Konformationsanalyse, III.exo-Anomerer Effekt und Circulardichroismus von Glycopyranosylaziden

Recent progress in the synthesis of natural product inspired bioactive glycohybrids

Carbohydrates are a basic structural component that are indispensable to all cellular processes. In addition to being employed as chiral starting materials in the synthesis of a variety of natural products, carbohydrates are recognized as naturally occurring molecules having an enormous variety of functional, stereochemical, and structural properties. The understanding and biological roles of carbohydrate derived molecules can be greatly improved by selectively synthesizing functional carbohydrates through incorporating them with privileged scaffolds. For a deeper understanding of their roles and the development of functional materials based on sugar, it is crucial to develop new techniques for efficiently synthesizing, functionalizing, and modifying carbohydrates. Glycohybrids have a wide range of structural and functional characteristics along with protein-carbohydrate interactions that are crucial to mammalian biology and a number of disease states. This review, consisting the literature from January 2017 to July 2023 and provide an overview of recent developments in the chemical synthesis of glycohybrids based on natural product scaffolds of coumarin, quinolone, naphthalene diimide, indole, isatin, naphthoquinone, imidazole and pyrimidine. The biological activity of active glycohybrids are discussed in this review.

Stereoselective synthesis of glycosyl azides from anomeric hydroxides via protecting group manipulations

Herein, we report the direct conversion of anomeric hydroxides to glycosyl azides in one step using diphenylphosphoryl azide. Protecting group manipulations on the hexose sugars have enabled the stereoselective synthesis of either the α-glycosyl azides or the β-anomeric azides in moderate to very good yields. The reaction has also been successfully used to enable the synthesis of β-2-deoxy-2-aminoglucosyl azides.

Carbohydrate-Functionalized Triazolylidene Iridium Complexes: Hydrogenation Catalysis in Water with Asymmetric Induction

Two sets of carbohydrate-NHC hybrid iridium complexes were synthesised in order to combine properties of carbohydrates and triazolylidene (trz) ligands in organometallic catalysis. One set features a direct trz linkage to the anomeric carbohydrate carbon, while the second set is comprised of an ethyl linker between the two functional units. Deprotection of the carbohydrate afforded hybrid complexes that efficiently catalyse the direct hydrogenation of ketones in water. The catalytic activity of the hybrid complexes was influenced by the pH of the aqueous medium and surpassed the activity of carbohydrate-free or acetyl-protected analogues (>90 % vs 13 % yield). While no enantiomeric induction was observed for the ethyl-linked hybrids, a moderate enantiomeric excess (ee) was induced by the directly linked systems. Moreover, these carbohydrate-trz hybrid complexes displayed mixed inhibitory activity towards a glycosidase from H. orenii that contain a glucose binding site.

Ruthenium Half-Sandwich Type Complexes with Bidentate Monosaccharide Ligands Show Antineoplastic Activity in Ovarian Cancer Cell Models through Reactive Oxygen Species Production

Ruthenium complexes are developed as substitutes for platinum complexes to be used in the chemotherapy of hematological and gynecological malignancies, such as ovarian cancer. We synthesized and screened 14 ruthenium half-sandwich complexes with bidentate monosaccharide ligands in ovarian cancer cell models. Four complexes were cytostatic, but not cytotoxic on A2780 and ID8 cells. The IC₅₀ values were in the low micromolar range (the best being 0.87 µM) and were similar to or lower than those of the clinically available platinum complexes. The active complexes were cytostatic in cell models of glioblastoma, breast cancer, and pancreatic adenocarcinoma, while they were not cytostatic on non-transformed human skin fibroblasts. The bioactive ruthenium complexes showed cooperative binding to yet unidentified cellular target(s), and their activity was dependent on reactive oxygen species production. Large hydrophobic protective groups on the hydroxyl groups of the sugar moiety were needed for biological activity. The cytostatic activity of the ruthenium complexes was dependent on reactive species production. Rucaparib, a PARP inhibitor, potentiated the effects of ruthenium complexes.

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.

Ruthenium-centred btp glycoclusters as inhibitors for Pseudomonas aeruginosa biofilm formation

Carbohydrate-decorated clusters (glycoclusters) centred on a Ru(ii) ion were synthesised and tested for their activity against Pseudomonas aeruginosa biofilm formation. These clusters were designed by conjugating a range of carbohydrate motifs (galactose, glucose, mannose and lactose, as well as galactose with a triethylene glycol spacer) to a btp (2,6-bis(1,2,3-triazol-4-yl)pyridine) scaffold. This scaffold, which possesses a C₂ symmetry, is an excellent ligand for d-metal ions, and thus the formation of the Ru(ii)-centred glycoclusters 7 and 8Gal was achieved from 5 and 6Gal; each possessing four deprotected carbohydrates. Glycocluster 8Gal, which has a flexible spacer between the btp and galactose moieties, showed significant inhibition of P. aeruginosa bacterial biofilm formation. By contrast, glycocluster 7, which lacked the flexible linker, didn't show significant antimicrobial effects and neither does the ligand 6Gal alone. These results are proposed to arise from carbohydrate–lectin interactions with LecA, which are possible for the flexible metal-centred multivalent glycocluster. Metal-centred glycoclusters present a structurally versatile class of antimicrobial agent for P. aeruginosa, of which this is, to the best of our knowledge, the first example.

Ruthenium-centred glycoclusters based on carbohydrate-functionalised bis(triazolyl)pyridine ligands show Pseudomonas aeruginosa biofilm inhibition, with activity that is dependent on ligand structure.

Low-budget 3D-printed equipment for continuous flow reactions

This article describes the development and manufacturing of lab equipment, which is needed for the use in flow chemistry. We developed a rack of four syringe pumps controlled by one Arduino computer, which can be manufactured with a commonly available 3D printer and readily available parts. Also, we printed various flow reactor cells, which are fully customizable for each individual reaction. With this equipment we performed some multistep glycosylation reactions, where multiple 3D-printed flow reactors were used in series.

Carbohydrate-functionalized N-heterocyclic carbene Ru(ii) complexes: synthesis, characterization and catalytic transfer hydrogenation activity

Triazolylidene NHCs decorated with a carbohydrate wingtip group were complexed to a ruthenium(ii) center. Deprotection of the carbohydrate in the metal complex affords a carbohydrate–NHC hybrid system for use as a transfer hydrogenation catalyst.

Synthesis of new triazole linked carbohybrids with ROS-mediated toxicity in breast cancer

Carbohybrids are an important class of molecules which exhibit diverse biological activities. New coumarins and quinolones linked carbohybrids are synthesised which are showing selective anticancer activity.

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.

A prodrug approach toward cancer-related carbonic anhydrase inhibition

The selective inhibition of cancer-associated human carbonic anhydrase (CA) enzymes, specifically CA IX and XII, has been validated as a mechanistically novel approach toward personalized cancer management. Herein we report the design and synthesis of a panel of 24 novel glycoconjugate primary sulfonamides that bind to the extracellular catalytic domain of CA IX and XII. These compounds were synthesized from variably acylated glycopyranosyl azides and either 3- or 4-ethynyl benzene sulfonamide using Cu(I)-catalyzed azide alkyne cycloaddition (CuAAC). The CA enzyme inhibition profile for all compounds was determined, while in vitro metabolic stability, plasma stability, and plasma protein binding for a representative set of compounds was measured. Our findings demonstrate the influence of the differing acyl groups on these key biopharmaceutical properties, confirming that acyl group protected carbohydrate-based sulfonamides have potential as prodrugs for selectively targeting the extracellular cancer-associated CA 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.

Determination of the Anomeric Configurations of 2,3,4,6-Tetra-O-Acetyl-D-Mannopyranosyl Azide

The structures of 2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl azide and 2,3,4,6-tetra-O-acetyl-β-d-mannopyranosyl azide were determined using X-ray crystallographic and one-dimensional NOESY techniques.

Synthesis of N-(beta-D-glucopyranosyl)- and N-(2-acetamido-2-deoxy-beta-D-glucopyranosyl) amides as inhibitors of glycogen phosphorylase

2,3,4,6-Tetra-O-acetyl-beta-D-glucopyranosyl- and 2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-glucopyranosyl azides were transformed into the corresponding per-O-acetylated N-(beta-D-glycopyranosyl) amides via a PMe(3) mediated Staudinger protocol (generation of N-(beta-D-glycopyranosyl)imino-trimethylphosphoranes followed by acylation with carboxylic acids, acid chlorides or anhydrides). The deprotected compounds obtained by Zemplén deacetylation were evaluated as inhibitors of rabbit muscle glycogen phosphorylase b. The best inhibitor of this series has been N-(beta-D-glucopyranosyl) 3-(2-naphthyl)-propenoic amide (K(i)=3.5microM).

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.

Convenient syntheses of symmetrical and unsymmetrical glycosyl carbodiimides and N,N-bis(glycosyl)cyanamides

Reaction of glycosyl trimethylphosphinimides with carbon disulfide under mild conditions (room temperature, short reaction time) leads to symmetrical glycosyl carbodiimides. Addition of bis(trimethylsilyl)carbodiimide to peracetylated aldoses under the influence of SnCl(4) afforded N,N-bis(glycosyl)cyanamides for the first time. Readily accessible unsymmetrical N,N'-bis(glycosyl)thioureas can be desulfurated and transformed into the corresponding carbodiimides using HgO in CHCl(3)/water at room temperature.

Investigation of the active site of the extracellular beta-D-xylosidase from Aspergillus carbonarius

The catalytic amino acid residues of the extracellular beta-D-xylosidase (beta-D-xyloside xylohydrolase, EC 3.2.1.37) from Aspergillus carbonarius was investigated by the pH dependence of reaction kinetic parameters and chemical modifications of the enzyme. The pH dependence curves gave apparent pK values of 2.7 and 6.4 for the free enzyme, while pK value of 4.0 was obtained for the enzyme-substrate complex using p-nitrophenyl beta-D-xyloside as a substrate. These results suggested that a carboxylate group and a protonated group--presumably a histidine residue--took part in the binding of the substrate but only a carboxylate group was essential in the substrate cleavage. Carbodiimide- and Woodward's reagent K-mediated chemical modifications of the enzyme also supported that a carboxylate residue, located in the active center, was fundamental in the catalysis. The pH dependence of inactivation revealed the involvement of a group with pK value of 4.4, proving that a carboxylate residue relevant for hydrolysis was modified. During modification V(max) decreased to 10% of that of the unmodified enzyme and K(m) remained unchanged, supporting that the modified carboxylate group participated in the cleavage and not in the binding of the substrate. We synthesized and tested a new, potential affinity label, N-bromoacetyl-beta-d-xylopyranosylamine for beta-D-xylosidase. The A. carbonarius beta-D-xylosidase was irreversible inactivated by N-bromoacetyl-beta-D-xylopyranosylamine. The competitive inhibitor beta-D-xylopyranosyl azide protected the enzyme from inactivation proving that the inactivation took place in the active center. Kinetic analysis indicated that one molecule of reagent was necessary for inactivation of one molecule of the enzyme.

Stereoselective syntheses of α-amino acid and peptide derivatives by the U-4CR of 5-desoxy-5-thio-D-xylopyranosylamine

Since 1961, the synthesis of α-amino acids derivatives by the four-component reaction of isocyanides (U-4CR) as a one-pot reaction has been developed. Only recently it was found that a variety of these α-amino acids compounds can be formed stereoselectively by the U-4CR using 1-amino-5-deoxy-5-thio-2,3,4-tri-O-isobutanoyl-β-D-xylopyranose as the amine component. The stereoselectivity inducing auxiliary 5-desoxy-5-thio-D-xylopyranosyl group of the so-formed products can be replaced selectively by hydrogen.Key words: stereoselective U-4CR, chiral amine component, amino carbohydrate, α-amino acid derivatives.

Reaction of D-glycals with azidotrimethylsilane

The treatment of 2,3,4,6-tetra-O-acetyl-1,5-anhydro-D-arabino-hex-1-enitol with azidotrimethylsilane by the aid of a catalytic amount of Yb(OTf)(3) afforded 2,4,6-tri-O-acetyl-2,3-dideoxy-alpha-D-erythro-hex-2-enopyranosyl azide in high yield.

Studies on the origin of stereoselectivity in the synthesis of 1,2-trans glycofuranosyl azides

The stereoselectivity of the 1,2-trans directed, Lewis acid-catalysed azidation of peracylated furanoses was found to depend on the reactivity of the azide donor (azide nucleophilicity) and the configuration at the anomeric centre relative to the neighbouring 2-O-acyl group. Reactions of 1,2-trans glycosyl esters with highly nucleophilic azide donors, generated from SnCl4 and Me3SiN3, were stereospecific. The results are interpreted in terms of the rapid reaction of the azide species with bicyclic 1,2-acyloxonium (1,2-O-alkyliumdiyl-D-glycofuranose) ions, which were the primarily formed reactive intermediates. When using 1,2-cis glycosyl esters as starting materials the selectivity was reduced (90-94% de); the same is true with 1,2-trans counterparts if less nucleophilic Me3SiN3 in combination with Me3SiOTf catalyst was used. This occurred due to the appearance of the more reactive but less selective oxocarbenium (glycofuranoxonium) ions either as primarily formed reactive intermediates in the former case or after equilibration with acyloxonium ions in the latter case. Protected 1,2-trans beta-D-glycofuranosyl azides with ribo, xylo and 3-deoxy-erythro-pento configurations were best prepared from the corresponding glycosyl esters using 0.05 equivalents of SnCl4, i.e., under anomerization-free conditions. Azidation of methyl glycofuranosides proceeds with inferior (80-90% de) and less predictable selectivity irrespective of the starting anomeric configuration.