: A mild reagent for the regioselective reductive ring opening of benzylidene acetals in carbohydrates

Sensitive and specific enzyme immunoassays for antigenic trisaccharide from Bacillus anthracis spores

A straightforward synthesis of an anthrose-containing trisaccharide derived from Bacillus anthracis was achieved. Antibodies raised against this hapten provide a highly sensitive enzyme immunoassay with a detection limit of 8.5 pmol mL(-1). By investigating the specificity of the antibodies obtained using different mono-, di- and trisaccharide synthetic analogues, we demonstrated that the epitope was mainly made up of the methyl group at C-5, the butamido group at C-4 and the hydroxyl at C-3 of the anthrose unit, the other parts of the trisaccharide appearing little involved in the recognition.

Synthesis of monodeoxy and mono-O-methyl congeners of methyl beta-D-mannopyranosyl-(1-->2)-beta-D-mannopyranoside for epitope mapping of anti-Candida albicans antibodies

A panel of six complementary monodeoxy and mono-O-methyl congeners of methyl beta-d-mannopyranosyl-(1-->2)-beta-d-mannopyranoside (1) were synthesized by stereoselective glycosylation of monodeoxy and mono-O-methyl monosaccharide acceptors with a 2-O-acetyl-glucosyl trichloroacetimidate donor, followed by a two-step oxidation-reduction sequence at C-2'. The beta-manno configurations of the final deprotected congeners 2-7 were confirmed by measurement of (1)J(C1,H1) heteronuclear and (3)J(1',2') homonuclear coupling constants. These disaccharide derivatives will be used to map the protective epitope recognized by a protective anti-Candida albicans monoclonal antibody C3.1 (IgG3) and to determine its key polar contacts with the binding site.

Synthesis and absolute structures of Mycoplasma pneumoniae beta-glyceroglycolipid antigens

Just recently, a pair of beta-glycolipids was isolated from the cell membrane of Mycoplasma pneumoniae as a mixture of the two compounds. They are the major immunodeterminants of this pathogenic Mycoplasma and indicate high medicinal potential. They have a beta-(1-->6)-linked disaccharide structure close to each other; one has beta-d-galactopyranoside (beta-Gal-type 1) at the non-reducing terminal, and another has beta-d-glucopyranoside (beta-Glc-type 2). In the present study, the first stereoselective synthesis was conducted for each of the two beta-glycolipids 1 and 2. (1)H NMR and TLC-immunostaining studies of the synthetic compounds enable us to establish the absolute structures having the beta-(1-->6)-linked disaccharides at the glycerol sn-3 position.

Probing the substrate specificity of Golgi alpha-mannosidase II by use of synthetic oligosaccharides and a catalytic nucleophile mutant

Inhibition of Golgi alpha-mannosidase II (GMII), which acts late in the N-glycan processing pathway, provides a route to blocking cancer-induced changes in cell surface oligosaccharide structures. To probe the substrate requirements of GMII, oligosaccharides were synthesized that contained an alpha(1,3)- or alpha(1,6)-linked 1-thiomannoside. Surprisingly, these oligosaccharides were not observed in X-ray crystal structures of native Drosophila GMII (dGMII). However, a mutant enzyme in which the catalytic nucleophilic aspartate was changed to alanine (D204A) allowed visualization of soaked oligosaccharides and led to the identification of the binding site for the alpha(1,3)-linked mannoside of the natural substrate. These studies also indicate that the conformational change of the bound mannoside to a high-energy B 2,5 conformation is facilitated by steric hindrance from, and the formation of strong hydrogen bonds to, Asp204. The observation that 1-thio-linked mannosides are not well tolerated by the catalytic site of dGMII led to the synthesis of a pentasaccharide containing the alpha(1,6)-linked Man of the natural substrate and the beta(1,2)-linked GlcNAc moiety proposed to be accommodated by the extended binding site of the enzyme. A cocrystal structure of this compound with the D204A enzyme revealed the molecular interactions with the beta(1,2)-linked GlcNAc. The structure is consistent with the approximately 80-fold preference of dGMII for the cleavage of substrates containing a nonreducing beta(1,2)-linked GlcNAc. By contrast, the lysosomal mannosidase lacks an equivalent GlcNAc binding site and kinetic analysis indicates oligomannoside substrates without non-reducing-terminal GlcNAc modifications are preferred, suggesting that selective inhibitors for GMII could exploit the additional binding specificity of the GlcNAc binding site.

Innate immune responses of synthetic lipid A derivatives of Neisseria meningitidis

Differences in the pattern and chemical nature of fatty acids of lipid A of Neisseria meningitides lipooligosaccharides (LOS) and Escherichia coli lipopolysaccharides (LPS) may account for differences in inflammatory properties. Furthermore, there are indications that dimeric 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) moieties of LOS and LPS enhance biological activities. Heterogeneity in the structure of lipid A and possible contaminations with other inflammatory components have made it difficult to confirm these observations. To address these problems, a highly convergent approach for the synthesis of a lipid A derivative containing KDO has been developed, which relies on the ability to selectively remove or unmask in a sequential manner an isopropylidene acetal, 9-fluorenylmethoxycarbonyl (Fmoc), allyloxycarbonate (Alloc), azide, and thexyldimethylsilyl (TDS) ether. The strategy was employed for the synthesis of N. meningitidis lipid A containing KDO (3). Mouse macrophages were exposed to the synthetic compound and its parent LOS, E. coli lipid A (2), and a hybrid derivative (4) that has the asymmetrical acylation pattern of E. coli lipid A, but the shorter lipids of meningococcal lipid A. The resulting supernatants were examined for tumor necrosis factor alpha (TNF-alpha) and interferon beta (IFN-beta) production. The lipid A derivative containing KDO was much more active than lipid A alone and just slightly less active than its parent LOS, indicating that one KDO moiety is sufficient for full activity of TNF-alpha and IFN-beta induction. The lipid A of N. meningitidis was a significantly more potent inducer of TNF-alpha and IFN-beta than E. coli lipid A, which is due to a number of shorter fatty acids. The compounds did not demonstrate a bias towards a MyD88- or TRIF-dependent response.

A facile method for oxidation of primary alcohols to carboxylic acids and its application in glycosaminoglycan syntheses

A convenient two-step, one-pot procedure was developed for the conversion of primary alcohols to carboxylic acids. The alcohol was first treated with NaOCl and TEMPO under phase-transfer conditions, followed by NaClO2 oxidation in one pot. This reaction is applicable to a wide range of alcohols and the mild reaction conditions are compatible with many sensitive functional groups, including electron-rich aromatic rings, acid-labile isopropylidene ketal and glycosidic linkages, and oxidation-prone thioacetal, p-methoxybenzyl, and allyl moieties. Several glycosaminoglycans such as heparin, chondroitin, and hyaluronic acid oligosaccharides have been synthesized in high yields by using this new oxidation protocol.

Facile and regioselective preparation of partly O-benzylated d-glucopyranose acetates via acid-mediated simultaneous debenzylation–acetolysis

Fully O-benzylated methyl alpha-D-glucopyranoside shows a steady order in stepwise debenzylation when it is treated with sulfuric acid in acetic anhydride. Based on the order of debenzylation, regioselective preparations of 2,3,4-tri-, 2,3-, 2,4-, 3,4-di-, and 2-O-benzyl-D-glucopyranose acetates were facilitated in greater than 80% yields. The key points of the preparative reactions were the control of the acid strength and choice of suitable substrates.

Stereocontrolled synthesis of the D- and L-glycero-beta-D-manno-heptopyranosides and their 6-deoxy analogues. Synthesis of methyl alpha-l-rhamno-pyranosyl-(1-->3)-D-glycero-beta-D-manno-heptopyranosyl- (1-->3)-6-deoxy-glycero-beta-D-manno-heptopyranosyl-(1-->4)-alpha-L- rhamno-pyranoside, a tetrasaccharide subunit of the lipopolysaccharide from Plesimonas shigelloides

The synthesis of d- and l-glycero-alpha-manno-thioheptopyranosides, protected with 4,6-O-alkylidene-type acetals is described. In glycosylations carried out with preactivation with the 1-benzenesulfinylpiperidine/trifluoromethanesulfonic anhydride couple, both the D- and L-glycero series exhibit excellent beta-selectivity with a range of glycosyl acceptors. In contrast, a 4,7-O-alkylidene acetal was found not to afford beta-selectivity. With a 4,6-O-[1-cyano-2-(2-iodophenyl)ethylidene] acetal protected thioglycoside, excellent beta-selectivity was obtained in glycosylation reactions, and subsequent treatment with tributyltin hydride and azoisobutyronitrile brought about clean fragmentation to the 6-deoxy-glycero-beta-D-manno-heptopyranosides. This chemistry was applied to the stereocontrolled synthesis of methyl alpha-L-rhamno-pyranosyl-(1-->3)-D-glycero-beta-D-manno-heptopyranosyl-(1-->3)-6-deoxy-glycero-beta-D-manno-heptopyranosyl-(1-->4)-alpha-L-rhamno-pyranoside, a component of the lipopolysaccharide from Plesimonas shigelloides.

Corrected order in the simultaneous debenzylation–acetolysis of methyl 2,3,4,6-tetra-O-benzyl-α-d-glucopyranoside

The benzyl groups of methyl 2,3,4,6-tetra-O-benzyl-alpha-d-glucopyranoside were cleaved in the order of 6-O-Bn>3-O-Bn>4-O-Bn>2-O-Bn under acid-mediated conditions in acetic anhydride. The order is a correction of that previously reported.

The 2-aminogluconate isomer of rhizobium sin-1 lipid A can antagonize TNF-alpha production induced by enteric LPS

The naturally occurring lipopolysaccharide (LPS) from Rhizobium sin-1, a nitrogen-fixing bacterial species, can prevent the induction of the tumor necrosis factor TNF-alpha induced by enteric LPS. The proximal saccharide moiety of R. sin-1 lipid A can exist in two forms, namely as a 2-aminogluconolactone or 2-aminogluconate. As it is unknown which of these forms is responsible for the antagonistic properties of R. sin-1 lipid A, compound 4 was prepared, and its inflammatory properties were studied. This compound contains a methyl ether at the C-5 hydroxyl, which prevents lactonization and therefore is ideally suited to determine whether the 2-aminogluconate possesses antagonistic properties. Compound 4 was synthesized by a highly convergent approach with a key disaccharide building block functionalized with a set of orthogonal protecting groups. The novel synthetic compound lacks proinflammatory properties, as indicated by an absence of TNF-alpha protein production. This compound was, however, able to antagonize the production of TNF-alpha induced by enteric LPS; this indicates that the 2-aminogluconate form of R. sin-1 lipid A is responsible for its biological properties.

Regioselective Reductive Openings of Acetals; Mechanistic Details and Synthesis of Fluorescently Labeled Compounds

Regioselective reductive openings of mixed phenolic-benzylic acetals, using BH3.NMe3-AlCl3, was investigated, and a mechanism where the outcome is directed by the electrostatic potential of the two oxygen atoms is presented. The regioselective acetal opening was used in the synthesis of a fluorescently labeled analogue to antiproliferative xylosides. The fluorescently labeled xyloside was tested for uptake, antiproliferative activity, and glycosaminoglycan priming in different cell lines. The xyloside was taken up by all cell lines but did not initiate glycosaminoglycan biosynthesis.

Synthesis of the tetrasaccharide repeat unit of the O-antigen polysaccharide from Escherichia coli O77 employing the 2′-carboxybenzyl glycoside

The synthesis of the suitably protected form (1) of the tetrasaccharide repeat unit, →2)-α-D-Manp-(1→2)-β-D-Manp-(1→3)-α-D-GlcpNAc-(1→6)-α-D-Manp-(1→ (A), of the O-antigen polysaccharide of the lipopolysaccharide from Escherichia coli O77 has been accomplished by latent–active glycosylation employing the 2′-carboxybenzyl (CB) gly coside method. In addition to previously used latent glycosyl donors, 2′-(benzyloxycarbonyl)benzyl (BCB) glycosides, new latent glycosyl donors, 2'-(allyloxycarbonyl)benzyl (ACB) glycosides, have been introduced as a direct precursor for the active CB glycosides. We also demonstrate that 4,6-O-benzylidene-2-azido-2-deoxy-α-D-mannopyranoside (7) has been readily prepared from D-glucosamine in good yield.Key words: Escherichia coli O77, glycosylation, 2′-carboxybenzyl (CB) glycosides, 2′-(allyloxycarbonyl)benzyl (ACB) glycosides, glycosyl donor.

Solid-Phase Oligosaccharide Synthesis of a Small Library of N-Glycans

Solid-phase oligosaccharide synthesis is based on a hydroxymethylbenzyl benzoate spacer linker which is connected to the Merrifield resin (1 P). Glycosylation was performed with O-glycosyl trichloroacetimidates of glucosamine, mannose, and galactose permitting chain extension (2e, 5e), branching (4b, 7b, 8b), and chain termination (3t, 6t, 9t) with the use of O-benzyl, O-benzoyl, and N-dimethylmaleoyl as permanent and O-fluorenylmethoxycarbonyl (Fmoc) and O-phenoxyacetyl (PA) as temporary protecting groups. The steps required on solid phase are i) glycosylation under TMSOTf catalysis, ii) selective cleavage of the temporary protecting groups, Fmoc with NEt3 and PA with 0.5 equivalents of NaOMe in CH2Cl2/MeOH, and iii) product cleavage from the resin with 4.0 equivalents of NaOMe in CH2Cl2/MeOH and following O-acetylation for convenient product isolation. Thus a highly successful synthesis of a small library of seventeen N-glycan structures was made possible comprising the N-glycan pentasaccharide core structure 53 and two further chain extended hexa- and heptasaccharide N-glycans with a glucosamine or a lactosamine residue, respectively, which is attached to one of the mannose residues of the core structure (56 and 59).

Synthesis and stereoselective glycosylation of D- and L-glycero-beta-D-manno-heptopyranoses

[reaction: see text] A method for the direct stereocontrolled synthesis of D- and L-glycero-beta-D-manno-heptopyranosides such as those found in the repeating unit of the O-specific polysaccharide from the CNCTC 113/92 LPS (serotype 54) is described. The method relies on the presence of a 4,6-O-benzylidene acetal to effect stereocontrol at the anomeric center; the configuration at C6 (L- or D-glycero) is of minimal importance.

Stripping off Water at Ambient Temperature: Direct Atom-Efficient Acetal Formation between Aldehydes and Diols Catalyzed by Water-Tolerant and Recoverable Vanadyl Triflate

[reaction: see text]. Aromatic aldehydes can be readily protected as acetals with 1,2- and 1,3-diols by using vanadyl triflate as a catalyst in CH(3)CN at ambient temperature. Carbohydrate-based 1,2- and 1,3-diols can similarly be protected in good to excellent yields. The catalyst can be readily recovered from the aqueous layer. In combination with vanadyl triflate-catalyzed sequential regioselective, reductive acetal opening and chemoselective acylations, the title method allows for differential functionalization of all four hydroxyl units in a given glucopyranoside.

Design and synthesis of inositolphosphoglycan putative insulin mediators

The binding modes of a series of molecules, containing the glucosamine (1-->6) myo-inositol structural motif, into the ATP binding site of the catalytic subunit of cAMP-dependent protein kinase (PKA) have been analysed using molecular docking. These calculations predict that the presence of a phosphate group at the non-reducing end in pseudodisaccharide and pseudotrisaccharide structures properly orientate the molecule into the binding site and that pseudotrisaccharide structures present the best shape complementarity. Therefore, pseudodisaccharides and pseudotrisaccharides have been synthesised from common intermediates using effective synthetic strategies. On the basis of this synthetic chemistry, the feasibility of constructing small pseudotrisaccharide libraries on solid-phase using the same intermediates has been explored. The results from the biological evaluation of these molecules provide additional support to an insulin-mediated signalling system which involves the intermediacy of inositolphosphoglycans as putative insulin mediators.

Temperature-controlled regioselectivity in the reductive cleavage of p-methoxybenzylidene acetals

The regioselective ring opening of pyranosidic 4,6-p-methoxybenzylidene acetals with BH(3)/Bu(2)BOTf in THF can be tuned by adjusting the reaction temperature and reagent concentrations. Reductive cleavage at 0 degrees C resulted in the exclusive formation of 4-O-p-methoxybenzyl (PMB) ethers, whereas reaction at -78 degrees C produced 6-O-PMB ethers in high yields. The latter condition was observed to be compatible with a variety of acid-sensitive functional groups, including allyl and enol ethers. The presence of water does not interfere with reductive ring opening and may contribute toward in situ generation of H(+) as a catalyst for 6-O-PMB ether formation. Reductive cleavage under rigorously aprotic conditions is greatly decelerated, and yields only the 4-O-PMB ether. The temperature-dependent reductive cleavage of the 4,6-acetal can be described in terms of kinetic versus thermodynamic control: Lewis-acid coordination of the more accessible O-6 is favored at higher temperatures, whereas protonation of the more basic but sterically encumbered O-4 predominates at low temperatures.

Novel ether derivatives of mannopeptimycin glycopeptide antibiotic

Novel ether derivatives of mannopeptimycin glycopeptide were synthesized to probe their SAR. Many of these derivatives exhibited potent antibacterial activity against methicillin resistant and vancomycin resistant strains. These ether derivatives were prepared via reductive ring cleavage of acetals to give a mixture of 6-O, 4-O, 3-O, and 2-O-ether isomers. Both 6-O-ether and 4-O-ether showed significantly enhanced antibacterial activity over the parent and the isovalerate esters.

Total synthesis of desferrisalmycin B

The first total synthesis of a naturally occurring siderophore antibiotic, desferrisalmycin B, is described, and the configuration of the unknown stereocenter is assigned. The synthesis features a synthetic strategy of constructing the novel amino-heptopyranoside component by stereoselective dihydroxylation followed by a Bose-modified Mitsunobu reaction. Through this convergent approach, other members of salmycins should also be synthetically accessible.

Stereoselective (2-naphthyl)methylation of sugar hydroxyls by the hydrogenolysis of diastereoisomeric dioxolane-type (2-naphthyl)methylene acetals

The cis axial/equatorial OH groups of methyl alpha-L- and ethyl 1-thio-alpha-L-rhamnopyranoside, 1,6-anhydro-beta-D-mannopyranose, and 1,6-anhydro-beta-D-galactopyranose were reacted with 2-naphthaldehyde dimethyl acetal to diastereomeric dioxolane-type 2,3-O-(2-naphthyl)methylene or 3,4-O-(2-naphthyl)methylene acetals. The glycosides yielded the exo- and endo-isomers in nearly 1:1 ratio, 1,6-anhydro-beta-D-mannopyranose gave predominantly the endo-, and 1,6-anhydro-beta-D-galactopyranose exclusively endo-isomer. The acetals and some of their fully protected derivatives bearing benzyl or tert-butyldimethylsilyl groups were hydrogenolised with AlH(3) (3LiAlH(4)-AlCl(3)) or with Me(3)N.BH(3)-AlCl(3) reagents. The endo-isomers were cleaved by both reagents to give axial NAP ethers, the exo-isomers of pyranosides furnished equatorial NAP ethers. However, the exo-isomers of pyranoses gave irregular axial ethers with a > 30-fold enhancement of the reaction rates with respect to the endo-isomer.

[Synthesis of biomimetic analogs of neomycin B]

Neomycin B has been found to block the binding of HIV-1 Rev protein to its viral RNA recognition site, thereby inhibiting the production of the virus. This paper describes the synthesis of analogues of neomycin B, which are potential anti-HIV compounds designed as inhibitors of Rev/RRE binding.