Preparative acetonation of pyranoid, vicinal trans-glycols under kinetic control: methyl 2,3:4,6-di-O-isopropylidene-α- and -β-d-glucopyranoside

Investigation on the Synthesis of Shigella flexneri Specific Oligosaccharides Using Disaccharides as Potential Transglucosylase Acceptor Substrates

Chemo-enzymatic strategies hold great potential for the development of stereo- and regioselective syntheses of structurally defined bioactive oligosaccharides. Herein, we illustrate the potential of the appropriate combination of a planned chemo-enzymatic pathway and an engineered biocatalyst for the multistep synthesis of an important decasaccharide for vaccine development. We report the stepwise investigation, which led to an efficient chemical conversion of allyl α-d-glucopyranosyl-(1→4)-α-l-rhamnopyranosyl-(1→3)-2-deoxy-2-trichloroacetamido-β-d-glucopyranoside, the product of site-specific enzymatic α-d-glucosylation of a lightly protected non-natural disaccharide acceptor, into a pentasaccharide building block suitable for chain elongation at both ends. Successful differentiation between hydroxyl groups features the selective acylation of primary alcohols and acetalation of a cis-vicinal diol, followed by a controlled per-O-benzylation step. Moreover, we describe the successful use of the pentasaccharide intermediate in the [5 + 5] synthesis of an aminoethyl aglycon-equipped decasaccharide, corresponding to a dimer of the basic repeating unit from the O-specific polysaccharide of Shigella flexneri 2a, a major cause of bacillary dysentery. Four analogues of the disaccharide acceptor were synthesized and evaluated to reach a larger repertoire of O-glucosylation patterns encountered among S. flexneri type-specific polysaccharides. New insights on the potential and limitations of planned chemo-enzymatic pathways in oligosaccharide synthesis are provided.

Acetalated dextran is a chemically and biologically tunable material for particulate immunotherapy

Materials that combine facile synthesis, simple tuning of degradation rate, processability, and biocompatibility are in high demand for use in biomedical applications. We report on acetalated dextran, a biocompatible material that can be formed into microparticles with degradation rates that are tunable over 2 orders of magnitude depending on the degree and type of acetal modification. Varying the degradation rate produces particles that perform better than poly(lactic-co-glycolic acid) and iron oxide, two commonly studied materials used for particulate immunotherapy, in major histocompatibility complex class I (MHC I) and MHC II presentation assays. Modulating the material properties leads to antigen presentation on MHC I via pathways that are dependent or independent of the transporter associated with antigen processing. To the best of our knowledge, this is the only example of a material that can be tuned to operate on different immunological pathways while maximizing immunological presentation.

Acetal-derivatized dextran: an acid-responsive biodegradable material for therapeutic applications

Dextran, a biocompatible, water-soluble polysaccharide, was modified at its hydroxyls with acetal moieties such that it became insoluble in water but freely soluble in common organic solvents enabling its use in the facile preparation of acid-sensitive microparticles. These particles degrade in a pH-dependent manner: FITC-dextran was released with a half-life at 37 degrees C of 10 h at pH 5.0 compared to a half-life of approximately 15 days at pH 7.4. Both hydrophobic and hydrophilic cargoes were successfully loaded into these particles using single and double emulsion techniques, respectively. When used in a model vaccine application, particles loaded with the protein ovalbumin (OVA) increased the presentation of OVA-derived peptides to CD8+ T-cells 16-fold relative to OVA alone. Additionally, this dextran derivative was found to be nontoxic in preliminary in vitro cytotoxicity assays. Owing to its ease of preparation, processability, pH-sensitivity, and biocompatibility, this type of modified dextran should find use in numerous drug delivery applications.

The interaction of cationic antimicrobial peptides with vesicles containing synthetic glycolipids as models of the outer membrane of gram-negative bacteria

Two simple lipid A analogues methyl 2,3-di-O-tetradecanoyl-alpha-D-glucopyranoside (GL1) and methyl 2,3-di-O-tetradecanoyl-alpha-D-glucopyranoside 4-O-phosphate (GL2) were synthesized and used for preparing mixed phosphocholine vesicles as models of the outer membrane of gram-negative bacteria. The interaction of these model membranes with magainin 2, a representative of the alpha-helical membrane active peptides, and apidaecin Ib and drosocin, two insect Pro-rich peptides which do not act at the level of the cellular membrane, were studied by CD and dye-releasing experiments. The CD spectra of apidaecin Ib and drosocin in the presence of GL1- or GL2-containing vesicles were consistent with largely unordered structures, whereas, according to the CD spectra, magainin 2 adopted an amphipathic alpha-helical conformation, particularly in the presence of negatively charged bilayers. The ability of the peptides to fold into amphipathic conformations was strictly correlated to their ability to bind and to permeabilize phospholipid as well as glycolipid membranes. Apidaecin Ib and drosocin, which are unable to adopt an amphipathic structure, showed negligible dye-leakage activity even in the presence of GL2-containing vesicles. It is reasonable to suppose that, as for the killing mechanism, the two classes of antimicrobial peptides follow different patterns to cross the bacterial outer membrane.

An efficient synthesis of methyl 1,3-O-isopropylidene-α-d-fructofuranoside and 2,3:5,6-di-O-isopropylidene-d-glucose dimethyl acetal derivatives from sucrose

Acetalation of sucrose with 2,2-dimethoxypropane in 1,4-dioxane in the presence of p-toluenesulfonic acid, followed by acetylation, afforded methyl 4,6-di-O-acetyl-1,3-O-isopropylidene-alpha-D-fructofuranoside and 4-O-acetyl-2,3:5,6-di-O-isopropylidene-D-glucose dimethyl acetal as major products, while tosylation of the intermediate acetals provided methyl 6-O-tosyl-1,3-O-isopropylidene-alpha-D-fructofuranose.

Mixed acetals of cyclodextrins. Preparation of hexakis-, heptakis- and octakis[2,6-di-O-(methoxydimethyl)methyl]-alpha-, beta- and gamma-cyclodextrins

The proton-catalyzed addition of 2-methoxypropene to alpha-, beta- and gamma-cyclodextrins resulted in hexakis-, heptakis-, and octakis[2,6-di-O-(methoxydimethyl)methyl]-alpha-, beta- and gamma-cyclodextrins, but no reaction was observed of CD-s with 2,2-dimethoxypropane. The mixed acetal-type compounds can be alkylated under basic conditions. The preparation of hexakis(3-O-benzyl)-alpha-cyclodextrin demonstrates the synthetic value of this methodology.

Synthesis of a hexasaccharide that relates to the arabinogalactan epitope

A hexasaccharide derivative of the arabinogalactan epitope, methyl beta-D-galactopyranosyl-(1-->6)-[alpha-L-arabinofuranosyl-(1-->3)]-beta-D-galactopyranosyl-(1-->6)-beta-D-galactopyranosyl-(1-->6)-[alpha-L-arabinofuranosyl-(1-->3)]-alpha-D-galactopyranoside, was synthesized efficiently using a 3+3 strategy. The key step is the preparation of the trisaccharide donor, isopropyl 2,3,4,6-tetra-O-benzoyl-beta-D-galactopyranosyl-(1-->6)-[2,3,5-tri-O-benzoyl-alpha-L-arabinofuranosyl-(1-->3)]-2,4-di-O-benzoyl-1-thio-beta-D-galactopyranoside, from isopropyl 1-thio-beta-D-galactopyranoside using a one-pot synthesis of a 3,6-differentially protected building block.

Synthesis of 2,3-di-O-glycosyl derivatives of methyl alpha- and beta-D-glucopyranoside

The syntheses are described of 2,3-di-O-glycosyl derivatives of methyl alpha- and beta-D-glucopyranoside having alpha-D-manno-, beta-D-galacto-, alpha-L-rhamno-, alpha-L-fuco-, and beta-L-fuco-pyranosyl substituents at O-2 and O-3. The syntheses involved glycosylation of methyl 4,6-O-benzylidene-alpha- (24) and -beta-D-glucopyranoside (21), and substituted derivatives of 21 bearing 2-O-(2,3,4,6-tetra-O-benzoyl-alpha-D-mannopyranosyl)-, -(2,3,4,6-tetra-O- acetyl-beta-D-galactopyranosyl)-, -(2,3,4,-tri-O-benzoyl-alpha-L- rhamnopyranosyl)-, and -(2,3,4-tri-O-benzoyl-beta-L-fucopyranosyl) groups.

Preparative synthesis of C-(alpha-D-glucopyranosyl)-alkenes and -alkadienes: Diels-Alder reaction

The reaction of 2,3,4,6-tetra-O-benzyl-1-O-(p-nitrobenzoyl)-alpha-D-glucopyranose with (E)-penta-2,4-dienyltrimethylsilane and boron trifluoride etherate in acetonitrile afforded stereoselectively (E)-5-(tetra-O-benzyl-alpha-D-glucopyranosyl)-1,3-pentadiene in good yield. The readily available penta-O-benzoyl-alpha-D-glucopyranose reacted with allyltrimethylsilane in the presence of boron trifluoride etherate in acetonitrile to give 3-(tetra-O-benzoyl-alpha-D-glucopyranosyl)-1-propene and its beta anomer in yields of 60% and 2.3%, respectively. Diels-Alder cycloaddition of maleic anhydride to diene 1 afforded the adduct cis,cis-3-(tetra-O-benzyl-alpha-D-glucopyranosylmethyl)cyclohex -4-ene- 1,2-dicarboxylic anhydride in high yield.

Synthesis of methyl pyranosides and furanosides of 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) by acid-catalysed solvolysis of the acetylated derivatives

Treatment of methyl 2,4,5,7,8-penta-O-acetyl-3-deoxy-alpha-D-manno-oct- 2-ulopyranosonic acid, or its methyl ester, with refluxing methanolic 0.1 M hydrogen chloride for 16 h gave 95% of methyl (methyl 3-deoxy-alpha-D-manno-oct-2-ulopyranosid)onate. Acetylation of the methyl ester of 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) gave mainly methyl 2,4,6,7,8-penta-O-acetyl-3-deoxy-alpha,beta-D-manno-oct-2-ulofuranoso nate. Treatment of this mixture with methanolic 0.02 M hydrogen chloride at room temperature gave methyl (methyl 3-deoxy-alpha, beta-D-manno-oct-2-ulofuranosid)onate and the corresponding 4-acetates which were isolated by reverse-phase column chromatography of their 7,8-O-isopropylidene derivatives. Confirmation of the position of the isopropylidene group was obtained by acetylation to give methyl (methyl 4,6-di-O-acetyl-3-deoxy-7,8-O-isopropylidene-alpha,beta-D-manno-oct-2-ul ofuranosid)onate. The furanose anomers were differentiated primarily by J3,4 values (alpha approximately 6.1 Hz, beta approximately 2.2 Hz). The anomeric configuration in the furanose series has been assigned on the basis of optical rotation.

Identification of pyruvated monosaccharides in polysaccharides by gas-liquid chromatography mass spectrometry

Twelve bacterial polysaccharides of known structure containing a representative range of pyruvated monosaccharides, were methanolysed, trimethylsilylated, and analysed by g.l.c. and g.l.c.-m.s. Except for 3,4-O-(1-carboxyethylidene)-L-rhamnose, which was unusually labile, the pyruvic acid substituents were largely retained during methanolysis and the Me3Si derivatives of the resulting pyruvated methyl glycosides gave distinctive g.l.c. peaks with characteristic mass spectra. The pyranose rings of 4,6-O-(1-carboxyethylidene)-D-glucose, 4,6-O-(1-carboxyethylidene)-D-mannose, 4,6-O-(1-carboxyethylidene)-D-galactose, and 3,4-O-(1-carboxyethylidene)-D-galactose survived the methanolysis, but that of 2,3-O-(1-carboxyethylidene)-D-glucuronic acid was cleaved to give the methyl ester of 2,3-O-(1-carboxyethylidene)-aldehydo-D-glucuronic acid dimethyl acetal. In the case of 2,3-O-(1-carboxyethylidene)-D-galactose, cleavage of the pyranose ring was less complete; under the conditions used in these experiments two-thirds of the pyranose rings were intact while one-third were cleaved to give the methyl ester of 2,3-O-(1-carboxyethylidene)-aldehydo-D-galactose dimethyl acetal. A very small amount of 3,4-O-(1-carboxyethylidene)-L-rhamnose from one polysaccharide retained its pyruvic acid substituent after gentle methanolysis to give the methyl ester of 3,4-O-(1-carboxyethylidene)-aldehydo-L-rhamnose dimethyl acetal. Susceptibility to cleavage of the pyranose ring during methanolysis appears to be a property of pyruvated monosaccharides with trans-fused 1,3-dioxolane rings.