Photocatalyzed Perfluoroalkylation of Endoglycals.

The visible light-induced perfluoroalkyl (RF) radical reactions on peracetylglycals derived from hexoses and pentoses (galactal, glucal, arabinal, and xylal derivatives) were investigated. Various photocatalysts and perfluoroalkyl iodides (RF-I) were employed as sources of RF radicals with LEDs as the irradiation source. Particularly noteworthy was the use of an Iridium photocatalyst, Ir[dF(CF3)ppy]2(dtbpy))PF6, which yielded two distinct product types when applied to glucal. On the one hand, the 2-RF-substituted glucal was formed, a trend observed even when utilizing organic dyes as photocatalysts. On the other hand, the unexpected addition product, namely the 1-RF-2-iodo-α-manno-configured C-glycosyl derivative, was also obtained, as a result of a highly regioselective addition reaction of the RF moiety into the anomeric carbon, followed by attachment of the iodine atom on C-2 in axial disposition. This result contrasted with other radical reactions carried out on 2-unsubstituted glycals, where the incipient radical adds to C-2, generating a stabilized 1-glycosyl radical. The photocatalyzed radical perfluoroalkylations of peracetyl glycals derived from galactose, arabinose, and xylose all afforded the 2-RF-substituted glycals in good yields as a result of the expected vinylic substitution reaction. Mechanistic studies revealed that the 1-RF-2-iodo-α-manno-configured C-glycosyl derivatives arise from a radical chain reaction, whereas the 2-RF-substituted glycals proceed from inefficient chain processes.

Synthesis of sugar enones and their use as powerful synthetic precursors of thiodisaccharides.

Monosaccharide derivatives having a double bond conjugated to a carbonyl (sugar enones or enuloses) are relevant synthetic tools. They are also suitable starting materials, or versatile intermediates, for the synthesis of a wide variety of natural or synthetic compounds with a broad spectrum of biological and pharmacological activities. The preparation of enones is mainly focused on the search for more efficient and diastereoselective synthetic methodologies. The usefulness of enuloses relies on the diverse reaction possibilities offered by alkene and carbonyl double bonds, which are prone to undergo varied reactions such as halogenation, nitration, epoxidation, reduction, addition, etc. The addition of thiol groups that led to sulfur glycomimetics, such as thiooligosaccharides, is particularly relevant. Therefore, the synthesis of enuloses and the Michael addition of sulfur nucleophiles to give thiosugars or thiodisaccharides are discussed here. Chemical modifications of the conjugate addition products to afford biologically active compounds are also reported.

Amphiphilic Low-Molecular-Weight Gelators Bearing ß-S-N-Acetylglucosamine Linked to a Tartaric Acid Scaffold: Synthesis, Self-Assembly and Wheat Germ Agglutinin Binding.

The self-assembly of carbohydrate-based amphiphiles can lead to colloidal soft materials such as supramolecular gels featuring highly desirable characteristics like biodegradability and biocompatibility. The report herein presents the synthesis, characterization and supramolecular self-assembly, physical gelation and wheat lectin binding of two structurally related amphiphilic compounds having ß-S-N-acetylglucosamine residues linked to a 2,3-diacyl-N,N'-dipropargylated-l-tartaric diamide. A 1-thio-ß-N-acetyl-d-glucosamine precursor attached to a conveniently functionalized linker with an azido group was synthesized by means of a one-pot procedure followed by deprotection. A click reaction successfully led to the two amphiphiles, which differed in length of the fatty acid attached to the tartaric acid scaffold. Although both compounds are poorly soluble in water and organic solvents, the difference in terms of hydrophilic moieties provided them with distinct supramolecular gelation properties. While the presence of an octadecyl chain produced a hydrogelator, the dodecadecyl homologue would only form weak gels in DMSO. SEM and rheology experiments confirmed the characteristic fibrillar morphology and viscoelastic properties, in agreement with the presence of physical gels. Both amphiphiles were able to interact reversibly with wheat germ agglutinin (WGA), a lectin that specifically recognizes GlcNAc residues, indicating a potential use in the food industry, as a gluten sensitivity manager, as well as in health-related industries, for example, for drug delivery systems.

Synthesis of Oligosaccharides Containing the S-Galp(α1 → 3)Galp Unit, Glycomimetic of the Epitope Recognized by Lytic Antibodies.

Two important activities take place in the surface of Trypanosoma cruzi, the agent of Chagas disease: the trans-sialidase (TcTS) catalyzes the transfer of sialic acid from the host glycoconjugates to the mucin-like glycoproteins from the parasite and the presence of lytic antibodies recognize the epitope α-Galp(1 → 3)-ß-Galp(1 → 4)-α-GlcNAcp. This antigenic structure is known to be present in the parasite mucins; however, in order to be substrates of trans-sialidase, some of the galactose residues should be in the ß-Galp configuration. To study the interaction between both activities, it is important to count the synthetic structures as well as the structural-related glycomimetics. With this purpose, we addressed the synthesis of a trisaccharide and two isomeric tetrasaccharides containing the 1-S-α-Galp(1 → 3)-ß-Galp motif, the thio analog of the epitope recognized by lytic antibodies. Starting with a common lactose precursor, the sulfur function was incorporated by double inversion of the configuration of the galactose residue that was further glycosylated using different activated donors. Both tetrasaccharides were good acceptors of sialic acid in the reaction catalyzed by TcTS, as determined by high-performance anion exchange chromatography.

Synthesis, self-assembly and Langerin recognition studies of a resorcinarene-based glycocluster exposing a hyaluronic acid thiodisaccharide mimetic.

Herein, we report the synthesis of an octavalent glycocluster exposing a thiodisaccharide mimetic of the repetitive unit of hyaluronic acid, ßSGlcA(1 → 3)ßSGlcNAc, constructed on a calix[4]resorcinarene scaffold by CuAAC reaction of suitable precursors. This glycocluster showed a strong tendency toward self-aggregation. DOSY-NMR and DLS experiments demonstrated the formation of spherical micelles of d ≅ 6.2 nm, in good agreement. TEM micrographs showed the presence of particles of different sizes, depending on the pH of the starting solution, thus evidencing that the negative charge on the micelle surface due to ionization of the GlcA residues plays an important role in the aggregation process. STD-NMR and DLS experiments provided evidence of the interaction between the synthetic glycocluster and Langerin, a relevant C-type lectin. This interaction was not observed in the STD-NMR experiments performed with the basic disaccharide, providing evidence of a multivalent effect.

Synthesis of Thiodisaccharides Bearing N-Acetylhexosamine Residues: Challenges, Achievements and Perspectives.

Carbohydrate-protein interactions are involved in a myriad of biological processes. Thus, glycomimetics have arisen as one of the most promising synthetic targets to that end. Within the broad variety of glycomimetics, thiodisaccharides have proven to be excellent tools to study these processes, and even more, some of them unveiled interesting biological activities. This review brings together research made on the introduction of N-acetylhexosamine residues into thiodisaccharides to date, passing through classic substitution (as SN 2, thioglycosylation and ring-opening reactions) and addition (as thiol-ene coupling and Michael-type additions) reactions. Recent and interesting developments regarding addition reactions to vinyl azides, cross-coupling reactions and novel chemoenzymatic methods are also discussed.

Approaches to the Synthesis of Perfluoroalkyl-Modified Carbohydrates and Derivatives: Thiosugars, Iminosugars, and Tetrahydro(thio)pyrans.

Fluoroalkyl-substituted carbohydrates play relevant roles in diverse areas such as supramolecular chemistry, glycoconjugation, liquid crystals, and surfactants, with direct applications as wetting, antifreeze, and coating agents. In light of these promising applications, new methodologies for the late-stage incorporation of fluoroalkyl RF groups into carbohydrates and derivatives are herein presented as they are relevant to the synthetic carbohydrate community. Previously reviewed protocols for the installation of RF groups onto carbohydrates and derivatives will be succinctly summarized in the light of the new achievements. Fluoroalkyl-substituted iminosugars, on the other hand, are also interesting glycomimetic derivatives with prominent roles as glycosidases and glycosyltransferases inhibitors, as has recently been demonstrated. Also, they positively contribute to the study of sugar-protein interactions and enzyme mechanisms. New advances in the syntheses of fluoroalkyl-substituted iminosugars will also be presented here.

Synthetic Studies on the Incorporation of N-Acetylallosamine in Hyaluronic Acid-Inspired Thiodisaccharides.

Two approaches for the synthesis of the thiodisaccharide ß-S-GlcA(1→3)ß-S-AllNAc are described here. The target disaccharide was a C-3 epimer and thio-analogue of the hyaluronic acid repetitive unit, tuned with a thiopropargyl anomeric group for further click conjugation. Thus, we analysed and tested two convenient sequences, combining the two key steps required to introduce the thioglycosidic bonds and consequently reach the target molecule: the SN2 substitution of a good leaving group (triflate) present at C-3 of a GlcNAc derivative and the introduction of the anomeric thiopropargyl substituent. The use of a 2-azido precursor showed to be a convenient substrate for the SN2 step. Nevertheless, further protecting group manipulation and the introduction of the thiopropargyl anomeric residue were then required. This approach showed to provide access to a variety of thiodisaccharide derivatives as interesting building blocks for the construction of neoglycoconjugates.

Structure-Based Virtual Screening of New Benzoic Acid Derivatives as Trypanosoma cruzi Trans-sialidase Inhibitors.

BACKGROUND: Chagas disease, caused by the parasite Trypanosoma cruzi, represents a worldwide epidemiological, economic, and social problem. In the last decades, the trans-sialidase enzyme of Trypanosoma cruzi has been considered an attractive target for the development of new agents with potential trypanocidal activity. OBJECTIVE: In this work, the aim was to find new potential non-sugar trans-sialidase inhibitors using benzoic acid as a scaffold. METHODS: A structure-based virtual screening of the ZINC15 database was carried out. Additionally, the enzyme and trypanocidal activity of the selected compounds was determined. RESULTS: The results of this work detected 487 compounds derived from benzoic acid as potential transsialidase inhibitors with a more promising binding energy value (< -7.7 kcal/mol) than the known inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (DANA). In particular, two lead compounds, V1 and V2, turned out to be promising trans-sialidase inhibitors. Even though the trypanocidal activity displayed was low, these compounds showed trans-sialidase inhibition values of 87.6% and 29.6%, respectively. CONCLUSION: Structure-based virtual screening using a molecular docking approach is a useful method for the identification of new trans-sialidase inhibitors.

Crystal structures of peanut lectin in the presence of synthetic ß-N- and ß-S-galactosides disclose evidence for the recognition of different glycomimetic ligands.

Carbohydrate-lectin interactions are involved in important cellular recognition processes, including viral and bacterial infections, inflammation and tumor metastasis. Hence, structural studies of lectin-synthetic glycan complexes are essential for understanding lectin-recognition processes and for the further design of promising chemotherapeutics that interfere with sugar-lectin interactions. Plant lectins are excellent models for the study of the molecular-recognition process. Among them, peanut lectin (PNA) is highly relevant in the field of glycobiology because of its specificity for ß-galactosides, showing high affinity towards the Thomsen-Friedenreich antigen, a well known tumor-associated carbohydrate antigen. Given this specificity, PNA is one of the most frequently used molecular probes for the recognition of tumor cell-surface O-glycans. Thus, it has been extensively used in glycobiology for inhibition studies with a variety of ß-galactoside and ß-lactoside ligands. Here, crystal structures of PNA are reported in complex with six novel synthetic hydrolytically stable ß-N- and ß-S-galactosides. These complexes disclosed key molecular-binding interactions of the different sugars with PNA at the atomic level, revealing the roles of specific water molecules in protein-ligand recognition. Furthermore, binding-affinity studies by isothermal titration calorimetry showed dissociation-constant values in the micromolar range, as well as a positive multivalency effect in terms of affinity in the case of the divalent compounds. Taken together, this work provides a qualitative structural rationale for the upcoming synthesis of optimized glycoclusters designed for the study of lectin-mediated biological processes. The understanding of the recognition of ß-N- and ß-S-galactosides by PNA represents a benchmark in protein-carbohydrate interactions since they are novel synthetic ligands that do not belong to the family of O-linked glycosides.

Photocatalyzed reductive fluoroalkylation of 2-acetoxyglycals towards the stereoselective synthesis of α-1-fluoroalkyl-C-glycosyl derivatives.

A benign, efficient, regio- and stereoselective protocol for the syntheses of α-1-fluoroalkyl-C-glycosyl compounds bearing CF3, C4F9, and C6F13 substituents on the anomeric carbon has been developed by a new methodology starting from 2-acetoxyglycals for the first time. Remarkably, the reactions proceeded in only one step, through the visible light-photocatalyzed reductive fluoroalkylation of 2-acetoxyglycals by means of an Ir photocatalyst and employed commercially available fluoroalkyl iodides n-CnF2n+1-I (n = 1, 4, 6) as a source of fluoroalkyl radicals.

Synthesis of N-acetylglucosamine and N-acetylallosamine resorcinarene-based multivalent ß-thio-glycoclusters: unexpected affinity of N-acetylallosamine ligands towards Wheat Germ Agglutinin.

Herein, we report the synthesis of calix[4]resorcinarene-based multivalent ligands bearing ß-S-GlcNAc and ß-S-AllNAc recognition elements. A clickable ß-S-AllNAc derivative was successfully prepared from a ß-thioalkynyl GlcNAc precursor, making use of a 2,3-oxazoline intermediate, easily formed by intramolecular displacement of a triflate group located at the 3-position by the 2-N-acetate group. By reaction of these alkynyl-functionalized derivatives with an octaazido-calix[4]resorcinarene macrocycle having undecyl chains, two octavalent glycoclusters exposing the epimeric N-acetylhexosamines were obtained. In addition, a related calix[4]resorcinarene-based glycocluster having methyl groups instead of undecyl chains and ß-S-GlcNAc residues was also synthesized. After an initial evaluation of the interaction of the undecyl-functionalized ß-S-GlcNAc octavalent derivative with Wheat Germ Agglutinin (WGA) by a turbidimetry experiment, the interaction of the three synthesized glycoclusters towards WGA was studied by Isothermal Titration Calorimetry. The results showed a favorable effect due to the presence of the undecyl chains in terms of affinity. Surprisingly, the ß-S-AllNAc octavalent compound showed the highest affinity among the evaluated glycoclusters, showing for the first time that WGA interacts with ß-AllNAc-bearing ligands. Molecular docking studies of ß-AllNAc with WGA in comparison with ß-GlcNAc contributed to the understanding of the atomic interactions responsible for this unexpected affinity.

Synthesis of (1→3) Thiodisaccharides of GlcNAc and the Serendipitous Formation of 2,3-Dideoxy-(1→2)-thiodisaccharides through a Vinyl Azide Intermediate.

The syntheses of ß-S-GlcA(1→3)GlcNAc and ß-S-Gal(1→3)GlcNAc thiodisaccharides, which can be considered mimetics of the repeating units of hyaluronan and keratan respectively, were achieved by SN2 displacement of a triflate group allocated at the 3-position of a convenient 2-azido-4,6-O-benzylidene-2-deoxy-ß-d-allopyranose precursor by the corresponding nucleophilic suitable protected thioaldoses derived from glucuronic acid (GlcA) and galactose (Gal). The study of the reaction led to the finding that the vinyl azide formed by competitive E2 reaction of the mentioned triflate was an interesting precursor of a new kind of 2,3-dideoxy-2-azido-(1→2) thiodisaccharides through an addition reaction. Determination of the stereochemistry of the new stereocenter at C-2 was achieved by NOESY experiments. Final protecting group manipulation of the (1→3) thiodisaccharides led to a family of derivatives that could be used as building blocks for the synthesis of complex glycomimetics.

Trypanosoma cruzi trans-sialidase alternative substrates: Study of the effect of substitution in C-6 in benzyl ß-lactoside.

Trypanosoma cruzi trans-sialidase (TcTS) is a cell surface protein that participates in the adhesion and invasion mechanisms of the parasite into the host cells, making it an attractive target for inhibitors design. In order to contribute to the knowledge of the interaction between TcTS and their acceptor substrates, we designed and synthesized a library of 20 benzyl lactosides substituted in C-6 of the glucose residue with a series of 1,2,3-triazole derivatives containing different aromatic substituents in the C-4 position. The library was prepared by alkyne-azide cycloaddition reaction catalyzed by Cu(I) ("click chemistry") between a benzyl ß-lactoside functionalized with an azide group in the C-6 position and a series of 2-propargyl phenyl ethers. Herein we analyzed the chromatographic behavior on high performance anion exchange chromatography (HPAEC) of the triazoyl-lactose derivatives and their activity as acceptors of TcTS and inhibitors of the sialylation of N-acetyllactosamine. The triazoyl derivatives were obtained with excellent yields and all of them behaved as moderate alternative substrates. The presence of bulky hydrophobic substituents dramatically increased the retention times in HPAEC but did not affect significantly their acceptor properties toward TcTS.

Multivalent assembly of a pyrene functionalized thio-N-acetylglucosamine: Synthesis, spectroscopic and WGA binding studies.

We introduce here a new fluorescent derivative of 1-thio-ß-N-acetylglucosamine linked to a pyrene system through a triazolylpentyl spacer, designed to self-assemble into a multivalent glycocluster. The synthesis was achieved by efficient CuAAC click reaction between a pyrene functionalized with an azide group and a suitable alkynyl thiomonosaccharide. Spectroscopic studies by fluorometry indicated that the self-assembly in aqueous medium is modulated by concentration and pH changes, the latter due to the presence of the amino group close to the π system. Circular dichroism experiments revealed a moderate positive signal, suggesting that the pyrene-thioGlcNAc conjugate can aggregate into a chiral supramolecular assembly. The sugar moiety showed to specifically and reversibly interact with the wheat germ agglutinin, a fact that was demonstrated by turbidity assay. SEM microscopy of a lyophilized solution at pH 10 revealed a fibrillar morphology compatible with the presence of tubular micelles, whereas crystalline and amorphous solids are formed at lower pHs.

Synthetic strategies for fluorination of carbohydrates.

This review article discusses different synthetic strategies for accomplishing regio- and stereoselective fluorinations of the sugar moiety, discussing the reaction mechanisms and some biological implications arising from such substitutions.

Synthesis, molecular docking and biological evaluation of novel phthaloyl derivatives of 3-amino-3-aryl propionic acids as inhibitors of Trypanosoma cruzi trans-sialidase.

In the last two decades, trans-sialidase of Trypanosoma cruzi (TcTS) has been an important pharmacological target for developing new anti-Chagas agents. In a continuous effort to discover new potential TcTS inhibitors, 3-amino-3-arylpropionic acid derivatives (series A) and novel phthaloyl derivatives (series B, C and D) were synthesized and molecular docking, TcTS enzyme inhibition and determination of trypanocidal activity were carried out. From four series obtained, compound D-11 had the highest binding affinity value (-11.1 kcal/mol) compared to reference DANA (-7.8 kcal/mol), a natural ligand for TS enzyme. Furthermore, the 3D and 2D interactions analysis of compound D-11 showed a hydrogen bond, π-π stacking, π-anion, hydrophobic and Van der Waals forces with all important amino acid residues (Arg35, Arg245, Arg314, Tyr119, Trp312, Tyr342, Glu230 and Asp59) on the active site of TcTS. Additionally, D-11 showed the highest TcTS enzyme inhibition (86.9% ±â€¯5) by high-performance ion exchange chromatography (HPAEC). Finally, D-11 showed better trypanocidal activity than the reference drugs nifurtimox and benznidazole with an equal % lysis (63 ±â€¯4 and 65 ±â€¯2 at 10 µg/mL) and LC50 value (52.70 ±â€¯2.70 µM and 46.19 ±â€¯2.36 µM) on NINOA and INC-5 strains, respectively. Therefore, D-11 is a small-molecule with potent TcTS inhibition and a strong trypanocidal effect that could help in the development of new anti-Chagas agents.

A Highly Sensitive Fluorogenic Probe for Imaging Glycoproteins and Mucine Activity in Live Cells in the Near-Infrared Region.

A novel fluorescent molecular probe is reported, which is able to detect glycoproteins, especially mucins, with high sensitivity and with a turn-on response along with a large Stokes shift (>130 nm), within the biologically active window. The probe contains an aminotricarbocyanine as the fluorescent reporter with a linked benzoboroxole as the recognition unit, which operates through a dynamic covalent reaction between the boronic hemiester residue of the receptor and cis-diols of the analyte. The superior selectivity of the probe is displayed by the labeling of mucins present in Calu-3 cells. The new benzoboroxole fluorescent derivative gathers together key properties to make it a highly rated molecular probe: specificity, excellent solubility in water, and off-on near infrared emission. This probe is expected to be an excellent tool for imaging intracellular mucin to evaluate mucus-related diseases as well as a sensing strategy towards glycosylated structures with a high potential for theranostics approaches in biological samples.

An in vitro and in vivo evaluation of new potential trans-sialidase inhibitors of Trypanosoma cruzi predicted by a computational drug repositioning method.

Chagas disease is one of the most important neglected parasitic diseases afflicting developed and undeveloped countries. There are currently limited options for inexpensive and secure pharmacological treatment. In this study, we employed a structure-based virtual screening protocol for 3180 FDA-approved drugs for repositioning of them as potential trans-sialidase inhibitors. In vitro and in vivo evaluations were performed for the selected drugs against trypomastigotes from the INC-5 and NINOA strains of T. cruzi. Also, inhibition of sialylation by the trans-sialidase enzyme reaction was evaluated using high-performance anion-exchange chromatography with pulse amperometric detection to confirm the mechanism of action. Results from the computational study showed 38 top drugs with the best binding-energies. Four compounds with antihistaminic, anti-hypertensive, and antibiotic properties showed better trypanocidal effects (LC50 range = 4.5-25.8 µg/mL) than the reference drugs, nifurtimox and benznidazole (LC50 range = 36.1-46.8 µg/mL) in both strains in the in vitro model. The anti-inflammatory, sulfasalazine showed moderate inhibition (37.6%) of sialylation in a trans-sialidase enzyme inhibition reaction. Sulfasalazine also showed the best trypanocidal effects in short-term in vivo experiments on infected mice. This study suggests for the first time that the anti-inflammatory sulfasalazine could be used as a lead compound to develop new trans-sialidase inhibitors.

Synthesis of ß-galactosylamides as ligands of the peanut lectin. Insights into the recognition process.

The synthesis of mono and divalent ß-galactosylamides linked to a hydroxylated chain having a C2 symmetry axis derived from l-tartaric anhydride is reported. Reference compounds devoid of hydroxyl groups in the linker were also prepared from ß-galactosylamine and succinic anhydride. After functionalization with an alkynyl residue, the resulting building blocks were grafted onto different azide-equipped scaffolds through the copper catalyzed azide-alkyne cycloaddition. Thus, a family of structurally related mono and divalent ß-N-galactopyranosylamides was obtained and fully characterized. The binding affinities of the ligands towards the model lectin PNA were measured by the enzyme-linked lectin assay (ELLA). The IC50 values were significantly higher than that of galactose but the presence of hydroxyl groups in the aglycone chain improved lectin recognition. Docking and molecular dynamics experiments were in accordance with the hypothesis that a hydroxyl group properly disposed in the linker could mimic the Glc O3 in the recognition process. On the other hand, divalent presentation of the ligands led to lectin affinity enhancements.