Interhalogens (ICl/IBr) and AgOTf in thioglycoside activation; synthesis of bislactam analogues of ganglioside GD3

Stereoselective Synthesis of Sialyl Lewisa Antigen and the Effective Anticancer Activity of Its Bacteriophage Qβ Conjugate as an Anticancer Vaccine

Sialyl Lewisa (sLea ), also known as cancer antigen 19-9 (CA19-9), is a tumor-associated carbohydrate antigen. The overexpression of sLea on the surface of a variety of cancer cells makes it an attractive target for anticancer immunotherapy. However, sLea -based anticancer vaccines have been under-explored. To develop a new vaccine, efficient stereoselective synthesis of sLea with an amine-bearing linker was achieved, which was subsequently conjugated with a powerful carrier bacteriophage, Qβ. Mouse immunization with the Qβ-sLea conjugate generated strong and long-lasting anti-sLea IgG antibody responses, which were superior to those induced by the corresponding conjugate of sLea with the benchmark carrier keyhole limpet hemocyanin. Antibodies elicited by Qβ-sLea were highly selective toward the sLea structure, could bind strongly with sLea -expressing cancer cells and human pancreatic cancer tissues, and kill tumor cells through complement-mediated cytotoxicity. Furthermore, vaccination with Qβ-sLea significantly reduced tumor development in a metastatic cancer model in mice, demonstrating tumor protection for the first time by a sLea -based vaccine, thus highlighting the significant potential of sLea as a promising cancer antigen.

Promoter-Controlled Synthesis and Conformational Analysis of Cyclic Mannosides up to a 32-mer

Cyclodextrins are widely used as carriers of small molecules for drug delivery owing to their remarkable host properties and excellent biocompatibility. However, cyclic oligosaccharides with different sizes and shapes are limited. Cycloglycosylation of ultra-large bifunctional saccharide precursors is challenging due to the constrained conformational spaces. Herein we report a promoter-controlled cycloglycosylation approach for the synthesis of cyclic α-(1→6)-linked mannosides up to a 32-mer. Cycloglycosylation of the bifunctional thioglycosides and (Z)-ynenoates was found to be highly dependent on the promoters. In particular, a sufficient amount of a gold(I) complex played a key role in the proper preorganization of the ultra-large cyclic transition state, providing a cyclic 32-mer polymannoside, which represents the largest synthetic cyclic polysaccharide to date. NMR experiments and a computational study revealed that the cyclic 2-mer, 4-mer, 8-mer, 16-mer, and 32-mer mannosides adopted different conformational states and shapes.

3,5-Di(trifluoromethyl)phenyl(cyano)iodonium triflate as a novel and potential activator for p-tolyl thioglycoside donors

3,5-Di(trifluoromethyl)phenyl(cyano)iodonium triflate is described as an accessible, stable, and powerful thiophile that can activate batches of p-tolyl thioglycoside donors at room temperature. Various alcoholic acceptors were efficiently glycosylated, providing the desired glycosides. The novel activation protocol features mild conditions as well as high compatibility with some classic strategies for the stereoselective construction of some biologically relevant glycosidic linkages, as exemplified by α-idosides, α-galactoamines, β-mannosides, and β-rhamnosides.

Glycopyranosylidene-Spiro-Morpholinones: Evaluation of the Synthetic Possibilities Based on Glyculosonamide Derivatives and a New Method for the Construction of the Morpholine Ring

Glycosylidene-spiro-morpholin(on)es are scarcely described skeletons in the literature. In this work, we have systematically explored the synthetic routes towards such morpholinones based on the reactions of O-peracylated hept-2-ulopyranosonamide derivatives of D-gluco and D-galacto configuration. Koenigs-Knorr type glycosylation of 2-chloroethanol, allylic and propargylic alcohols by (glyculosylbromide)onamides furnished the expected glycosides. The 2-chloroethyl glycosides were ring closed to the corresponding spiro-morpholinones by treatment with K₂CO₃. The (allyl glyculosid)onamides gave diastereomeric mixtures of spiro-5-hydroxymorpholinones by ozonolysis and 5-iodomethylmorpholinones under iodonium ion mediated conditions. The ozonolytic method has not yet been known for the construction of morpholine rings, therefore, it was also extended to O-allyl mandelamide. The 5-hydroxymorpholinones were subjected to oxidation and acid catalyzed elimination reactions to give the corresponding morpholine-3,5-dions and 5,6-didehydro-morpholin-3-ones, respectively. Base induced elimination of the 5-iodomethylmorpholinones gave 5-methyl-2H-1,4-oxazin-3(4H)-ones. O-Acyl protecting groups of all of the above compounds were removed under Zemplén conditions. Some of the D-gluco configured unprotected compounds were tested as inhibitors of glycogen phosphorylase, but showed no significant effect.

Guidelines for O-Glycoside Formation from First Principles

With a view to reducing the notorious complexity and irreproducibility of glycosylation reactions, 12 guidelines for the choice of concentration, temperature, and counterions are adumbrated.

Improved α-Sialylation through the Synergy of 5-N,4-O-Oxazolidinone Protection and Exocyclic C-1 Neighboring Group Participation

Stereoselective construction of α-sialyl linkages is one of the most significant challenges in carbohydrate chemistry. In this research, we developed a novel strategy for stereoselective synthesis of α-linked sialosides by protecting the 5-N,4-O-positions of a sialyl donor with an oxazolidinone group and its C-1 carboxylic functionality with a cyanoethyl ester to promote α-glycosylation. We also adopted the more electrophilic N-bromosuccinimide as a promoter to readily activate p-tolyl thiosialoside at -78 °C. The sialylation using this sialyl donor gave excellent yields and α-selectivity. The new synthetic method was used to successfully construct naturally occurring α-sialosides having sialic acid linked to the 6-O- or 3-O-position of galactoside, or 8-O-position of another sialic acid, respectively, as well as other α-linked sialosides.

Glucopyranosylidene-spiro-benzo[ b][1,4]oxazinones and -benzo[ b][1,4]thiazinones: Synthesis and Investigation of Their Effects on Glycogen Phosphorylase and Plant Growth Inhibition

Glucopyranosylidene-spiro-benzo[ b][1,4]oxazinones were obtained via the corresponding 2-nitrophenyl glycosides obtained by two methods: (a) AgOTf-promoted glycosylation of 2-nitrophenol derivatives by O-perbenzoylated methyl (α-d-gluculopyranosyl bromide)heptonate or (b) Mitsunobu-type reactions of O-perbenzoylated methyl (α-d-gluculopyranose)heptonate with bulky 2-nitrophenols in the presence of diethyl azodicarboxylate (DEAD) and PPh₃. Catalytic hydrogenation (H₂-Pd/C) or partial reduction (e.g., H₂-Pd/C, pyridine) of the 2-nitro groups led to spiro-benzo[ b][1,4]oxazinones and spiro-benzo[ b][1,4]-4-hydroxyoxazinones by spontaneous ring closure of the intermediate 2-aminophenyl or 2-hydroxylamino glycosides, respectively. The analogous 2-aminophenyl thioglycosides, prepared by reactions of O-perbenzoylated methyl (α-d-gluculopyranosyl bromide)heptonate with 2-aminothiophenols, were cyclized in m-xylene at reflux temperature to the corresponding spiro-benzo[ b][1,4]thiazinones. O-Debenzoylation was effected by Zemplén transesterification in both series. Spiro-configurations were determined by NMR and electronic circular dichroism time-dependent density functional theory (ECD-TDDFT) methods. Inhibition assays with rabbit muscle glycogen phosphorylase b showed (1' R)-spiro{1',5'-anhydro-d-glucitol-1',2-benzo[ b][1,4]oxazin-3(4 H)-one} and (1' R)-spiro{1',5'-anhydro-d-glucitol-1',2-benzo[ b][1,4]thiazin-3(4 H)-one} to be the most efficient inhibitors (27 and 28% inhibition at 625 μM, respectively). Plant growth tests with white mustard and garden cress indicated no effect except for (1' R)-4-hydroxyspiro{1',5'-anhydro-d-glucitol-1',2-benzo[ b][1,4]oxazin-3(4 H)-one} with the latter plant to show modest inhibition of germination (95% relative to control).

The use of the novel glycosyl acceptor and supramer analysis in the synthesis of sialyl-α(2-3)-galactose building block

A new glycosyl acceptor to be used in sialylation was designed as a 3-hydroxy derivative of 4-methoxyphenyl β-d-galactopyranoside with 2-O-acetyl group and O-4 and O-6 protected as benzylidene acetal. Two alternative syntheses of this compound were compared. Sialylation of 3-OH group of the glycosyl acceptor with O-chloroacetylated N-trifluoroacetylneuraminic acid phenyl thioglycoside (NIS, TfOH, MeCN, MS 3 Å, -40 °C) was studied in a wide concentration range (5-150 mmol L^-1). The outcome of sialylation generally followed the predictions of supramer analysis of solutions of sialyl donor in MeCN, which was performed by polarimetry and static light scattering and revealed two concentration ranges differing in solution structure and the structures of supramers of glycosyl donor. The optimized conditions of sialylation (C = 50 mmol L^-1) were used to synthesize protected Neu-α(2-3)-Gal disaccharide (78%, α:β = 13:1), which was then converted to sialyl-α(2-3)-galactose imidate building block useful for the synthesis of complex sialo-oligosaccharides.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.

Chemical Synthesis of Glycosides of N-Acetylneuraminic Acid

Investigations of methodologies aimed on improving the stereoselective synthesis of sialosides and the efficient assembly of sialic acid glycoconjugates has been the mission of dedicated research groups from the late 1960s. This review presents major accomplishments in the field, with the emphasis on significant breakthroughs and influential synthetic strategies of the last decade.

Synthesis and intramolecular glycosylation of sialyl mono-esters of o-xylylene glycol. The importance of donor configuration and nitrogen protecting groups on cyclization yield and selectivity; isolation and characterization of a N-sialyl acetamide indicative of participation by acetonitrile

The synthesis and cyclization reactions, leading to spirocyclic medium ring-sized diolides, of o-(hydroxymethyl)xylylene monoesters of sialyl thioglycosides is described. Cyclization yields and stereoselectivities are found to vary as a function of the anomeric stereochemistry of the thioglycoside and of the N5 protecting group, and these effects are discussed in terms of the reaction mechanism. Cyclization in the presence of acetonitrile results in the isolation and characterization of a Ritter-type N-sialyl acetamide, which affords strong evidence for the participation of acetonitrile in the form of sialyl nitrilium ions.

Is an acyl group at O-3 in glucosyl donors able to control α-stereoselectivity of glycosylation? The role of conformational mobility and the protecting group at O-6

The stereodirecting effect of a 3-O-acetyl protecting group, which is potentially capable of the remote anchimeric participation, and other protecting groups in 2-O-benzyl glucosyl donors with flexible and rigid conformations has been investigated. To this aim, an array of N-phenyltrifluoroacetimidoyl and sulfoxide donors bearing either 3-O-acetyl or 3-O-benzyl groups in combination with 4,6-di-O-benzyl, 6-O-acyl-4-O-benzyl, or 4,6-O-benzylidene protecting groups was prepared. The conformationally flexible 3-O-acetylated glucosyl donor protected at other positions with O-benzyl groups demonstrated very low or no α-stereoselectivity upon glycosylation of primary or secondary acceptors. On the contrary, 3,6-di-O-acylated glucosyl donors proved to be highly α-stereoselective as well as the donor having a single potentially participating acetyl group at O-6. The 3,6-di-O-acylated donor was shown to be the best α-glucosylating block for the primary acceptor, whereas the best α-selectivity of glycosylation of the secondary acceptor was achieved with the 6-O-acylated donor. Glycosylation of the secondary acceptor with the conformationally constrained 3-O-acetyl-4,6-O-benzylidene-protected donor displayed under standard conditions (-35°C) even lower α-selectivity as compared to the 3-O-benzyl analogue. However, increasing the reaction temperature essentially raised the α-stereoselectivities of glycosylation with both 3-O-acetyl and 3-O-benzyl donors and made them almost equal. The stereodirecting effects of protecting groups observed for N-phenyltrifluoroacetimidoyl donors were also generally proven for sulfoxide donors.

Organoiridium complexes: efficient catalysts for the formation of sugar acetals and ketals

[Cp(∗)IrCl(2)](2) is used as an efficient promoter for the synthesis of sugar acetals and ketals with good to excellent yields. The catalyst is found to be general for a wide range of sugars.

Chemical approaches to glycobiology

Glycans are ubiquitous components of all organisms. Efforts to elucidate glycan function and to understand how they are assembled and disassembled can reap benefits in fields ranging from bioenergy to human medicine. Significant advances in our knowledge of glycan biosynthesis and function are emerging, and chemical biology approaches are accelerating the pace of discovery. Novel strategies for assembling oligosaccharides, glycoproteins, and other glycoconjugates are providing access to critical materials for interrogating glycan function. Chemoselective reactions that facilitate the synthesis of glycan-substituted imaging agents, arrays, and materials are yielding compounds to interrogate and perturb glycan function and dysfunction. To complement these advances, small molecules are being generated that inhibit key glycan-binding proteins or biosynthetic enzymes. These examples illustrate how chemical glycobiology is providing new insight into the functional roles of glycans and new opportunities to interfere with or exploit these roles.

Efficient dehydrative sialylation of C-4-aminated sialyl-hemiketal donors with Ph2SO/Tf2O

An efficient approach to the dehydrative sialylation of various substrates with C-4-aminated sialyl-hemiketal donors by using the reagent combination of diphenyl sulfoxide and triflic anhydride is reported. By using a C-4-hindered non-nucleophilic amine auxiliary, excellent yields and high alpha-stereoselectivities were obtained for coupling with a wide range of primary and secondary acceptors.

Efficient synthesis of Idraparinux, the anticoagulant pentasaccharide

An efficient [DEF+GH] route was developed to the synthesis of Idraparinux, which is a fully O-sulfated, O-methylated mimic of the unique Antithrombin III binding domain of heparin.

Synthesis of a tetra- and a trisaccharide related to an anti-tumor saponin “Julibroside J28” from Albizia julibrissin

Simple and convergent synthesis of a tetra- and a trisaccharide portions of an antitumor compound Julibroside J(28), isolated from Albizia julibrissin, that showed significant in vitro antitumor activity against HeLa, Bel-7402 and PC-3M-1E8 cancer cell lines is reported. The tetrasaccharide has been synthesized as its p-methoxyphenyl glycoside starting from commercially available D-glucose, L-rhamnose and L-arabinose. The trisaccharide part has been synthesized from commercially available N-acetyl D-glucosamine, D-fucose and D-xylose using simple protecting group manipulations. Sulfuric acid immobilized on silica has been used successfully as a Brönsted acid catalyst for the crucial glycosylation steps.

Synthesis of ketopyranosyl glycosides and determination of their anomeric configuration on the basis of the three-bond carbon–proton couplings

Anomeric pairs of ketopyranosyl glycosides with various substituents at C(alpha), C(beta) and C(gamma) were synthesized from the corresponding thioglycosides, and the influence of the C(alpha)-C(beta)-C(gamma)-H(gamma) torsion angle and substituent effects on the three-bond carbon-proton couplings was studied. The cis coupling constants range from 1 to 2Hz. The trans couplings are generally as small as 2.3-2.6Hz; however, for compounds bearing an unsubstituted gamma-carbon, a relatively large trans coupling was measured (4.8Hz). An S-ethyl group at the beta-position increases the cis coupling (up to 3.2Hz) compared to the corresponding O-glycosides.

O-sialylation with N-acetyl-5-n,4-o-carbonyl-protected thiosialoside donors in dichloromethane: facile and selective cleavage of the oxazolidinone ring

An N-acetyl-5-N,4-O-carbonyl-protected thiosialoside donor, the structure of which has been defined through X-ray crystallography, was prepared and tested in couplings to a wide range of acceptors. This donor gives excellent yields and alpha-selectivities in linking with various primary alkyl and carbohydrate acceptors under the N-iodosuccinimide and trifluoromethanesulfonic acid in situ activation method at -40 degrees C in dichloromethane. The favorable affect of the oxazolidinone substructure for alpha-sialylation is illustrated by a comparison study with a N,N-diacetylsialyl donor, which exhibited inferior yields and alpha-selectivities. The sialylation selectivity is independent of the anomeric configuration of the donor, but is highly related to the reaction temperature under the NIS/TfOH activation method. In contrast to the NIS/TfOH method, the Ph2SO/Tf2O promotion gives beta-selective couplings in dichloromethane. The oxazolidinone of the N-acetyl-5-N,4-O-carbonyl protected sialosides, both alpha- and beta-anomers, could be cleaved cleanly by treatment with sodium methoxide under mild conditions without removal of the acetamide.

Removal of Acid-Labile Protecting Groups on Carbohydrates Using Water-Tolerant and Recoverable Vanadyl Triflate Catalyst

Acetal, trityl, and TBDMS protecting groups on saccharides were subjected to alcoholysis using a catalytic amount of vanadyl triflate in an MeOH-CH2Cl2 solvent system. The configuration at the anomeric positions of saccharides was retained, and no glycosidic bond cleavage and oxidation of sulfides were observed. The presented method was easily implemented, compatible with diverse functional groups, and regioselective in some cases.

Efficient glycosidation of a phenyl thiosialoside donor with diphenyl sulfoxide and triflic anhydride in dichloromethane

The formation of sialic acid glycosides with a thiosialic acid derivative, diphenyl sulfoxide, and trifluoromethanesulfonic anhydride is reported. With an excess of diphenyl sulfoxide, glycal formation can be completely suppressed and excellent yields are obtained for coupling to a wide range of primary, secondary, and tertiary acceptors.

N-Trifluoroacetyl Sialyl Phosphite Donors for the Synthesis of α(2 → 9) Oligosialic Acids

[reaction: see text] A new method for the synthesis of alpha(2 --> 9) oligosialic acids is developed using phosphite sialyl donors that are protected with a C-5 N-trifluoroacetyl (NHTFA) substituent. Compared with conventional donors, these donors gave a higher degree of alpha-anomeric selectivity during glycosidic bond formation and better yields during iterative sialylation in the synthesis of oligosialic acids.