A simple method for the preparation of thioglycosides of N-acetylneuraminic acid

Triflic acid-mediated synthesis of thioglycosides

An efficient synthesis of thioglycosides from per-acetates in the presence of triflic acid is described. The developed protocol features high reaction rates and product yields. Some reactive sugar series give high efficiency in the presence of sub-stoichiometric trifluoromethanesulfonic acid (TfOH) in contrast to other known protocols that require multiple equivalents of Lewis acids to reach high conversion rates.

Synthesis and Evaluation of 1,5-Dithia-d-laminaribiose, Triose, and Tetraose as Truncated β-(1→3)-Glucan Mimetics

The preparation and characterization of a series of di-, tri-, and tetrasaccharide analogues of β-(1→3)-glucans is described in which each pyranoside ring is replaced by a 5-thiopyranosyl ring and each glycosidic oxygen by a thioether. These oligomeric 1,5-dithio-d-glucopyranose derivatives were shown to inhibit the staining of human neutrophils and of mouse macrophages by fluorescent anti-CR3 and anti-Dectin-1 antibodies, respectively. The compounds were also demonstrated to stimulate phagocytosis and pinocytosis indicative of binding to the carbohydrate binding domains of complement receptor 3 (CR3) and Dectin-1. Activity in all three assays was optimum at the level of the trisaccharide mimic, suggesting that, while the replacement of ethereal oxygens by thioethers results in a greater affinity for the aromatic lined hydrophobic binding pockets, the presence of multiple longer C-S bonds eventually results in a mismatch and a loss of affinity.

Second generation of thiazolylmannosides, FimH antagonists for E. coli-induced Crohn's disease

The chemical stability of potentE. colianti-adhesives was improved by substitution of the anomeric nitrogen by short linkers.

Synthesis and Biological Evaluation of Thioglycosylated meso-Arylporphyrins

A new class of porphyrins bearing thioglycosyl groups is presented in order to improve targeting of malignant cells and resistance to glycosidases. These compounds were synthesized in four steps. They produced 1 O 2 and their photocytotoxicity against the yeast Saccharomyces cerevisiae was compared with the effect of haematoporphyrin.

Synthesis and biological evaluation of galactofuranosyl alkyl thioglycosides as inhibitors of mycobacteria

As part of our research interest directed toward the development of antimycobacterial agents, we have investigated compounds based on galactofuranose (Galf), an essential cell wall component of mycobacteria. The objective of this study was to explore structure activity relationships of Galf thioglycosides with straight chain and branched aglycons. Acylated Galf 9-heptadecyl thioglycoside was prepared by Lewis acid-catalyzed thioglycosidation of 1,2,3,5,6-penta-O-acyl-D-galactofuranose with 9-heptadecanethiol, and subsequently converted to the corresponding sulfone using m-CPBA. Both Galf 9-heptadecyl thioglycoside and sulfone displayed in vitro inhibition (MIC) of the growth of Mycobacterium smegmatis below 5 microg/mL, while Galf 1-octyl thioglycoside gave no inhibition at or below 32 microg/mL.

Modelling, synthesis and biological evaluation of novel glucuronide-based probes of Vibrio cholerae sialidase

The development of sialidase inhibitors is an area of continuing interest due to their potential use as therapeutic agents to combat viral and bacterial infections. Herein, we report our studies involving the sialidase from the pathogen Vibrio cholerae, through the modelling, synthesis and biological evaluation of mimetics of 5-acetamido-2,6-anhydro-3,5-dideoxy-D-glycero-D-galacto-non-2-enonic acid (Neu5Ac2en, 1), a naturally occurring sialidase inhibitor. These mimetics are O- and S-glycosides of N-acetyl-D-glucosaminuronic acid in which the aglycone portion effectively replaces the C-6 glycerol side chain of Neu5Ac2en (1). The choice of aglycones was aided by use of the X-ray crystal structure of V. cholerae sialidase complexed with Neu5Ac2en (1). All Neu5Ac2en mimetics tested were found to inhibit V. cholerae sialidase as determined using a standard fluorometric assay.

S-Linked Ganglioside Analogues for Use in Conjugate Vaccines

[structure: see text] Glycosidase resistant thioglycoside precursors of the melanoma-associated ganglioside GM(2) have been synthesized starting from lactose. Syntheses of several analogues of ganglioside GM(3) and a positional isomer have been developed. These compounds contain thio-linked sialic acid residues and a modified ceramide aglycon functionalized for coupling to proteins, surfaces, or matrices. The hydrolytic stability of these oligosaccharides enhances the immunogenicity of the corresponding conjugate vaccines by ensuring their integrity in the acidic compartments of antigen processing cells.

Synthesis and biological evaluation of thioglycosylated porphyrins for an application in photodynamic therapy

The aim of this work is the synthesis of a new family of glycosylated porphyrins in which the sugar moieties are linked to the tetrapyrrole ring by a thioglycosidic bond. Two series have been designed. The first one corresponds to meso-aryl porphyrin derivatives. The second one has been obtained from protoporphyrin IX derivatization. Aryl-porphyrins were prepared from tristolyl o- and p-hydroxyporphyrins followed by bromoallylation and thioglycosylation with peracetylated S-glucose, mannose and galactose and deprotection. The other series has been synthesized from protoporphyrin IX dimethylester with a regioselective glycosylation of terminal alkenyl carbon. The UV-visible, NMR and MALDI mass spectra are presented. Photocytotoxicities of the synthesized compounds against K562 chronic leukaemia cell line has been evaluated.

Synthesis and biological evaluation of sialylmimetics as rotavirus inhibitors

Rotaviruses cause severe gastroenteritis in infants and are estimated to be responsible for over 600 000 deaths annually, primarily in developing countries. The development of potential inhibitors of this virus is therefore of great interest, particularly since the safety and efficacy of rotaviral vaccines has recently been questioned. This study describes the synthesis of a variety of compounds that can be considered as mimetics of N-acetylneuraminic acid thioglycosides and the subsequent in vitro biological evaluation of these sialylmimetics as inhibitors of rotaviral infection. Our results show that readily accessible carbohydrate-based compounds have the potential to act as inhibitors of rotaviral replication in vitro, presumably through inhibition of the rotaviral adhesion process.

Synthesis of novel sialylmimetics as biological probes

Glycomimetics are increasingly being recognised as powerful tools in the search for novel compounds that possess useful biological properties. This paper describes our preliminary efforts towards the development of novel mimetics of sialic acid thioglycosides. These sialylmimetics are readily prepared and have been shown, in some instances, to have biological properties similar to sialic acid thioglycosides.

Mixed-linkage cellooligosaccharides: a new class of glycoside hydrolase inhibitors

A new class of inhibitors for beta-D-glycoside hydrolases, in which a single alpha-(1-->4)-glycosidic bond is incorporated into an otherwise all-beta-(1-->4)-linked oligosaccharide, is described. Such mixed beta/alpha-linkage cellooligosaccharides are not transition-state mimics, but instead are capable of utilising binding energy from numerous subsites, spanning either side of the catalytic centre, without the need for substrate distortion. This binding is significant; a mixed alpha/beta-D-tetrasaccharide acts competitively on a number of cellulases, displaying inhibition constants in the range of 40-300 microM. Using the Bacillus agaradhaerens enzyme Cel5A as a model system, one such mixed beta/alpha-cellooligosaccharide, methyl 4(II),4(III)-dithio-alpha-cellobiosyl-(1-->4)-beta-cellobioside, displays a K(i) value of 100 microM, an inhibition at least 150 times better than is observed with an equivalent all-beta-linked compound. The three-dimensional structure of B. agaradhaerens Cel5A in complex with methyl 4(II),4(III)-dithio-alpha-cellobiosyl-(1-->4)-beta-cellobioside has been determined at 1.8 A resolution. This confirms the expected mode of binding in which the ligand, with all four pyranosides in the (4)C(1) chair conformation, occupies the -3, -2 and +1 subsites whilst evading the catalytic (-1) subsite. Such "by-pass" compounds offer great scope for the development of a new class of beta-D-glycoside hydrolase inhibitors.

The synthesis of biotinylated carbohydrates as probes for carbohydrate-recognizing proteins

The intimate involvement of carbohydrate-protein interactions in a number of important biological processes has prompted several research efforts towards developing new methods of investigating these glycobiological interactions. Biotinylated oligosaccharides are emerging as a new and powerful tool in this area of research, primarily due to their high affinity towards streptavidin and their ease of immobilization on matrices. Here we describe a novel synthetic approach towards biotinylated saccharides which incorporate a UV absorbing group into the final compounds. The synthetic strategy described is applicable to a variety of saccharides, with examples of biotinylated mono-, di-, and trisaccharides being prepared with overall high efficiency.

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.

A one-pot synthesis of novel N,N-dialkyl-S-glycosylsulfenamides

An efficient new entry into N,N-dialkyl-S-glycosylsulfenamides is reported. The reaction of bis-activated alkyl halides in the presence of a secondary amine base with glycosylic S-acetyl derivatives (1-S-acetyl-1-thioaldoses or 2-S-acetyl-2-thioketoses) results in the formation of novel carbohydrate sulfonamides. These new carbohydrate-based sulfonamides may provide useful derivatives with biological activity, as well as provide reactive carbohydrate sulfonylating agents.

Synthesis of a polymeric 4-N-linked sialoside which inhibits influenza virus hemagglutinin

A multiple sialic acid-bearing polymer 7 has been made in which a novel 4-N-substituted sialoside 5 has been coupled to polyacrylamide. The conjugate 7 has been found to inhibit the agglutination of influenza virus to red blood cells with HAI inhibition constants of around 10(-6) M, based on the sialic acid concentration.

The synthesis and evaluation of novel sialic acid analogues bound to matrices for the purification of sialic acid-recognising proteins

A novel N-acetylneuraminic acid analogue, 2-S-(5'-aminopentyl) 5-acetamido-3,5-dideoxy-2-thio-D-glycero-alpha-D-galacto-2- nonulopyranosidonic acid, as well as the thiosialoside 2-S-(2'-aminoethyl) 5-acetamido-3,5-dideoxy-2-thio-D-glycero-alpha-D-galacto-2- nonulopyranosidonic acid, have been synthesised and successfully coupled to CNBr-activated Sepharose 4B through the terminal amino group. The resultant affinity resins have proved efficient in purifying a number of sialic acid-recognising proteins such as Vibrio cholerae sialidase, sialidase-L from leech, trans-sialidase from Trypanosoma cruzi, and sialyltransferases from rat liver, all in high yield.

Investigation of the stability of thiosialosides toward hydrolysis by sialidases using NMR spectroscopy

[formula: see text] 1H NMR spectroscopy has been used to investigate whether the alpha(2-->6)-linked thiosialoside 3 and the alpha(2-->3)-linked thiosialoside 9 are hydrolyzed in the presence of Vibrio cholerae sialidase. Similarly, the hydrolysis of the O-ketosides Neu5Ac-2-O-alpha-(2-->3)-Gal beta Me (4) and the alpha-(2-->6)-sialyllactoside 7, representing natural alpha(2-->3)- and alpha(2-->6)-linked sialosides, respectively, was investigated. The results of the 1H NMR experiments clearly demonstrate that the thiosialosides are not hydrolyzed by Vibrio cholerae sialidase. As expected, the O-sialosides are hydrolyzed to give N-acetyl-alpha-D-neuraminic acid as the first product of substrate cleavage.

Solid-phase extraction on C18 silica as a purification strategy in the solution synthesis of a 1-thio-beta-D-galactopyranoside library

A novel strategy for the purification of carbohydrate-based chemical libraries synthesized in solution was developed. Purification of reaction products was accomplished by means of solid-phase extraction enabled by protecting the 2-, 3-, 4-, and 6-hydroxyl groups of a galactose derivative as their hydrophobic O-laurates. The presence of multiple O-laurates allowed adsorption of reaction products onto C18 silica while reagents and by-products were washed away with MeOH. Products were quantitatively eluted with pentane. Purification of products using solid-phase extraction offers the combined advantages of solution synthesis (normal solution reactivity and ease of reaction monitoring) with those of solid-phase synthesis (facile product isolation permitting the use of large excesses of reagents). To demonstrate the utility of the hydrophobic recovery-procedure, tetra-O-lauroyl-beta-D-galactopyranose-1-thiol was subjected to high-yielding reactions with a panel of Michael-acceptors and an alpha-chloro ketone. The resulting ketone adducts were then either reduced to the alcohols or reductively aminated with a selection of amino acids to give 30 different 1-thio-beta-D-galactosides as mixtures of four diastereomers after removal of protecting groups. At each step, the product was separated from the reagents and their by-products by simple adsorption onto C18 silica, washing with MeOH and elution of product with pentane. After completion of the combinatorial chemistry sequence, the O-laurates were cleaved by methanolysis and the product methyl laurate in turn removed from the desired water-soluble products by C18 adsorption. Individual library members were thus conveniently produced on 10-30 mg scales at purity levels of > 90%. One of the 1-thio-beta-D-galactosides thus produced was found to be a competitive inhibitor of the beta-galactosidase from E. coli with Ki value of 1.7 microM.

Synthesis and evaluation of N-acetylneuraminic acid-based affinity matrices for the purification of sialic acid-recognizing proteins

The synthesis of 2-S-(2-aminoethyl) 5-acetamido-3,5-dideoxy-2-thio-D-galacto-2-nonulopyranosidonic acid (1) has been successfully achieved from the precursors methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-2-S-acetyl-3,5-dideoxy-2-thio-D-glyce ro-alpha-D-galacto-2-nonulopyranosonate (2) and 2-bromo-N-(tert-butoxycarbonyl)-ethylamine (5). Compounds 1 and 2 were coupled, via amino and thioglycosidic linkages, respectively, to epoxy-activated Sepharose 6B. The resultant affinity adsorbents have proved efficient in purifying the sialic acid-recognizing enzyme Vibrio cholerae sialidase, in a one-step process with yields in the order of 60%.

Design and chemoenzymatic synthesis of thiooligosaccharide inhibitors of 1,3:1,4-beta-D-glucanases

A successful chemoenzymatic synthesis of oligosaccharides with an interglucosidic sulfur atom as inhibitors of 1,3:1,4-D-glucanases is described. The key compound 3a was synthesized from acetylated 1-thio-beta-laminaribiose 4 and the methyl 4'-O-triflyl-lactoside 5. After de-O-acylation, the tetrasaccharide 3b was used as an acceptor and glucose-1-P as a donor in a phosphorolytic elongation catalysed by cellodextrin phosphorylase from Clostridium thermocellum. The expected pentasaccharide 2a and hexasaccharide 1 were isolated in 56% and 13% yield, respectively. As expected, the thiooligosaccharides 1, 2a, and 3b were resistant to enzymatic cleavage by 1,3:1,4-beta-D-glucanase isolated from Bacillus licheniformis. Furthermore, they have been shown to act as competitive inhibitors of the hydrolysis of the chromophoric trisaccharide substrate 11 by this enzyme.

Synthesis and biological evaluation of N-acetylneuraminic acid-based rotavirus inhibitors

Rotavirus can cause severe gastrointestinal disease, especially in infants and young children, and is particularly prevalent in Third-World countries. Therefore, the development of potential inhibitors of this virus is of great interest. The present study describes the synthesis and in vitro biological evaluation of a number of N-acetylneuraminic acid-based compounds as potential rotavirus inhibitors. Our data suggests that it is indeed possible to inhibit adhesion of the virus, and hence in vitro replication, with carbohydrate-based molecules, although this inhibition does appear to be strain dependent.