Synthesis, Structure and Reactions of Glycosyl Azides

Stereoselective synthesis of α-glycosyl azides: allyl glycosyl sulfones as radical precursors

Despite their critical importance in drug development and biochemistry, efficiently synthesizing α-glycosyl azides has continued to pose significant challenges. In this report, we introduce a universal and practical radical reaction for the stereoselective synthesis of α-glycosyl azides using bench-stable allyl glycosyl sulfones as the donor. This method is characterized by its mild reaction conditions, high stereoselectivity, and extensive scope of glycosyl units. Moreover, the accessibility of several structurally complex drug-sugar conjugates underscores the practicality of our approach.

Synthesis of C,N-Glycosides via Brønsted Acid-Catalyzed Azidation of exo-Glycals

The reaction of exo-glycals with azidotrimethylsilane in the presence of a Brønsted acid leads to the generation of the corresponding C,N-glycosyl azides. The majority of these glycosylation reactions proceed at room temperature with short reaction times. In addition, the targeted products were obtained in high yields with exclusive diastereoselectivity to the α-anomer in pyranose-based derivatives. Carbohydrate units based on mannose, galactose, arabinose, and ribose were also shown to proceed in high yields.

Mechanism-based inhibition of GH127/146 cysteine glycosidases by stereospecifically functionalized l-arabinofuranosides

To understand the precise mechanism of the glycoside hydrolase (GH) family 127, a cysteine β-l-arabinofuranosidase (Arafase) - HypBA1 - has been isolated from Bifidobacterium longum in the human Gut microbiota, and the design and synthesis of the mechanism-based inhibitors such as l-Araf-haloacetamides have been carried out. The α-l-Araf-azide derivative was used as the monoglycosylamine equivalent to afford the l-Araf-chloroacetamides (α/β-1-Cl) as well as bromoacetamides (α/β-1-Br) in highly stereoselective manner through Staudinger reaction followed by amide formation with/without anomerization. Against HypBA1, the probes 1, especially in the case of α/β-1-Br inhibited the hydrolysis. Conformational implications of these observations are discussed in this manuscript. Additional examinations using l-Araf-azides (α/β-5) resulted in further mechanistic observations of the GH127/146 cysteine glycosidases, including the hydrolysis of β-5 as the substrate and oxidative inhibition by α-5 using the GH127 homologue.

Chemoenzymatic and Protecting-Group-Free Synthesis of 1,4-Substituted 1,2,3-Triazole-α-d-glucosides with Potent Inhibitory Activity toward Lysosomal α-Glucosidase

α-Glucosyl triazoles have rarely been tested as α-glucosidase inhibitors, partly due to inefficient synthesis of their precursor α-d-glucosylazide (αGA1). Glycosynthase enzymes, made by nucleophile mutations of retaining β-glucosidases, produce αGA1 in chemical rescue experiments. Thermoanaerobacterium xylanolyticus glucosyl hydrolase 116 β-glucosidase (TxGH116) E441G nucleophile mutant catalyzed synthesis of αGA1 from sodium azide and pNP-β-d-glucoside (pNPGlc) or cellobiose in aqueous medium at 45 °C. The pNPGlc and azide reaction product was purified by Sephadex LH-20 column chromatography to yield 280 mg of pure αGA1 (68% yield). αGA1 was successfully conjugated with alkynes attached to different functional groups, including aryl, ether, amine, amide, ester, alcohol, and flavone via copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry reactions. These reactions afforded the 1,4-substituted 1,2,3-triazole-α-d-glucoside derivatives AGT2-14 without protection and deprotection. Several of these glucosyl triazoles exhibited strong inhibition of human lysosomal α-glucosidase, with IC50 values for AGT4 and AGT14 more than 60-fold lower than that of the commercial α-glucosidase inhibitor acarbose.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

On the anomeric preference of the isothiocyanato group

Anomeric effect of the isothiocyanato group has been quantified for the first time in xylopyranose triacetates; both anomers being synthesized.

Does targeting Arg98 of FimH lead to high affinity antagonists?

Bacterial resistance has become an important challenge in the treatment of urinary tract infections. The underlying resistance mechanisms can most likely be circumvented with an antiadhesive approach, antagonizing the lectin FimH located at the tip of fimbriae of uropathogenic E. coli. Here we report on a novel series of FimH antagonists based on the 1-(α-d-mannopyranosyl)-4-phenyl-1,2,3-triazole scaffold, designed to incorporate carboxylic acid or ester functions to interact with FimH Arg98. The most potent representative of the series, ester 11e, displayed a K_d value of 7.6 nM for the lectin domain of FimH with a general conclusion that all esters outperform carboxylates in terms of affinity. Surprisingly, all compounds from this new series exhibited improved binding affinities also for the R98A mutant, indicating another possible interaction contributing to binding. Our study on 1-(α-d-mannopyranosyl)-4-phenyl-1,2,3-triazole-based FimH antagonists offers proof that targeting Arg98 side chain by a "chemical common sense", i.e. by introduction of the acidic moiety to form ionic bond with Arg98 is most likely unsuitable approach to boost FimH antagonists' potency.

Bivalent glycoconjugates based on 1,5-diazabicyclo[3.3.0]octa-3,6-diene-2,8-dione ("bimane") as a central scaffold

The heteroaromatic fused diazabicyclic "bimane" ring system, discovered four decades ago, is endowed with remarkable chemical and photophysical properties. No carbohydrate derivatives of bimanes have, however, been described thus far. Here we report on the syntheses of a range of bimanes decorated with various glycosyl residues. Mono- and disaccharide residues were attached to syn- or anti-bimane central cores via thio-, disulfido- or selenoglycosidic linkages to obtain novel fluorescent or nonfluorescent glycoconjugates. Cu(I)-catalyzed cycloaddition of glycosyl azides to a bimane diethynyl derivative furnished further bivalent glycoconjugates with sugar residues linked to the central bimane core via 1,2,3-triazole rings. We have determined the crystal and molecular structures of several glycosylated and non-glycosylated bimanes and report fluorescence data for the new compounds.

AuBr3-catalyzed azidation of per-O-acetylated and per-O-benzoylated sugars

Herein we report, for the first time, the successful anomeric azidation of per-O-acetylated and per-O-benzoylated sugars by catalytic amounts of oxophilic AuBr3 in good to excellent yields. The method is applicable to a wide range of easily accessible per-O-acetylated and per-O-benzoylated sugars. While reaction with per-O-acetylated and per-O-benzoylated monosaccharides was complete within 1-3 h at room temperature, the per-O-benzoylated disaccharides needed 2-3 h of heating at 55 °C.

Conversion of glycals into vicinal-1,2-diazides and 1,2-(or 2,1)-azidoacetates using hypervalent iodine reagents and Me3SiN3. Application in the synthesis of N-glycopeptides, pseudo-trisaccharides and an iminosugar

Glycals react with PIFA (or PIDA)–TMSN₃in presence of TMSOTf to form sugar derived 1,2-diazides and vicinal azidoacetates. Synthesis of 2-azido-N-glycopeptides, pseudotrisaccharides, and a piperidine triol derivative is reported.

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

Photoresponsive carbohydrate-based giant surfactants: automatic vertical alignment of nematic liquid crystal for the remote-controllable optical device

Photoresponsive carbohydrate-based giant surfactants (abbreviated as CELAnD-OH) were specifically designed and synthesized for the automatic vertical alignment (VA) layer of nematic (N) liquid crystal (LC), which can be applied for the fabrication of remote-controllable optical devices. Without the conventional polymer-based LC alignment process, a perfect VA layer was automatically constructed by directly adding the 0.1 wt % CELA1D-OH in the N-LC media. The programmed CELA1D-OH giant surfactants in the N-LC media gradually diffused onto the substrates of LC cell and self-assembled to the expanded monolayer structure, which can provide enough empty spaces for N-LC molecules to crawl into the empty zones for the construction of VA layer. On the other hand, the CELA3D-OH giant surfactants forming the condensed monolayer structure on the substrates exhibited a planar alignment (PA) rather than a VA. Upon tuning the wavelength of light, the N-LC alignments were reversibly switched between VA and PA in the remote-controllable LC optical devices. Based on the experimental results, it was realized that understanding the interactions between N-LC molecules and amphiphilic giant surfactants is critical to design the suitable materials for the automatic LC alignment.

β-Xylopyranosides: synthesis and applications

In recent years, β-xylopyranosides have attracted interest due to the development of biomass-derived molecules. This review focuses on general routes for the preparation of β-xylopyranosides by chemical and enzymatic pathways and their main uses.

Synthesis and characterization of maltose-based amphiphiles as supramolecular hydrogelators

Low molecular mass amphiphilic glycolipids have been prepared by linking a maltose polar head and a hydrophobic linear chain either by amidation or copper(I)-catalyzed azide-alkyne [3 + 2] cycloaddition. The liquid crystalline properties of these amphiphilic materials have been characterized. The influence of the chemical structure of these glycolipids on the gelation properties in water has also been studied. Glycolipids obtained by the click coupling of the two components give rise to stable hydrogels at room temperature. The fibrillar structure of supramolecular hydrogels obtained by the self-assembly of these gelators have been characterized by electron microscopy. Fibers showed some torsion, which could be related with a chiral supramolecular arrangement of amphiphiles, as confirmed by circular dichroism (CD). The sol-gel transition temperature was also determined by differential scanning calorimetry (DSC) and NMR.

Iron(III) chloride as an efficient catalyst for stereoselective synthesis of glycosyl azides and a cocatalyst with Cu(0) for the subsequent click chemistry

A highly efficient and mild method for azido glycosylation of glycosyl β-peracetates to 1,2-trans glycosyl azides was developed by using inexpensive FeCl(3) as the catalyst. In addition, we demonstrated, for the first time, that FeCl(3) in combination with copper powder can promote 1,3-dipolar cycloaddition (click chemistry) of azido glycosides with terminal alkynes. Good to excellent yields were obtained with exclusive formation of a single isomer in both glycosyl azidation and subsequent cycloaddition processes.

RGD-xyloside conjugates prime glycosaminoglycans

Glycosaminoglycans (GAG) play decisive roles in various cardio-vascular & cancer-associated processes. Changes in the expression of GAG fine structures, attributed to deregulation of their biosynthetic and catabolic enzymes, are hallmarks of vascular dysfunction and tumor progression. The wide spread role of GAG chains in blood clotting, wound healing and tumor biology has led to the development of modified GAG chains, GAG binding peptides and GAG based enzyme inhibitors as therapeutic agents. Xylosides, carrying hydrophobic aglycone, are known to induce GAG biosynthesis in various systems. Given the important roles of GAG chains in vascular and tumor biology, we envision that RGD-conjugated xylosides could be targeted to activated endothelial and cancer cells, which are known to express α(v)β(3) integrin, and thereby modulate the pathological processes. To accomplish this vision, xylose residue was conjugated to linear and cyclic RGD containing peptides using click chemistry. Our results demonstrate that RGD-conjugated xylosides are able to prime GAG chains in various cell types, and future studies are aimed toward evaluating potential utility of such xylosides in treating myocardial infarction as well as cancer-associated thrombotic complications.

Total synthesis of N-acetylglucosamine-1,6-anhydro-N-acetylmuramylpentapeptide and evaluation of its turnover by AmpD from Escherichia coli

The bacterial cell wall is recycled extensively during the course of cell growth. The first recycling event involves the catalytic action of the lytic transglycosylase enzymes, which produce an uncommon 1,6-anhydropyranose moiety during separation of the muramyl residues from the peptidoglycan, the major constituent of the cell wall. This product, an N-acetyl-beta-D-glucosamine-(1-->4)-1,6-anhydro-N-acetyl-beta-D-muramylpeptide, is either internalized to initiate the recycling process or diffuses into the milieu to cause stimulation of the pro-inflammatory responses by the host. We report the total syntheses of N-acetyl-beta-D-glucosamine-(1-->4)-1,6-anhydro-N-acetyl-beta-D-muramyl-L-Ala-gamma-D-Glu-meso-DAP-D-Ala-D-Ala (compound 1, the product of lytic transglycosylase action on the cell wall of gram-negative bacteria) and N-acetyl-beta-D-glucosamine-(1-->4)-1,6-anhydro-N-acetyl-beta-D-muramyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala (compound 2, from lytic transglycosylase action on the cell wall of gram-positive bacteria). The syntheses were accomplished in 15 linear steps. Compound 1 is shown to be a substrate of the AmpD enzyme of the gram-negative bacterium Escherichia coli, an enzyme that removes the peptide from the disaccharide scaffold in the early cytoplasmic phase of cell wall turnover.

Design and Synthesis of Lipopeptide - Carbohydrate Assembled Multivalent Vaccine Candidates Using Native Chemical Ligation

Development of a synthetic vaccine against group A streptococcal infection is increasingly paramount due to the induction of autoimmunity by the main virulent factor – M protein. Peptide vaccines, however, are generally poorly immunogenic, necessitating administration with carriers and adjuvants. One of the promising approaches to deliver antigenic peptides is to assemble peptides on a suitable template which directs the attached peptides to form a well defined tertiary structure. For self-adjuvanting human vaccines, the conjugation of immunostimulatory lipids has been demonstrated as a potentially safe method. This study describes the design and optimized synthesis of two lipopeptide conjugated carbohydrate templates and the assembling of peptide antigens. These lipopeptide–carbohydrate assembled multivalent vaccine candidates were obtained in high yield and purity when native chemical ligation was applied. Circular dichroism studies indicated that the template-assembled peptides form four α-helix bundles. The developed technique extends the use of carbohydrate templates and lipopeptide conjugates for producing self-adjuvanting and topology-controlled vaccine candidates.

Development of a liposaccharide-based delivery system and its application to the design of group A streptococcal vaccines

Group A streptococcus (GAS) is associated with many human diseases, ranging in severity from benign to life-threatening. A promising strategy for developing vaccines against GAS involves the use of carbohydrates as carriers for peptide antigens. This study describes the optimized synthesis of d-glucose and d-galactose derived carriers, bearing an adipate linker and four tert-butoxycarbonyl protected aminopropyl groups. Prophylactic GAS vaccine candidates were synthesized by conjugating multiple copies of a single GAS M protein derived peptide antigen (either J8 or J14) onto the carbohydrate carriers. These antigens contain peptide sequences, which are highly conserved and offer the potential to prevent infections caused by up to 70% of GAS strains. Lipophilic amino acids were also conjugated to the d-glucose anomeric carbon to produce a self-adjuvanting liposaccharide vaccine. High serum IgG antibody titers against each of the incorporated peptide epitopes were detected following subcutaneous immunization of B10.BR (H-2 (k)) mice with the liposaccharide vaccine candidates.

Synthesis of furanosyl α-C-glycosides derived from 4-chloro-4-deoxy-α-d-galactose and their cytotoxic activities

Condensation of a new unnatural sugar 1 with 1,3-dicarbonyl compounds in the presence of anhydrous zinc chloride gave the polyhydroxyalkyl-furans in excellent yields. Further modification afforded the corresponding furanosyl alpha-C-glycoside derivatives. The absolute configuration of 3-acetyl-2-methyl-5-(2'-chloro-D-galacto-tetritol-1-yl)-furan was confirmed by single-crystal X-ray analysis. The in vitro cytotoxic activities of these furanosyl C-glycosides were also investigated.

A facile stereoselective synthesis of α-glycosyl ureas

Alpha-glycosyl ureas can be synthesised directly from tetra-O-benzyl glycosyl azides and isocyanates, using a one-pot procedure that is simple and general in scope. The benzyl protecting groups are easily removed from the urea products by catalytic hydrogenation. The synthesised alpha-glycosyl ureas represent a new class of neo-glycoconjugates with the potential of being resistant towards carbohydrate processing enzymes.

Synthesis of N-(β-d-glucopyranosyl) monoamides of dicarboxylic acids as potential inhibitors of glycogen phosphorylase

O-peracetylated N-(beta-D-glucopyranosyl)imino trimethylphosphorane obtained in situ from 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl azide and PMe3 was reacted with saturated and unsaturated aliphatic and aromatic dicarboxylic acids, or their anhydrides, or monoesters to give the corresponding N-(beta-D-glucopyranosyl) monoamides of dicarboxylic acids or derivatives. The acetyl protecting groups were removed according to the Zemplén protocol to give a series of compounds which showed moderate inhibitory effects against rabbit muscle glycogen phosphorylase b. The best inhibitor was 3-(N-beta-D-glucopyranosyl-carbamoyl)propanoic acid (7) with Ki = 20 microM.

Traceless Staudinger Ligation of Glycosyl Azides with Triaryl Phosphines: Stereoselective Synthesis of Glycosyl Amides

Alpha-glycosyl amides can be synthesized from the corresponding O-benzyl-alpha-glycosyl azides using a traceless Staudinger ligation with diphenylphosphanyl-phenyl esters 4. All the phosphines employed and their phenol precursors are stable to air at 4 degrees C for months. Fast intramolecular trapping of the reduction intermediates results in the direct formation of the amide link, which, in turn, prevents epimerisation and allows retention of configuration at the anomeric carbon. Yields and alpha-selectivity are high when the reaction is performed in polar aprotic solvents. Removal of the benzyl ether protecting groups is achieved by catalytic hydrogenation. Alpha-glycosyl amides represent a class of virtually unexplored nonhydrolyzable monosaccharide derivatives that may find a useful application as sugar mimics. Conformational studies by NMR spectroscopy confirm that deprotected alpha-glycosyl amides in the gluco, galacto, and fuco series retain the normal pyranose conformation of the monosaccharide. The reaction of phosphines 4 with tetra-O-acetyl-glycosyl azides is nonstereoconservative, and beta-glycosyl amides are obtained in good yields and with complete stereoselectivity starting from both alpha and beta azides.