An improved method for synthesizing antennary β-d-mannopyranosyl disaccharide units

The merits of an indirect protecting method for hydroxyl groups using allyl groups via allyloxycarbonyl groups in the synthesis of antennary β-d-mannopyranosyl disaccharides from β-d-galactopyranosyl disaccharides were studied. Regioselective allyloxycarbonylation and conversion reactions involving simultaneous double S_N2 nucleophilic substitution at C-2′ and C-4′ of benzyl O-[β-d-galactopyranosyl]-(1-4)-3,6-di-O-benzyl-2-deoxy-2-N-phthalimido-β-d-glucopyranoside were examined for comparison with the direct allylation method. The required β-d-mannopyranosyl disaccharide having proper protecting groups was obtained using this indirect method in 52% yield. In contrast, the reported direct allylation method using methyl O-(β-d-galactopyranosyl) disaccharide gave the corresponding β-d-mannopyranosyl disaccharide in only 7.5% yield.     

Fluoridolysis of 5,6-epoxy carbohydrates: application to the synthesis of 5-fluoro lactosamine and isolactosamine glycosides

The synthesis of 6-selenophenyl derivatives of β-1,3 and β-1,4 disaccharides has been explored for the purpose of extending our epoxide fluoridolysis methodology to the synthesis of 5-fluoro analogues of N-acetyl isolactosamine (isoLacNAc, lacto-N-biose) and N-acetyl lactosamine (LacNAc) glycosides. Successful synthesis of the C-6 selenium-containing disaccharides was achieved via Lewis acid-mediated donor and acceptor substrates, the latter containing a selectively protected C-6 hydroxyl group for ultimate conversion to the desired 6-selenophenyl disaccharides. In contrast, the use of selenium-containing acceptor substrates under a variety of conditions failed to yield the desired selenium-containing disaccharides. Oxidation of the 6-selenophenyl derivatives to the corresponding selenoxides followed by thermal elimination yielded the exocyclic olefins, which were converted to the 5,6-epoxides. Epoxide fluoridolysis yielded the desired target compounds, 5-fluoro β-octyl glycoside analogues of type 1 and type 2 glycans. The newly synthesized fluorine-containing disaccharides have potential application as fucosyltransferase substrates, both for mechanistic studies and in the chemoenzymatic synthesis of fluorine-containing oligosaccharides.     

Synthesis of a Tetrasaccharide Related to the Repeating Unit of the Antigen from Shigella dysenteriae Type 9 in the Form of Its 2‐(Trimethylsilyl)ethyl Glycoside

Abstract

Starting from D‐mannose, D‐galactose and D‐glucosamine hydrochloride, two disaccharide blocks were synthesized. Schmidt's inverse addition technique for trichloroacetimidate was utilized for the construction of a disaccharide with a β‐mannosidic linkage in good yield. The two disaccharides in the appropriate form were then allowed to react in the presence of N‐iodosuccinimide (NIS) and trifluoromethanesulfonic acid (TfOH) to give the tetrasaccharide derivative from which removal of protecting groups gave the desired tetrasaccharide in the form of its 2‐(trimethylsilyl)ethyl glycoside.     

Isolation and characterization of an exopolygalacturonase from Fusarium oxysporum f.sp. cubense race 1 and race 4

Background

Previously, we have reported the presence of highly sulfated dermatans in solitary ascidians from the orders Phlebobranchia (Phallusia nigra) and Stolidobranchia (Halocynthia pyriformis and Styela plicata). Despite the identical disaccharide backbone, consisting of [→4IdoA(2S)β-1→3GalNAcβ-1→], those polymers differ in the position of sulfation on the N-Acetyl galactosamine, which can occur at carbon 4 or 6. We have shown that position rather than degree of sulfation is important for heparin cofactor II activity. As a consequence, 2,4- and 2,6-sulfated dermatans have high and low heparin cofactor II activities, respectively. In the present study we extended the disaccharide analysis of ascidian dermatan sulfates to additional species of the orders Stolidobranchia (Herdmania pallida, Halocynthia roretzi) and Phlebobranchia (Ciona intestinalis), aiming to investigate how sulfation evolved within Tunicata. In addition, we analysed how heparin cofactor II activity responds to dermatan sulfates containing different proportions of 2,6- or 2,4-disulfated units.

Results

Disaccharide analyses indicated a high content of disulfated disaccharide units in the dermatan sulfates from both orders. However, the degree of sulfation decreased from Stolidobranchia to Phlebobranchia. While 76% of the disaccharide units in dermatan sulfates from stolidobranch ascidians are disulfated, 53% of disulfated disaccharides are found in dermatan sulfates from phlebobranch ascidians. Besides this notable difference in the sulfation degree, dermatan sulfates from phlebobranch ascidians contain mainly 2,6-sulfated disaccharides whereas dermatan sulfate from the stolidobranch ascidians contain mostly 2,4-sulfated disaccharides, suggesting that the biosynthesis of dermatan sulfates might be differently regulated during tunicates evolution. Changes in the position of sulfation on N-acetylgalactosamine in the disaccharide [→4IdoA(2-Sulfate)β-1→3GalNAcβ-1→] modulate heparin cofactor II activity of dermatan sulfate polymers. Thus, high and low heparin cofactor II stimulating activity is observed in 2,4-sulfated dermatan sulfates and 2,6-sulfated dermatan sulfates, respectively, confirming the clear correlation between the anticoagulant activities of dermatan sulfates and the presence of 2,4-sulfated units.

Conclusions

Our results indicate that in ascidian dermatan sulfates the position of sulfation on the GalNAc in the disaccharide [→4IdoA(2S)β-1→3GalNAcβ-1→] is directly related to the taxon and that the 6-O sulfation is a novelty apparently restricted to the Phlebobranchia. We also show that the increased content of [→4IdoA(2S)β-1→3GalNAc(4S)β-1→] disaccharide units in dermatan sulfates from Stolidobranchia accounts for the increased heparin cofactor II stimulating activity.     

A new pyrrolidine-derived atropisomeric amino alcohol as a highly efficient chiral ligand for the asymmetric addition of diethylzinc to aldehydes

A highly efficient pyrrolidine-derived atropisomeric amino alcohol, (S_a)-1-[2-diphenylhydroxymethyl-6-(trifluoromethyl)phenyl]-2-(1-pyrrolido)methyl-1H-pyrrole, has been synthesized as a chiral ligand for the enantioselective addition of diethylzinc to some prochiral aldehydes to afford (S)-alcohols. The conversion rates were close to quantitative with good to excellent enantiomeric excesses (up to 95% ee).     

Chiral memory effects in catalytic hydrogenations with dynamically chiral ligands

The low barrier for interconversion of chiral conformations of the dynamically chiral 2,2′-biphenyl ligand NMe₂C₆H₄C₆H₄PCy₂ is raised upon coordination. The individual enantiomers of the planar chiral arene-tethered complex Ru(η⁶:η¹- NMe₂C₆H₄C₆H₄PCy₂)Cl₂ (1), however, do not undergo racemization readily. A second source of chirality, such as a chiral diamine, can be included by conversion of 1 into a dicationic analogue [Ru(η⁶:η¹-NMe₂C₆H₄C₆H₄PCy₂)((1S,2S)-DPEN)](SbF₆)₂ (2), which is a catalyst precursor for the hydrogenation of aryl ketones. Two epimers of 2, R_Ar,S,S and S_Ar,S,S, are formed when starting from racemic 1; this 1:1 mixture of diastereomers catalyzed the asymmetric hydrogenation of acetophenone. The enantiomerically pure diastereomers were obtained from resolved 1 and used separately to catalyze the reaction. Each diastereomer showed different selectivity, with S_Ar,S,S-2 being the more selective (61% ee for the hydrogenation of acetophenone). Our studies suggest that ruthenium hydride formation is accompanied by a decrease in hapticity of the η⁶-arene and probable detachment of the ring from the metal. Nevertheless, the original conformational chirality of the biphenyl ligand appears to be at least partially retained during the catalysis.     

Enantioselective Darzens reaction using organoselenide-lithium hydroxide complexes

Asymmetric Darzens reaction catalyzed by chiral selenides is described. A novel Lewis acid/Brønsted base catalyst formed by C₂ symmetric chiral selenide-bearing isoborneol skeletons, which were readily prepared from (1S)-10-camphorsulfonic acid, and LiOH promoted the reaction of phenacyl bromide with aldehydes to afford the desired trans oxiranes with up to 62% ee.     

Synthetic Disialyl Hexasaccharides Protect Neonatal Rats from Necrotizing Enterocolitis

Two novel synthetic α2–6‐linked disialyl hexasaccharides, disialyllacto‐N‐neotetraose (DSLNnT) and α2–6‐linked disialyllacto‐N‐tetraose (DS′LNT), were readily obtained by highly efficient one‐pot multienzyme (OPME) reactions. The sequential OPME systems described herein allowed the use of an inexpensive disaccharide and simple monosaccharides to synthesize the desired complex oligosaccharides with high efficiency and selectivity. DSLNnT and DS′LNT were shown to protect neonatal rats from necrotizing enterocolitis (NEC) and are good therapeutic candidates for preclinical experiments and clinical application in treating NEC in preterm infants.     

Stereoselective synthesis of 3-deoxy-piperidine iminosugars from l-lysine

A new method using electrochemical oxidation and/or OsO₄ oxidation has been used for the stereoselective synthesis of 2,3,6-trihydroxylated (5S)-piperidine derivatives. The electrochemical method was successively used for the conversion of N-protected piperidines to N-protected 1-methoxypiperidines and for the conversion of 2,3-didehydro-1-methoxypiperidine derivatives to 2,3-trans-1,2,3-triacetoxypiperidine derivatives. These triacetates were easily transformed into (2S,3S)-6-triacetoxy-(5S)-methylpiperidine and (2R,3R)-6-triacetoxy-(5S)-methylpiperidine. In addition, the 2,3-cis-dihydroxylation of 2,3-didehydro-1-methoxypiperidine derivatives with OsO₄ afforded (2R,3S)-6-triacetoxy-(5S)-methylpiperidine and (2S,3R)-6-triacetoxy-(5S)-methylpiperidine.     

Synthesis and conformational analysis of novel N(OCH3)-linked disaccharide analogues

N(OMe)-linked disaccharide analogues, isosteric to the corresponding natural disaccharides, have been synthesized by chemoselective assembly of unprotected natural monosaccharides with methyl 6-deoxy-6-methoxyamino-alpha-D-glucopyranoside in an aqueous environment. The coupling reactions were found to be chemo- and stereoselective affording beta-(1-->6) disaccharide mimics when using Glc and GlcNAc; in the case of Gal, the beta-anomer was prevalent (beta:alpha=7:1). An iterative method for the synthesis of linear N(OMe) oligosaccharide analogues was demonstrated, based on the use of an unprotected monosaccharide building block in which an oxime functionality at C-6 is converted during the synthesis into the corresponding methoxyamino group. The conformational analysis of these compounds was carried out by using NMR spectroscopy, ab initio, molecular mechanics, and molecular dynamics methods. Optimized geometries and energies of fourteen conformers for each compound have been calculated at the B3LYP/6-31G* level. Predicted conformational equilibria were compared with the results based on NMR experiments and good agreement was found. It appears that N(OMe)-linked disaccharide analogues exhibit a slightly different conformational behavior to their parent natural disaccharides.     

Tandem mass spectrometry of alkali cationized polysaccharides in a quadrupole ion trap

Quadrupole ion trap mass spectrometry is used to study the linkage type dependent dissociation pathways of alkali-cationized disaccharides, mostly of the type glucosyl(1 → X)glucose (X = 1, 2, 3, 4, or 6). The reaction mechanisms of a set of disaccharides containing all possible α anomeric linkage types and some β anomers are probed with tandem mass spectrometry, MS ⁿ , and double resonance experiments. Tandem mass spectrometry experiments on an ¹⁸O-labeled disaccharide show that the dissociation paths for Li and Na cationized species are the same. Experiments on three trisaccharides (isomaltotriose, maltotriose, and panose), a tetrasaccharide (isomaltotetraose), and a pentasaccharide (maltopentaose) show that tandem mass spectrometry provides all available linkage information and MS ⁿ can provide selected linkage information. The mode of alkali binding is examined via semiempirical calculations and by measuring alkali-carbohydrate relative cation affinities.     

New chiral amino alcohol ligands derived from 1-phenylethylamine for efficient Ru-catalyzed asymmetric transfer hydrogenation

A series of chiral amino alcohols have been prepared from cheap and readily available (S)-1-phenylethylamine through a one-step transformation. The ability of these newly developed amino alcohols as chiral ligands was evaluated in the Ru-catalyzed asymmetric transfer hydrogenation of aromatic alkyl ketones, providing chiral secondary alcohols with good to excellent conversions (71-100%) and moderate to good enantioselectivities (67-95% ee). The results also showed that the structure of these amino alcohols has a significant influence on the conversion and enantioselectivity.     

Palladium-catalyzed asymmetric allylic alkylation of indoles by C–N bond axially chiral phosphine ligands

The palladium-catalyzed asymmetric allylic alkylation of indoles with 1,3-diphenyl-2-propenyl acetate using P/N-type ligands such as N-aryl indole, C–N bond axially chiral aminophosphine (aS)-L4, gave the desired products 1 in good yields and with moderate to high enantioselectivities (up to 90% ee).     

Efficient asymmetric synthesis of long chain methyl-branched hydrocarbons,components of the contact sex pheromone of females of the cerambycid beetle,Neoclytus acuminatus acuminatus

Both enantiomers of 7-methylheptacosane and 7-methylpentacosane, the major components of the female-produced contact sex pheromone of the red-headed ash borer beetle (Neoclytus acuminatus accuminatus) were synthesized in 62–68% overall yield (six steps) via alkynylation of the triflate of a shared chiral alcohol intermediate (R)- or (S)-2-methyloctanol. The enantiomers of the shared intermediate were prepared via acylation and subsequent asymmetic methylation of (R)- and (S)-4-isopropyloxazolidin-2-one chiral auxiliaries, respectively. The methodology can be readily adapted for the preparation of analogues with any desired chain length and methyl branch positions.     

Magnetic properties of the Fe spin crossover complex in emulsion polymerization of trifluoroethylmethacrylate using poly(vinyl alcohol)

Influence of chemical substitution in the Fe^II spin crossover complex on magnetic properties in emulsion polymerization of trifluoroethylmethacrylate using poly(vinyl alcohol) as a protective colloid was investigated near its high spin/low spin (HS/LS) phase transition. The obvious bi-stability of the HS/LS phase transition was considered by the identification of multiple spin states between the quintet (S=2) states to single state (S=0) across the excited triplet state (S=1). Magnetic parameters of gradual shifts of anisotropy g-tensor supported by the molecular distortion of the spin crossover complex would arise from a Jahn-Teller effect regarding ligand field theory on the basis of a B3LYP density functional theory using electron spin resonance (ESR) spectrum and X-ray powder diffraction.     

Toward the assembly of heparin and heparan sulfate oligosaccharide libraries: efficient synthesis of uronic acid and disaccharide building blocks

The monosaccharide moieties found in heparin (HP) and heparan sulfate (HS), glucosamine and two kinds of uronic acids, glucuronic and iduronic acids, were efficiently synthesized by use of glucosamine hydrochloride and glucurono-6,3-lactone as starting compounds. In the synthesis of the disaccharide building block, the key issues of preparation of uronic acids (glucuronic acid and iduronic acid moieties) were achieved in 12 steps and 15 steps, respectively, without cumbersome C-6 oxidation. The resulting monosaccharide moieties were utilized to the syntheses of HP/HS disaccharide building blocks possessing glucosamine-glucuronic acid (GlcN-GlcA) or iduronic acid (GlcN-IdoA) sequences. The disaccharide building blocks were also suitable for further modification such as glycosylation, selective deprotection, and sulfation.     

Novel fungi-catalyzed reduction of α-alkyl-β-keto esters

A screening of 15 fungi and yeast strains was carried out in fermentation processes to perform the diastereo- and enantioselective reduction of ethyl 2-methyl-3-oxobutanoate, to the corresponding (R¹,S¹)-3-hydroxy-2-methyl esters. Overall, biotransformations led to excellent conversions, as well as good to excellent diastereo- and enantioselectivities. A strain of Aureobasidium pullulans (CCM H1) was found to be the most efficient biocatalyst in terms of conversion (100%), syn:anti ratio (3:97), and enantiomeric excess (94% anti-(2S,3S) isomer). This biotransformation was successfully carried out on a preparative level as well. Other microorganisms, such as Fusarium graminearum (CCM HH 224), Aspergillus terreus (BFQU 121), Geotrichum candidum (CCM H38), Trichoderma koningii (ATCC 76666), and Aspergillus niger (CCM H21) also showed excellent diastereo- and enantioselectivities, combined with high conversions (>95% conversion, ?95% ee, and excellent syn:anti ratios). Many of the strains used in this work had scarcely been described as oxido-reducing agents, or had never been used with the substrates reported herein.     

Synthesis of three novel chiral diamines derived from (S)-proline and their evaluation as precursors of diazaborolidines for the catalytic borane-mediated enantioselective reduction of prochiral ketones

A series of chiral diazaborolidine catalysts are readily prepared in situ at 75 °C in toluene solvent and under microwave irradiation (100 W, 15 min, air cooling) using chiral diamines derived from inexpensive and commercially available (S)-proline and borane-dimethyl sulfide. Special mention deserves the synthesis of potentially versatile diamine (S)-8 [(S)-(pyrrolidin-2-yl)diphenylmethanamine], with the key step being the conversion of tertiary alcohol (S)-(1-benzylpyrrolidin-2-yl)diphenyl methanol, (S)-12, to azide (S)-13. The chiral diazaborolidine/BH₃ reagent system was successfully employed in the enantioselective reduction of prochiral ketones to give the corresponding secondary alcohols in excellent yield and with up to 96% enantiomeric purities.     

Isolation and Characterization of Heparan Sulfate from Human Lung Tissues

Glycosaminoglycans are a class of linear, highly negatively charged, O-linked polysaccharides that are involved in many (patho)physiological processes. In vitro experimental investigations of such processes typically involve porcine-derived heparan sulfate (HS). Structural information about human, particularly organ-specific heparan sulfate, and how it compares with HS from other organisms, is very limited. In this study, heparan sulfate was isolated from human lung tissues derived from five donors and was characterized for their overall size distribution and disaccharide composition. The expression profiles of proteoglycans and HS-modifying enzymes was quantified in order to identify the major core proteins for HS. In addition, the binding affinities of human HS to two chemokines—CXCL8 and CCL2—were investigated, which represent important inflammatory mediators in lung pathologies. Our data revealed that syndecans are the predominant proteoglycan class in human lungs and that the disaccharide composition varies among individuals according to sex, age, and health stage (one of the donor lungs was accidentally discovered to contain a solid tumor). The compositional difference of the five human lung HS preparations affected chemokine binding affinities to various degrees, indicating selective immune cell responses depending on the relative chemokine–glycan affinities. This represents important new insights that could be translated into novel therapeutic concepts for individually treating lung immunological disorders via HS targets.