furanoside rearrangement. In this review we summarize the changes recently revealed for the conformations of common pyranosides and furanosides upon total O-sulfation which were studied by means of NMR spectroscopy as well as molecular modelling. It was found that pentoses, being more flexible, undergo complete conformational chair inversion. Meanwhile, for hexoses the situation strongly depends on the monosaccharide configuration. Conformational changes are most pronounced in gluco-compounds though quantum chemical calculations helped to establish that no complete chair inversion occurred. In furanosides distortions of two types were observed: either the ring conformation or the conformation of the side chain changed. The presented data may be used for the analysis of chemical, physical and biological properties of sulfated carbohydrates.

Synthesis of oligosaccharides related to galactomannans from Aspergillus fumigatus and their NMR spectral data

The synthesis of model oligosaccharides related to antigenic galactomannans of the dangerous fungal pathogen Aspergillus fumigatus has been performed employing pyranoside-into-furanoside (PIF) rearrangement and controlled O(5) → O(6) benzoyl migration as key synthetic methods. The prepared compounds along with some previously synthesized oligosaccharides were studied by NMR spectroscopy with the full assignment of ¹H and ¹³C signals and the determination of ¹³C NMR glycosylation effects. The obtained NMR database on ¹³C NMR chemical shifts for oligosaccharides representing galactomannan fragments forms the basis for further structural analysis of galactomannan related polysaccharides by a non-destructive approach based on the calculation of the ¹³C NMR spectra of polysaccharides by additive schemes.

Influence of per-O-sulfation upon the conformational behaviour of common furanosides

The studies on the recently discovered pyranoside-into-furanoside rearrangement have led us to conformational investigations of furanosides upon their total sulfation. Experimental NMR data showed that in some cases drastic changes of the ring conformation occurred while sometimes only the conformation of the exocyclic C4-C5 linkage changed. Herein we describe a combined quantum chemical and NMR conformational investigation of three common monosaccharide furanosides as their propyl glycosides: α-mannose, β-glucose and β-galactose. Full exploration of the furanoside ring by means of ab initio calculations was performed and coupling constants were calculated for each of the low-energy conformers. The results demonstrated preferred trans-orientation of H4-H5 protons in the non-sulfated molecules which changed to gauche-orientation upon sulfation. The effect is less pronounced in the galactosides. For all the studied structures changes in the conformational distribution were revealed by quantum mechanical calculations, that explained the observed changes in intraring coupling constants occurring upon introduction of sulfates.

Reinvestigation of carbohydrate specificity of EB-A2 monoclonal antibody used in the immune detection of Aspergillus fumigatus galactomannan

Great progresses have been made in the recent years in the detection of circulating galactofuranose-bearing molecules for the diagnosis of aspergillosis. However, the test used in the clinical practice is hampered by the occurrence of false positives. A glycoarray with dozens of oligosaccharides structurally related to the Aspergillus fumigatus galactomannan has allowed us to reinvestigate the carbohydrate specificity of the EB-A2 monoclonal antibody used in the Platelia^TM Aspergillus sandwich immune assay. We have now demonstrated that the mAb can recognize shorter oligosaccharides than the previously reported tetrasaccharide Galf-β-(1→5)-Galf-β-(1→5)-Galf-β-(1→5)-Galf-β and oligosaccharides which contains alternating β-(1→5)/β-(1→6)-linkages. This result could explain the occurrence of false-positive signals due to the presence of the abovementioned epitopes not only in A. fumigatus galactomannan but also in other bacteria and fungi.

Driving Force of the Pyranoside-into-Furanoside Rearrangement

Ab initio calculations of fully O-sulfated model monosaccharides, including common hexoses (glucose, galactose, fucose, and mannose) and pentoses (arabinose and xylose), were performed to study the energetic properties of the recently discovered pyranoside-into-furanoside (PIF) rearrangement. It was shown that the per-O-sulfated derivatives of furanoside isomers generally had lower energies than the corresponding per-O-sulfated pyranosides, while nonsulfated furanosides were always less favored than nonsulfated pyranosides. Mannose, which is known to be unreactive in PIF rearrangement, was the only exception. The results of the theoretical calculations were confirmed by experimental studies of monosaccharide models and explained the driving force of such unusual ring contraction process as PIF rearrangement. The conclusions of performed investigation can be used for prediction of new substrates applicability for PIF rearrangement.

A Route to Polyprenol Pyrophosphate-Based Probes of O-Polysaccharide Biosynthesis in Klebsiella pneumoniae O2a

An approach for the assembly of polyprenol pyrophosphate-based probes of O-polysaccharide biosynthesis in Klebsiella pneumoniae serotype O2a is described. This convergent route features high-yielding, diastereoselective glycosylations and the late-stage installation of the polyprenol pyrophosphate moiety. Although applied to the synthesis of a nonasaccharide bearing a farnesyl group (1), the modular nature of the route makes it amenable to the synthesis of additional derivatives containing either larger glycans or different lipid domains.

Synthesis of vicinal dideoxy-difluorinated galactoses

Three novel dideoxydifluorinated galactose derivatives are described.

Lewis acid promoted anomerisation of alkyl O- and S-xylo-, arabino- and fucopyranosides

Pentopyranoside and 6-deoxyhexopyranosides, such as those from d-xylose, l-arabinose and l-fucose are components of natural products, oligosaccharides or polysaccharides. Lewis acid promoted anomerisation of some of their alkyl O- and S-glycopyranosides is reported here. SnCl₄ was more successful than TiCl₄, with the latter giving the glycosyl chloride by-product in some cases, and both were superior to BF₃OEt₂. Kinetics study using ¹H NMR spectroscopy showed an order of reactivity: O-xylopyranoside > O-arabinopyranoside > O-fucopyranoside. Benzoylated glycosides were more reactive than acetylated glycosides. The reactivity of S-glycosides was greater than that of O-glycosides for both arabinose and fucose derivatives; the reactivity of O- and S-xylopyranosides was similar. The highest stereoselectivities were observed for fucopyranosides. The β-d-xylopyranoside and α-l-arabinopyranoside reactants are conformationally more flexible than β-l-fucopyranosides.

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.

Lysozyme's lectin-like characteristics facilitates its immune defense function

Interactions between human lysozyme (HL) and the lipopolysaccharide (LPS) of Klebsiella pneumoniae O1, a causative agent of lung infection, were identified by surface plasmon resonance. To characterize the molecular mechanism of this interaction, HL binding to synthetic disaccharides and tetrasaccharides representing one and two repeating units, respectively, of the O-chain of this LPS were studied. pH-dependent structural rearrangements of HL after interaction with the disaccharide were observed through nuclear magnetic resonance. The crystal structure of the HL-tetrasaccharide complex revealed carbohydrate chain packing into the A, B, C, and D binding sites of HL, which primarily occurred through residue-specific, direct or water-mediated hydrogen bonds and hydrophobic contacts. Overall, these results support a crucial role of the Glu35/Asp53/Trp63/Asp102 residues in HL binding to the tetrasaccharide. These observations suggest an unknown glycan-guided mechanism that underlies recognition of the bacterial cell wall by lysozyme and may complement the HL immune defense function.

Novel mouse monoclonal antibodies specifically recognize Aspergillus fumigatus galactomannan

A panel of specific monoclonal antibodies (mAbs) against synthetic pentasaccharide β-D-Galf-(1→5)-[β-D-Galf-(1→5)]₃-α-D-Manp, structurally related to Aspergillus fumigatus galactomannan, was generated using mice immunized with synthetic pentasaccharide-BSA conjugate and by hybridoma technology. Two selected mAbs, 7B8 and 8G4, could bind with the initial pentasaccharide with affinity constants of approximately 5.3 nM and 6.4 nM, respectively, based on surface plasmon resonance-based biosensor assay. The glycoarray, built from a series of synthetic oligosaccharide derivatives representing different galactomannan fragments, demonstrated that mAb 8G4 could effectively recognize the parental pentasaccharide while mAb 7B8 recognizes its constituting trisaccharide parts. Immunofluorescence studies showed that both 7B8 and 8G4 could stain A. fumigatus cells in culture efficiently, but not the mutant strain lacking galactomannan. In addition, confocal microscopy demonstrated that Candida albicans, Bifidobacterium longum, Lactobacillus plantarum, and numerous gram-positive and gram-negative bacteria were not labeled by mAbs 7B8 and 8G4. The generated mAbs can be considered promising for the development of a new specific enzyme-linked assay for detection of A. fumigatus, which is highly demanded for medical and environmental controls.

Immunobiological Activity of Synthetically Prepared Immunodominant Galactomannosides Structurally Mimicking Aspergillus Galactomannan

The study is oriented at the in vitro evaluation of the immunobiological activity and efficacy of synthetically prepared isomeric pentasaccharides representing fragments of Aspergillus fumigatus cell-wall galactomannan and containing β-(1→5)-linked tetragalactofuranoside chain attached to O-6 (GM-1) or O-3 (GM-2) of a spacer-armed mannopyranoside residue. These compounds were studied as biotinylated conjugates which both demonstrated immunomodulatory activities on the RAW 264.7 cell line murine macrophages as in vitro innate immunity cell model. Immunobiological studies revealed time- and concentration-dependent efficient immunomodulation. The proliferation of RAW 264.7 macrophages was induced at higher concentration (100 µg/mL) of studied glycoconjugates and longer exposure (48 h), with more pronounced efficacy for GM-1. The increase of proliferation followed the previous increase of IL-2 production. The cytokine profile of the macrophages treated with the glycoconjugates was predominantly pro-inflammatory Th1 type with significant increase of TNFα, IL-6, and IL-12 release for both glycoconjugates. The RAW 264.7 macrophages production of free radicals was not significantly affected by glycoconjugates stimulation. The phagocytic activity of RAW 264.7 cells was reduced following GM-1 treatment and was significantly increased after 24 h stimulation with GM-2, contrary to 48 h stimulation. Moreover, the synthetically prepared galactomannoside derivatives have been evaluated as efficient serodiagnostic antigens recognized by specific Ig isotypes, and significant presence of specific IgM antibodies in serum of patients suffering from vulvovaginitis was observed.

Recent advances in the synthesis of fungal antigenic oligosaccharides

The driving force for the constant improvement and development of new synthetic methodologies in carbohydrate chemistry is a growing demand for biologically important oligosaccharide ligands and neoglycoconjugates thereof for numerous biochemical investigations such as cell-to-pathogen interactions, immune response, cell adhesion, etc. Here we report our syntheses of the spacer-armed antigenic oligosaccharides related to three groups of the polysaccharides of the fungal cell-wall including α- and β-mannan, α- and β-glucan and galactomannan chains, which include new rationally designed synthetic blocks, efficient solutions for the stereoselective construction of glycoside bonds, and novel strategy for preparation of furanoside-containing oligosaccharides based on recently discovered pyranoside-into-furanoside (PIF) rearrangement.

Synthetic utility of endocyclic cleavage reaction

Existence of endocyclic cleavage reaction is now clearly shown from experimental evidence of endocyclic cleavage reaction as well as computational chemistry. Not only stereoelectronic factor, several factors could be main factors for endocyclic cleavage reaction. Endocyclic cleavage reaction is useful for 1,2-cis aminoglycoside formation, which is difficult by conventional glycosylation. By using endocyclic cleavage reaction, several glycosides with 1,2-cis aminoglycoside were prepared.

Orthogonal protection of saccharide polyols through solvent-free one-pot sequences based on regioselective silylations

tert-Butyldimethylsilyl (TBDMS) and tert-butyldiphenylsilyl (TBDPS) are alcohol protecting groups widely employed in organic synthesis in view of their compatibility with a wide range of conditions. Their regioselective installation on polyols generally requires lengthy reactions and the use of high boiling solvents. In the first part of this paper we demonstrate that regioselective silylation of sugar polyols can be conducted in short times with the requisite silyl chloride and a very limited excess of pyridine (2-3 equivalents). Under these conditions, that can be regarded as solvent-free conditions in view of the insolubility of the polyol substrates, the reactions are faster than in most examples reported in the literature, and can even be further accelerated with a catalytic amount of tetrabutylammonium bromide (TBAB). The strategy proved also useful for either the selective TBDMS protection of secondary alcohols or the fast per-O-trimethylsilylation of saccharide polyols. In the second part of the paper the scope of the silylation approach was significantly extended with the development of unprecedented "one-pot" and "solvent-free" sequences allowing the regioselective silylation/alkylation (or the reverse sequence) of saccharide polyols in short times. The developed methodologies represent a very useful and experimentally simple tool for the straightforward access to saccharide building-blocks useful in organic synthesis.

The Use of Pyranoside-into-Furanoside Rearrangement and Controlled O(5) → O(6) Benzoyl Migration as the Basis of a Synthetic Strategy To Assemble (1→5)- and (1→6)-Linked Galactofuranosyl Chains

A new pyranoside-into-furanoside (PIF) rearrangement of selectively protected galactopyranosides, followed by controlled O(5) → O(6) benzoate migration, gives either 5-OH or 6-OH products. It has been applied for the synthesis of four oligosaccharides related to the galactomannan from Aspergillus fumigatus. The assembly of target oligosaccharides containing both (1→5) and (1→6) linkages between galactofuranosyl residues was performed by applying terminal mannoside and digalactofuranoside blocks, forming a versatile approach toward fungal and bacterial carbohydrate antigens containing both 5-O- and 6-O-substituted galactofuranoside residues.

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.

Mycothiol synthesis by an anomerization reaction through endocyclic cleavage

Mycothiol is found in Gram-positive bacteria, where it helps in maintaining a reducing intracellular environment and it plays an important role in protecting the cell from toxic chemicals. The inhibition of the mycothiol biosynthesis is considered as a treatment for tuberculosis. Mycothiol contains an α-aminoglycoside, which is difficult to prepare stereoselectively by a conventional glycosylation reaction. In this study, mycothiol was synthesized by an anomerization reaction from an easily prepared β-aminoglycoside through endocyclic cleavage.

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.

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.

(13)C-NMR glycosylation effects in (1→3)-linked furanosyl-pyranosides

Synthesis, theoretical conformational analysis (molecular mechanics and DFT calculations) and NMR spectral data including the (13)C-NMR glycosylation effects for six pairs of isomeric furanosyl-(1→3)-pyranosides with different anomeric and absolute configurations of furanosyl units as well as configurations of C2 and C4 in the pyranoside units are described. The determined (13)C-NMR glycosylation effects were shown to correlate with the pattern of intramolecular interactions around the inter-unit bonds.