C-3 epimers of sugar amino acids as foldameric building blocks: improved synthesis, useful derivatives, coupling strategies.

To obtain key sugar derivatives for making homooligomeric foldamers or α/ß-chimera peptides, economic and multigram scale synthetic methods were to be developed. Though described in the literature, the cost-effective making of both 3-amino-3-deoxy-ribofuranuronic acid (H-t X-OH) and its C-3 epimeric stereoisomer, the 3-amino-3-deoxy-xylofuranuronic acid (H-c X-OH) from D-glucose is described here. The present synthetic route elaborated is (1) appropriate for large-scale synthesis; (2) reagent costs reduced (e.g. by a factor of 400); (3) yields optimized are ~80% or higher for all six consecutive steps concluding -t X- or -c X- and (4) reaction times shortened. Thus, a new synthetic route step-by-step optimized for yield, cost, time and purification is given both for D-xylo and D-ribo-amino-furanuronic acids using sustainable chemistry (e.g. less chromatography with organic solvents; using continuous-flow reactor). Our study encompasses necessary building blocks (e.g. -X-OMe, -X-OiPr, -X-NHMe, Fmoc-X-OH) and key coupling reactions making -Aaa-t X-Aaa- or -Aaa-t X-t X-Aaa- type "inserts". Completed for both stereoisomers of X, including the newly synthesized Fmoc-c X-OH, producing longer oligomers for drug design and discovery is more of a reality than a wish.

Benzylidene Acetal Protecting Group as Carboxylic Acid Surrogate: Synthesis of Functionalized Uronic Acids and Sugar Amino Acids.

Direct oxidation of the 4,6-O-benzylidene acetal protecting group to C-6 carboxylic acid has been developed that provides an easy access to a wide range of biologically important and synthetically challenging uronic acid and sugar amino acid derivatives in good yields. The RuCl3 -NaIO4 -mediated oxidative cleavage method eliminates protection and deprotection steps and the reaction takes place under mild conditions. The dual role of the benzylidene acetal, as a protecting group and source of carboxylic acid, was exploited in the efficient synthesis of six-carbon sialic acid analogues and disaccharides bearing uronic acids, including glycosaminoglycan analogues.

Synthesis of Staphylococcus aureus type 5 capsular polysaccharide repeating unit using novel L-FucNAc and D-FucNAc synthons and immunochemical evaluation.

Staphylococcus aureus is a major cause of nosocomial infections. Glycoconjugates of type 5 and 8 capsular polysaccharides have been investigated for vaccine application. The proposed structure of type 5 polysaccharide is: →4-ß-D-ManNAcA-(1→4)-α-L-FucNAc(3OAc)-(1→3)-ß-D-FucNAc-(1→. The stereocontrolled insertion of these three glycosydic bonds is a real synthetic challenge. In the present paper we report the preparation of two novel versatile L- and D-fucosamine synthons from commercially available starting materials. In addition we applied the two building blocks to the synthesis of type 5 trisaccharide repeating unit. The immunochemical properties of the synthesized trisaccharide were assessed by competitive ELISA and by immunodot blot analysis using sera of mice immunized with type 5 polysaccharide conjugated to CRM(197). The results suggest that although the type 5 S. aureus trisaccharide is recognized by specific anti polysaccharide antibodies in dot blot, structures longer than the trisaccharide may be needed in order to significantly compete with the native type 5 polymer in the binding with sera from mice immunized with S. aureus type 5 polysaccharide-CRM(197) conjugate.

Benzoylated uronic acid building blocks and synthesis of N-uronate conjugates of lamotrigine.

A chemoenzymatic approach towards benzoylated uronic acid building blocks has been investigated starting with benzoylated hexapyranosides using regioselective C-6 enzymatic hydrolysis as the key step. Two of the building blocks were reacted with the antiepileptic drug lamotrigine. Glucuronidation of lamotrigine using methyl (2,3,4-tri-O-benzoyl-α-D-glycopyranosyl bromide)uronate proceeded to give the N2-conjugate. However, lamotrigine-N2-glucuronide was most efficiently synthesised from methyl (2,3,4-tri-O-acetyl-α-D-glucopyranosyl bromide)uronate. Employing nitromethane as solvent with CdCO(3) as a base lamotrigine-N2 glucuronide was prepared in a high yield (41%). Also methyl (2,3-di-O-benzoyl-4-deoxy-4-fluoro-α-D-glucosyl bromide)uronate underwent N-glucuronidation, but the product was unstable, eliminating hydrogen fluoride to give the corresponding enoate conjugate.

Stereoselective synthesis of 2,3-diamino-2,3-dideoxy-ß-D-mannopyranosyl uronates.

With the aim to find an efficient synthetic procedure for the construction of 2,3-diamino-2,3-dideoxy-ß-D-mannuronic acids, we evaluated three mannosyl donors: (S)-phenyl 4,6-di-O-acetyl-2,3-diazido mannopyranoside, (S)-phenyl 2,3-diazido-4,6-O-benzylidene mannopyranoside, and (S)-phenyl 2,3-diazido mannopyranosyl methyl uronate. The first two mannosylating agents are rather unselective or slightly α-selective in their condensation with three different acceptors. The mannuronic acid donor on the other hand reliably provides the desired ß-mannosidic linkage. A mechanistic rationale is put forward to account for the different behavior of the three donor types. Suitably protected 2,3-diazido mannuronic acids were employed to construct the all-cis-linked tetrasaccharide repeating unit of the capsular polysaccharide of Bacillus stearothermophilus , featuring two 2,3-diacetamido-2,3-dideoxy-ß-D-mannuronic acids.

Mannosazide methyl uronate donors. Glycosylating properties and use in the construction of ß-ManNAcA-containing oligosaccharides.

Mannosazide methyl uronate donors equipped with a variety of anomeric leaving groups (ß- and α-S-phenyl, ß- and α-N-phenyltrifluoroacetimidates, hydroxyl, ß-sulfoxide, and (R(s))- and (S(s))-α-sulfoxides) were subjected to activating conditions, and the results were monitored by (1)H NMR. While the S-phenyl and imidate donors all gave a conformational mixture of anomeric α-triflates, the hemiacetal and ß- and α-sulfoxides produced an oxosulfonium triflate and ß- and α-sulfonium bistriflates, respectively. The ß-S-phenyl mannosazide methyl uronate performed best in both activation experiments and glycosylation studies and provided the 1,2-cis mannosidic linkage with excellent selectivity. Consequently, an α-Glc-(1→4)-ß-ManN(3)A-SPh disaccharide, constructed by the stereoselective glycosylation of a 6-O-Fmoc-protected glucoside and ß-S-phenyl mannosazide methyl uronate, was used as the repetitive donor building block in the synthesis of tri-, penta-, and heptasaccharide fragments corresponding to the Micrococcus luteus teichuronic acid.

Effects of carboxyl group on the anticoagulant activity of oxidized carrageenans.

In this paper, carrageenans having distinct sulfation patterns (κ-, ι-, ι/ν-, θ- and λ-carrageenans), were fully or partially oxidized at C-6 of the ß-d-Galp units using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and trichloroisocyanuric acid (TCCA) in bicarbonate buffer. The modified carrageenans were characterized by mono- and bidimensional 1H and 13C NMR spectroscopy. The influence of the sulfate and carboxyl groups onto anticoagulant activity was evaluated using Activated Partial Thromboplastin Time (aPTT) in vitro assay. The results showed a synergic effect of the carboxyl groups on the anticoagulant activity, which was dependent on the regiochemistry of the sulfate groups in the polysaccharide backbone. Sulfate groups at C2 of the ß-d-GalAp units appeared to positively influence the anticoagulant effect in comparison to C4-sulfate samples. Also, the partially oxidized κ-carrageenan derivative (κLO) showed better anticoagulant effect than the fully oxidized carrageenan (κHO).

Synthesis of both enantiomers of hydroxypipecolic acid derivatives equivalent to 5-azapyranuronic acids and evaluation of their inhibitory activities against glycosidases.

We have synthesized 3-hydroxy- and 3,4,5-trihydroxypipecolic acid derivatives corresponding to 5-aza derivatives of uronic acids and evaluated their inhibitory activities against various glycosidases including beta-glucuronidase. Compounds 4 and 5 were chosen as common intermediates for the synthesis of 3,4,5-trihydroxypipecolic acids and 3-hydroxypipecolic acids as well as for 3-hydroxybaikiain, a unique natural product isolated from a toxic mushroom. Cross aldol reaction of N-Boc-allylglycine derivative with acrolein followed by the ring-closing metathesis gave 4 and 5 as a mixture of diastereomers which could be separated by silica gel column chromatography. By employing lipase-catalyzed kinetic resolution, the synthesis of both L- and D-isomers of 3,4,5-trihydroxy- and 3-hydroxypipecolic acids was achieved. None of the compounds tested showed inhibitory activity against alpha- and beta-glucosidases. On the other hand, L-23 and L-29 were found to have potent inhibitory activity against beta-glucuronidase. In addition, it is interesting that some uronic-type azasugar derivatives showed moderate inhibitory activities against beta-N-acetylglucosaminidase.

Direct oxidation of sugar nucleotides to the corresponding uronic acids: TEMPO and platinum-based procedures.

The direct oxidation of UDP-alpha-d-glucose and UDP-N-acetyl-alpha-d-glucosamine to the corresponding uronic acids was explored using either TEMPO or platinum-catalysed oxidation with molecular oxygen. Whilst TEMPO-based procedures gave rise to substantial over-oxidation and/or degradation of UDP-glucose, oxidation of UDP-N-acetyl-glucosamine to UDP-N-acetyl-glucosaminuronic acid was achieved with >90% conversion and ca. 65% isolated yield using a platinum-catalysed procedure.

Structure-activity relationships of 2-chloro-N6-substituted-4'-thioadenosine-5'-uronamides as highly potent and selective agonists at the human A3 adenosine receptor.

We have established structure-activity relationships of novel 4'-thionucleoside analogues as the A(3) adenosine receptor (AR) agonists. Binding affinity, selectivity toward other AR subtypes, and efficacy in inhibition of adenylate cyclase were studied. From this study, 2-chloro-N(6)-methyl-4'-thioadenosine-5'-methyluronamide (36a) emerged as the most potent and selective agonist at the human A(3) AR. We have also revealed that, similar to 4'-oxoadenosine analogues, at least one hydrogen on the 5'-uronamide moiety was necessary for high-affinity binding at the human A(3) AR, presumably to allow this group to donate a H bond within the binding site. Furthermore, bulky substituents on the 5'-uronamide reduced binding affinity, but in some cases large 5'-uronamide substituents, such as substituted benzyl and 2-phenylethyl groups, maintained moderate affinity with reduced efficacy, leading to A(3) AR partial agonists or antagonists. In several cases for which the corresponding 4'-oxonucleosides have been studied, the 4'-thionucleosides showed higher binding affinity to the A(3) AR.

Synthesis and characterization of the hexenuronic acid model methyl 4-deoxy-beta-L-threo-hex-4-enopyranosiduronic acid.

A facile synthetic scheme for the preparation of methyl 4-deoxy-beta-L-threo-hex-4-enopyranosiduronic acid utilizing the commercially available methyl alpha-D-galactopyranoside as starting material has been developed. The synthesis sequence comprises six high yielding reaction steps: TEMPO oxidation, acetylation, methanolysis of the lactone, acetylation, beta-elimination, and final removal of the protecting groups. Only one column chromatographic purification is needed throughout the whole sequence. The overall yield is 60%. The final product has been characterized by NMR, Raman, UVRR, FTIR, and HRMS.

Preparation of 1-thio uronic acid lactones and their use in oligosaccharide synthesis.

The chemo- and regioselective TEMPO/BAIB-mediated oxidation of 2,6- and 3,6-dihydroxy 1-thio glycopyranosides to the corresponding 1-thio uronic acid lactones is described. These locked 1-thio glycuronides can directly be used as donors in glycosidation reactions using the Ph(2)SO/Tf(2)O reagent system. Alternatively, selective opening of the lactone bridge liberates a hydroxyl function for ensuing glycosylations.

Antioxidant activities of a polyglucuronic acid sodium salt obtained from TEMPO-mediated oxidation of xanthan.

A xanthouronic acid sodium salt called xanthouronan was produced from xanthan by regioselective oxidation with NaOCl/NaBr using 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) as catalyst. The efficiency of the one pot TEMPO-mediated oxidation was confirmed by HPAEC-PAD, (13)C NMR, and FT-IR. The oxidation degree was close to 98% and the mass yield of this new polyglucuronic acid was higher than 90% (w/w). The macromolecular characterization of xanthouronan using SEC-MALLS showed a molecular size reduced by a third due to the oxidation treatment and the degree of polymerization (DP) of the xanthouronan form was about 665. The evaluation of the enzymatic degradation of this C-6 carboxylated xanthan by various polysaccharide hydrolases and one polysaccharide lyase showed its high resistant to biodegradation. The antioxidant activity of xanthouronan was also tested by using the 2,2'-diphenyl-1-picrylhydrazyle (DPPH) and hydroxyl radical procedures. At 1 g/L, xanthouronan presented 75% of the ascorbic acid antioxidant activity.

Synthesis and biological activities of some uronic acids, uronates, uronamides, and urononitriles of pyrimidine nucleosides.

The 5'-hydroxymethylene function of several uracil and cytosine nucleosides has been modified to produce a variety of uronic acids, uronates, uronamides, and urononitriles of 2'-deoxy-beta-D-erythro-pentofuranosyl- and beta-D-arabino-pentofuranosylpyrimidines. In addition, the 5 position in many of these nucleosides has been substituted by a halogen atom. Twenty-one of the 35 compounds synthesized and examined for biological activity have not been previously reported. The purity of the products was measured by a high-pressure liquid chromatographic method. They were then evaluated as potential growth inhibitors of murine Sarcoma 180 cells in culture, of herpes simplex virus type 1 in vitro, and of Streptococcus faecium, a folic acid or deoxythymidine dependent bacterial strain. The ability of these nucleoside analogues to inhibit the phosphorylation of deoxythymidine by herpes simplex virus type 1 encoded pyrimidine deoxyribonucleoside kinase was also investigated and a structure-activity relationship examined.

Total synthesis of hexuronic acids.

Six stereoisomeric hexuronic acids (tert-butyl esters of methyl pyranosides) having the altro, manno, gluco, gulo, galacto, and talo configurations were obtained from cis,trans-tert-butyl 2-methoxy-5,6-dihydro-2H-pyran-6-carboxylate (5). The synthesis involved the following successive steps: epoxidation of the double bond in 5, opening of the epoxides with dimethylamine, Cope degradation of the dimethyl-amino derivatives, and hydroxylation of the double bond in the tert-butyl hex-3-enuronates. All compounds were obtained as pure diastereoisomers in racemic form.

Rapid access to uronic acid-based mimetics of Kdn2en from D-glucurono-6,3-lactone.

A concise route to novel mimetics of Kdn2en, based on delta4-uronic acids, from D-glucurono-6,3-lactone is presented. Uronic acid-based mimetics in which an aliphatic ether (O-glycoside), a thioether (S-glycoside), or acetamide takes the place of the natural C-6 glycerol sidechain of the sialic acid were synthesized from the key intermediate, methyl 2,3,4-tri-O-acetyl-alpha-D-glucopyranosyluronate bromide.

General methods for the synthesis of glycopyranosyluronic acid azides.

Per-O-acetylated D-glycopyranoses derived from both mono- and disaccharides were first converted to glycosyl iodides and subsequently reacted with an azide source to achieve the stereoselective synthesis of beta-D-glycosyl azides after deacetylation. Low-temperature (4 degrees C) TEMPO oxidation of the monosaccharides provided the corresponding uronic acids, which were purified as the free acids. Oxidation of the lactosyl- and cellobiosyl azides resulted in diacid formation. However, 4',6'-O-benzylidene protection enabled selective oxidation of the C-6 hydroxyl. 2-Acetamido-2-deoxy-D-glycopyranosyl azides were also prepared and converted to uronic acids completing the library synthesis.

Synthesis and glycosaminoglycan priming activity of three disaccharides related to the linkage region tetrasaccharide of proteoglycans.

To test if disaccharides might serve as primers of oligosaccharide synthesis in animal cells, we synthesized 2-naphthyl O-(beta-D-galactopyranosyl)-(1 --> 4)-beta-D-xylopyranoside, 2-naphthyl O-(beta-D-galactopyranosyl)-(1 --> 3)-beta-D-galactopyranoside, and 2-naphthyl O-(beta-D-glucopyranosyluronic acid)-(1 --> 3)-beta-D-galactopyranoside. These three disaccharides are related to subunits of the linkage tetrasaccharide of heparan sulfate and chondroitin sulfate chains in animal cell proteoglycans. The disaccharides were synthesized with coupling efficiencies of 40-70% using thioglycosides or by activating the monosaccharides with trichloroacetimidate. The structures of these compounds were confirmed by 1H NMR, 13C NMR and elemental analysis. The ability of these disaccharides to prime glycosaminoglycan chains was examined in a Chinese hamster ovary cell mutant, p gsA 745, which lacks xylosyltransferase. The missing enzyme renders the cells dependent on exogenous primers for making glycosaminoglycan chains. 2-Naphthyl O-(beta-D-galactopyranosyl)-(1 --> 3)-beta-D-galactopyranoside and 2-naphthyl O-(beta-D-glucopyranosyluronic acid)-(1 --> 3)-beta-D-galactopyranoside did not stimulate glycosaminoglycan synthesis, but 2-naphthyl O-(beta-D-galactopyranosyl)-(1 --> 4)-beta-D-xylopyranoside at high concentration primed chains. The peracetylated derivative (2-naphthyl O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-(1 --> 4)- 2,3-di-O-acetyl-beta-D-xylopyranoside) primed chains at lower concentration (100 microM), suggesting that cells took up the compound and removed the acetyl groups apparently in the compartment where glycosaminoglycan synthesis occurs.