Synthesis of protoescigenin glycoconjugates with O-28 triazole linker

New triazole linked conjugates were obtained from protoescigenin monopropargyl ethers and sugar azides, under Cu(II) salt promotion in good yield, without losing isopropylidene protection.

Electrochemical synthesis of glycoconjugates from activated sterol derivatives

Several derivatives of cholesterol and other 3β-hydroxy-Δ(5)-steroids were prepared and tested as sterol donors in electrochemical reactions with sugar alcohols. The reactions afforded glycoconjugates with sugar linked to a steroid moiety by an ether bond. Readily available sterol diphenylphosphates yielding up to 54% of the desired glycoconjugate were found to be the best sterol donors.

Integrating ReSET with glycosyl iodide glycosylation in step-economy syntheses of tumor-associated carbohydrate antigens and immunogenic glycolipids

Carbohydrates mediate a wide range of biological processes, and understanding these events and how they might be influenced is a complex undertaking that requires access to pure glycoconjugates. The isolation of sufficient quantities of carbohydrates and glycolipids from biological samples remains a significant challenge that has redirected efforts toward chemical synthesis. However, progress toward complex glycoconjugate total synthesis has been slowed by the need for multiple protection and deprotection steps owing to the large number of similarly reactive hydroxyls in carbohydrates. Two methodologies, regioselective silyl exchange technology (ReSET) and glycosyl iodide glycosylation have now been integrated to streamline the synthesis of the globo series trisaccharides (globotriaose and isoglobotriaose) and α-lactosylceramide (α-LacCer). These glycoconjugates include tumor-associated carbohydrate antigens (TACAs) and immunostimulatory glycolipids that hold promise as immunotherapeutics. Beyond the utility of the step-economy syntheses afforded by this synthetic platform, the studies also reveal a unique electronic interplay between acetate and silyl ether protecting groups. Incorporation of acetates proximal to silyl ethers attenuates their reactivity while reducing undesirable side reactions. This phenomenon can be used to fine-tune the reactivity of silylated/acetylated sugar building blocks.

Click chemistry inspired highly facile synthesis of triazolyl ethisterone glycoconjugates

Numerous deoxy-azido sugars 3 were prepared by the reaction of tosyl/bromo sugars with NaN3 in dry DMF under heating condition. The 1,3-dipolar cycloaddition of deoxy-azido sugars 3 with ethisterone 4 to afford regioselective triazole-linked ethisterone glycoconjugates 5 was investigated in the presence of CuI and DIPEA in dichloromethane or CuSO4·5H2O and sodium ascorbate in aqueous medium. All the developed compounds were characterized by spectroscopic analysis (IR, (1)H &(13)C NMR, and MS spectra). Structure of triazolyl ethisterone glycoconjugate 5a has been further confirmed by its Single Crystal X-ray analysis.

Modulation of ocular surface glycocalyx barrier function by a galectin-3 N-terminal deletion mutant and membrane-anchored synthetic glycopolymers

Background

Interaction of transmembrane mucins with the multivalent carbohydrate-binding protein galectin-3 is critical to maintaining the integrity of the ocular surface epithelial glycocalyx. This study aimed to determine whether disruption of galectin-3 multimerization and insertion of synthetic glycopolymers in the plasma membrane could be used to modulate glycocalyx barrier function in corneal epithelial cells.

Methodology/Principal Findings

Abrogation of galectin-3 biosynthesis in multilayered cultures of human corneal epithelial cells using siRNA, and in galectin-3 null mice, resulted in significant loss of corneal barrier function, as indicated by increased permeability to the rose bengal diagnostic dye. Addition of β-lactose, a competitive carbohydrate inhibitor of galectin-3 binding activity, to the cell culture system, transiently disrupted barrier function. In these experiments, treatment with a dominant negative inhibitor of galectin-3 polymerization lacking the N-terminal domain, but not full-length galectin-3, prevented the recovery of barrier function to basal levels. As determined by fluorescence microscopy, both cellobiose- and lactose-containing glycopolymers incorporated into apical membranes of corneal epithelial cells, independently of the chain length distribution of the densely glycosylated, polymeric backbones. Membrane incorporation of cellobiose glycopolymers impaired barrier function in corneal epithelial cells, contrary to their lactose-containing counterparts, which bound to galectin-3 in pull-down assays.

Conclusions/Significance

These results indicate that galectin-3 multimerization and surface recognition of lactosyl residues is required to maintain glycocalyx barrier function at the ocular surface. Transient modification of galectin-3 binding could be therapeutically used to enhance the efficiency of topical drug delivery.

Building biologics by chemical synthesis: practical preparation of di- and triantennary N-linked glycoconjugates

A unified strategy for the syntheses of bi- and triantennary fully sialylated N-glycans is described. The synthesis capitalizes on a global glycosylation strategy that delivers the desired undeca- and tetradecasaccharide in excellent yields. Finally, conjugation of the glycan to PSMA oligopeptide is described.

Poly(amido amine)-based mannose-glycodendrimers as multielectron redox probes for improving lectin sensing

An easy-to-prepare series of electroactive poly(amido amine) (PAMAM)-based dendrimers of generations G0 to G2 having mannopyranosylferrocenyl moieties in the periphery to detect carbohydrate-protein interactions is reported. The synthesis involved the functionalization of the PAMAM surface with azidomethylferrocenyl groups and subsequent coupling of mannoside units by the Cu(I)-catalyzed Huisgen reaction. The binding affinity of the series of electroactive glycodendrimers was studied by isothermal titration calorimetry (ITC) and differential pulse voltammetry (DPV). Upon complexation of the glycodendrimers conjugates with prototypical concanavalin A (Con A), voltammograms showed a decrease of the peak current. Such dendrimers showed a notable improvement of redox sensing abilities toward Con A when compared with mono- and divalent analogues, based on both the glycoside multivalent and ferrocene dendritic effects.

Synthesis of genistein 2,3-anhydroglycoconjugates -- potential antiproliferative agents

The title compounds, variously protected 2.3-anhydrosugars linked with genistein through an alkyl chain, were synthesized in a sequence of reactions. First step involved Ferrier rearragement of 3,4-di-O-acetyl-L-rhamnal with 3-bromopropanol to obtain 2,3-unsaturated bromoalkylglycosides. The next step was epoxidation with m-CPBA and finally these compounds were connected with genistein in reaction of 7-O-genistein tetra-butylamonium salt with 2,3-anhydro bromoalkylglycosides. Obtained glycoconjugates differ in orientation of an oxirane ring and the protecting group in a sugar moiety. All compounds were tested in vitro for antiproliferative potential in cancer cells.

Synthesis of galactothiophosphoesters of uridine and preliminary tests to evaluate their activity against selected glycosyltransferases

Glycoconjugates (8-13) were obtained in condensation reaction of selectively protected thiophosphoesters of uridine with 2,3,4.6-tetra-O-acetyl-1-thio-beta-D-galactose. In sequence of reactions: selective protection of esculine, glycosylation reaction and deprotecion beta-D-galactopiranoside-(1-4)-esculine (20) was obtained as a model of the product reaction catalyzed by GTs. Prepared glycococnjugates (8-13) were checked using TLC methods in terms of inhibition of bovine milk beta-1,4-galactosyltransferase.

Synthesis of glycinated glycoconjugates based on 1-thioglycosides and their preliminary studies as potential immunomodulatory factor

The biological importance of lipopolysaccharides (LPS), components of bacterial cell wall has not been explained sufficiently. The glycine present in these structures could play an important role in the immunological response after bacterial infections and during sepsis. In our studies we obtained synthetic and stable substituted glycinated 1-thioglycosides derivatives of monosaccharides, e.g., D-glucose or D-galactose as well as disaccharides, e.g., melibiose and lactose. The conditions of acylation reactions were validated and specific products were separated by using chromatography methods. Their structures were confirmed by NMR. These compounds were conjugated with carrier proteins e.g., bovine serum albumin and horse myoglobin. Prior to conjugation proteins were modified with glycidol to create the protein-diol intermediates and subsequent periodate oxidation of the glycol moieties to generate the reactive aldehyde functionalities. Modified and formylated carrier proteins were conjugated with acylated thioglycosides in the presence of sodium cyanoborohydride. Subsequently, the products obtained were analyzed in SDS-PAGE and separated by using HW-55S gel-filtration chromatography. The immunoreactivity of selected glycinated glycoconjugates were studied in ELISA assays with specific anti-aminoacylated glyconjugate antibodies obtained after rabbit immunization with Escherichia coli K12 C600 core oligosaccharide glycine-containing glycoconjugate. The differences in the immunoreactivity of different glycinated 1-thioglycosides were observed. The received glycine-acylated glycoconjugates could mimic the non-sugar substituents localized in various bacterial LPS. These synthetic compounds could be candidates for their use as glycoconjugate vaccines in protection against serious bacterial infections, e.g.. sepsis.

Application of different alpha-1-thioglycosides preparation methods in synthesis of 5-nitro-2-pyridyl 1-thioglycosides substrates in synthesis of conjugates with uridine moiety. Part I

The synthesis of (5-nitro-2-pyridyl) 2-deoxy-1-thioglycosides is presented. Obtained compounds were used in the synthesis of uridine derivatives, potential glycosyltransferases inhibitors. In the first stage of the research 2-deoxyglucose and 2-deoxygalactose were connected to aglycone (nitropyridine derivative) via alpha-1-thioglycosidic bond. Therefore, protected 1.2-unsaturated D-glucose or D-galactose derivatives were treated with 2-thio-5-nitropyridine in the presence of the catalyst. In the next step nitro group in the aglycone was reduced in the reaction with the use of zinc dust in acetic acid. Then, these compounds were connected to selectively protected uridine derivatives by amide bond with or without succinic spacer. The obtained glycoconjugates differ in the protecting group of the carbohydrate part and in the presence of the spacer between the sugar and uridine moiety.

Polar dibenzocyclooctynes for selective labeling of extracellular glycoconjugates of living cells

Although strain-promoted alkyne-azide cycloadditions (SPAAC) have found wide utility in biological and material sciences, the low polarity and limited water solubility of commonly used cyclooctynes represent a serious shortcoming. To address this problem, an efficient synthetic route has been developed for highly polar sulfated dibenzocyclooctynylamides (S-DIBO) by a Friedel-Crafts alkylation of 1,2-bis(3-methoxyphenyl)ethylamides with trichlorocyclopropenium cation followed by a controlled hydrolysis of the resulting dichlorocyclopropenes to give bis(3-methoxyphenyl)cyclooctacyclopropenones, which were subjected to methoxy group removal of the phenols, O-sulfation, and photochemical unmasking of the cyclopropenone moiety. Accurate rate measurements of the reaction of benzyl azide with various dibenzylcyclooctyne derivatives demonstrated that aromatic substitution and the presence of the amide function had only a marginal impact on the rate constants. Biotinylated S-DIBO 8 was successfully used for labeling azido-containing glycoconjugates of living cells. Furthermore, it was found that the substitution pattern of the dibenzylcyclooctynes influences subcellular location, and in particular it has been shown that DIBO derivative 4 can enter cells, thereby labeling intra- and extracellular azido-modified glycoconjugates, whereas S-DIBO 8 cannot pass the cell membrane and therefore is ideally suited for selective labeling of cell surface molecules. The ability to selectively label cell surface molecules will yield unique opportunities for glycomic analysis and the study of glycoprotein trafficking.

Synthetic neoglycoconjugates of cell-surface phosphoglycans of Leishmania as potential anti-parasite carbohydrate vaccines

Leishmania are a genus of sandfly-transmitted protozoan parasites that cause a spectrum of debilitating and often fatal diseases in humans throughout the tropics and subtropics. During the parasite life cycle, Leishmania survive and proliferate in highly hostile environments. Their survival strategies involve the formation of an elaborate and dense cell-surface glycocalyx composed of diverse stage-specific glycoconjugates that form a protective barrier. Phosphoglycans constitute the variable structural and functional domain of major cell-surface lipophosphoglycan and secreted proteophosphoglycans. In this paper, we discuss structural aspects of various phosphoglycans from Leishmania with the major emphasis on the chemical preparation of neoglycoconjugates (neoglycoproteins and neoglycolipids) based on Leishmania lipophosphoglycan structures as well as the immunological evaluation for some of them as potential anti-leishmaniasis vaccines.

Convenient synthesis of 3,4-dichloro-5-hydroxy-2(5H)-furanone glycoconjugates

3,4-Dichloro-5-hydroxy-2(5H)-furanone treated with methyl chloroformate in the presence of diisopropylethylamine (Hünig’s base) gave the corresponding carbonate. The labile methoxycarbonyloxy group smoothly undergoes substitution by amino alcohols. The obtained 5-(ω-hydroxyalkylamino) mucochloric acid derivatives reacted with peracetylated glucals using triphenylphosphine hydrobromide as a catalyst to give the title muchloric acid glycoconjugates.

Glycoconjugates, products of uridine derivatives phosphitylation and oxidation as glycosyltransferases potential inhibitors

The title compounds, variously protected 5'-uridine derivatives connected with 1-thiosugar with thio-phosphoesters fragment (17-22) were synthesized in sequence of reactions: phosphitylation--reaction of 5'-hydroxyl group of selectively protected nucleoside with a phosphitylating agent (N,N-diisopropyl chlorophosphoamidite), connection an phosphoroamidites with 2-bromoethanol or 3-bromopropanol and secondary oxidation with sulfur presence and finally condensation reaction of obtained products with 1-thiosugar. Received glycoconjugates (17-22) had a structure which mimic to structure of natural glycosyltransferases substrates.

Design and creativity in synthesis of multivalent neoglycoconjugates

From the authors' opinion, this chapter constitutes a modest extension of the seminal and inspiring contribution of Stowell and Lee on neoglycoconjugates published in this series [C. P. Stowell and Y. C. Lee, Adv. Carbohydr. Chem. Biochem., 37 (1980) 225-281]. The outstanding progresses achieved since then in the field of the "glycoside cluster effect" has witnessed considerable creativity in the design and synthetic strategies toward a vast array of novel carbohydrate structures and reflects the dynamic activity in the field even since the recent chapter by the Nicotra group in this series [F. Nicotra, L. Cipolla, F. Peri, B. La Ferla, and C. Radaelli, Adv. Carbohydr. Chem. Biochem., 61 (2007) 353-398]. Beyond the more classical neoglycoproteins and glycopolymers (not covered in this work) a wide range of unprecedented and often artistically beautiful multivalent and monodisperse nanostructures, termed glycodendrimers for the first time in 1993, has been created. This chapter briefly surveys the concept of multivalency involved in carbohydrate-protein interactions. The topic is also discussed in regard to recent steps undertaken in glycobiology toward identification of lead candidates using microarrays and modern analytical tools. A systematic description of glycocluster and glycodendrimer synthesis follows, starting from the simplest architectures and ending in the most complex ones. Presentation of multivalent glycostructures of intermediate size and comprising, calix[n]arene, porphyrin, cyclodextrin, peptide, and carbohydrate scaffolds, has also been intercalated to better appreciate the growing synthetic complexity involved. A subsection describing novel all-carbon-based glycoconjugates such as fullerenes and carbon nanotubes is inserted, followed by a promising strategy involving dendrons self-assembling around metal chelates. The chapter then ends with those glycodendrimers that have been prepared using commercially available dendrimers possessing varied functionalities, or systematically synthesized using either divergent or convergent strategies.

Opportunities and challenges in synthetic oligosaccharide and glycoconjugate research

Synthetic oligosaccharides and glycoconjugates are increasingly used as probes for biological research and as lead compounds for drug and vaccine discovery. These endeavors are, however, complicated by a lack of general methods for the routine preparation of this important class of compounds. Recent development such as one-pot multi-step protecting group manipulations, the use of unified monosaccharide building blocks, the introduction of stereoselective glycosylation protocols, and convergent strategies for oligosaccharide assembly, are beginning to address these problems. Furthermore, oligosaccharide synthesis can be facilitated by chemo-enzymatic methods, which employ a range of glycosyl transferases to modify a synthetic oligosaccharide precursor. Glycosynthases, which are mutant glycosidases, that can readily form glycosidic linkages are addressing a lack of a wide range glycosyltransferases. The power of carbohydrate chemistry is highlighted by an ability to synthesize glycoproteins.

Recent development in carbohydrate-based cancer vaccines

Tumor-associated carbohydrate antigens (TACAs) are important molecular markers on the cancer cell surface, useful for the development of therapeutic cancer vaccines or cancer immunotherapies. However, because of their poor immunogenicity and/or immunotolerance, most TACAs fail to induce T cell-mediated immunity that is critical for cancer therapy. This review summarizes the recent effort to overcome this problem via constructing TACA conjugates with improved immunogenicity, such as by covalently coupling TACAs to proper carrier molecules to form clustered or multi-epitopic conjugate vaccines, coupling TACAs to a T cell peptide epitope and/or an immunostimulant epitope to form fully synthetic multi-component glycoconjugate vaccines, and developing vaccines based on chemically modified TACAs, which is combined with metabolic engineering of cancer cells.

Synthesis and applications of a light-fluorous glycosyl donor

A new method using a light-fluorous glycosyl donor and an orthogonal tagging strategy to synthesize oligosaccharides and glycoconjugates has been developed. The glycosyl donor orthogonally protected with a C8F17-silyl tag and benzoyl groups was reacted with excess amounts of glycosyl acceptor. Fluorous solid-phase extraction separated the glycosylated product and unreacted glycosyl acceptor. This new protocol has high reaction efficiency and easy separation, which was demonstrated in the synthesis of an unprotected trisaccharide and an O-glycosylated serine in this paper.

Synthesis of glucoconjugates of oleanolic acid as inhibitors of glycogen phosphorylase

Synthesis and biological evaluation of glucoconjugates of oleanolic acid, linked by either a triazole moiety or an ester function, as novel inhibitors of glycogen phosphorylase have been described. Several triterpene-glycoside conjugates exhibited moderate-to-good inhibitory activity against rabbit muscle GPa. Compound 12 showed the best inhibition with an IC(50) value of 1.14 microM. Structure-activity relationship (SAR) analysis of these inhibitors is also discussed. Possible binding modes of compound 12 were explored by molecular docking simulations.

Sugar-phosphole oxide conjugates as "turn-on" luminescent sensors for lectins

A mixture of sugar-phosphole oxide conjugates and lectins in a buffer solution displays an intense blue emission, thereby acting as a "turn-on" fluorescent sensor for lectins as they form aggregates.

Synthesis of neoglycoconjugates by the desulfurative rearrangement of allylic disulfides

Two series of neoglycosyl donors are prepared on the basis of connection of an allylic disulfide motif to the anomeric center via a simple O-glycosyl linkage or N-glycosyl amide unit. Conjugation of both sets of donors to cysteine in peptides is demonstrated through classical disulfide exchange followed by the phosphine-mediated desulfurative allylic rearrangement resulting in neoglycopeptides characterized by a simple thioether spacer. The conjugation reaction functions in the absence of protecting groups on both the neoglycosyl donor and peptide in aqueous media at room temperature.

Iterative Synthesis of Spacered Glycodendrons as Oligomannoside Mimetics and Evaluation of Their Antiadhesive Properties

Dendrimer chemistry is an attractive concept for mimicry of the highly branched character of the bioactive carbohydrates found as part of a cell's sugar coat, called the glycocalyx. Glycodendrimers have thus been used to study biological processes occurring on cell surfaces, such as bacterial adhesion. This paper details a new approach in glycodendrimer synthesis, in which a 3,6-diallylated carbohydrate is utilised as core molecule, hydroboration-oxidation is the activating step, and glycosylation with branched and unbranched sugar trichloroacetimidates is used for dendritic growth. To obtain pure dendritic pseudo-tri- and -heptasaccharides in good yields, radical addition of mercaptoethanol to peripheral double bonds was also evaluated with great success. A collection of six new hyperbranched glycodendrons was tested for their potential as inhibitors of type 1 fimbriae-mediated bacterial adhesion in an ELISA and the results were interpreted with regard to sugar valency and spacer characteristics.

Development of sensitive chemical and immunochemical methods for detecting sulfated sialic acids and their application to glycoconjugates from sea urchin sperm and eggs

Sulfated sialic acid (SiaS) is a unique sialic acid (Sia) derivative in which an additional anionic group is attached to a carboxylated monosaccharide. Very little is known about the occurrence and biologic function of SiaS, due to the limitations of analytical methods to detect it in minute amounts. In this study, to develop methods and probes for detecting and pursuing the functions of SiaS, we developed sensitive chemical and immunochemical detection methods. First, we synthesized as model compounds 4-methylumbelliferyl glycosides of 8-O- and 9-O-sulfated Sia consisting of N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc), and deaminoneuraminic acid (Kdn). Second, we applied fluorometric high performance liquid chromatography (HPLC) analysis to these synthetic glycosides. After acid hydrolysis of the samples, the liberated SiaS were labeled with a fluorescent reagent, 1,2-diamino-4,5-methylenedioxybenzene, and analyzed on fluorometric HPLC. We established an optimal elution condition for successful separation of 8-O- and 9-O-sulfated Neu5Ac, Neu5Gc, and Kdn on HPLC. Third, we generated a monoclonal antibody (mAb) 2C4 against SiaS using sea urchin egg components as the immunogen. mAb.2C4 recognizes both 8-O-sulfated Neu5Ac (Neu5Ac8S) and Neu5Gc8S, whereas the previously prepared mAb.3G9 only recognizes Neu5Ac8S. Finally, using the fluorometric HPLC and monoclonal antibodies, we demonstrated that glycoconjugates from sea urchin sperm exclusively contained Neu5Ac8S, whereas those from eggs contained Neu5Gc8S. Furthermore, we clarified the quantitative differences in the SiaS content in eggs and sperm from two different species of sea urchins. Immunostaining using mAb.2C4 showed that Neu5Gc8S is localized in the cortical granules in unfertilized eggs, whereas it is localized in the outer surface of the fertilization layer as well as in the inner surface of fertilized eggs. Thus, 8-O-sulfation is dependent on the species, gametic cell-type, site-localization of the eggs, and glycoconjugates.