Glycosylation reactions with ‘disarmed’ thioglycoside donors promoted by N-iodosuccinimide and HClO4–silica

Synthesis of the tetrasaccharide repeating unit of the O-specific polysaccharide of Azospirillum doebereinerae type strain GSF71T using linear and one-pot iterative glycosylations

A straightforward synthetic strategy was developed for the synthesis of the tetrasaccharide repeating unit corresponding to the O-specific polysaccharide of Azospirillum doebereinerae type strain GSF71^T in a very good yield adopting sequential glycosylation followed by removal of the p-methoxybenzyl (PMB) group in the same pot. Further, the synthetic strategy was modified by carrying out three stereoselective iterative glycosylations followed by in situ removal of the PMB group in one pot. The stereochemical outcome of the newly formed glycosidic linkages was excellent using thioglycoside derivatives as glycosyl donors and a combination of N-iodosuccinimide (NIS) and perchloric acid supported on silica (HClO₄-SiO₂) as the glycosyl activator.

Convenient synthesis of the pentasaccharide repeating unit corresponding to the cell wall O-antigen of Escherichia albertii O4

A straightforward sequential synthetic strategy has been developed for the synthesis of a pentasaccharide repeating unit corresponding to the cell wall O-antigen of the Escherichia albertii O4 strain in very good yield with the desired configuration at the glycosidic linkages using thioglycosides and trichloroacetimidate derivatives as glycosyl donors and perchloric acid supported over silica (HClO4/SiO2) as a solid supported protic acid glycosyl activator. The expected configuration at the glycosidic linkages was achieved using a reasonable selection of protecting groups in the manosaccharide intermediates.

Synthesis of a hexasaccharide repeating unit of the cell wall polysaccharide of Bifidobacterium animalis subsp. lactis LKM512

A convergent synthesis of the hexasaccharide as its 2-aminoethyl glycoside corresponding to the repeating unit of the cell wall polysaccharide of Bifidobacterium animalis subsp. lactis LKM512 has been achieved applying a [4 + 2] glycosylation strategy. The disaccharide thioglycoside donor was prepared by combining a d-galactofuranosyl thioglycoside with another l-rhamnosyl thioglycoside acceptor. The yields of the individual glycosylation steps were highly satisfactory with excellent stereo outcome. An α-glycosidic linkage of the d-galactofuranosyl moiety in the hexasaccharide was achieved in very good yield.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Convergent Synthesis of Oligosaccharide Fragments Corresponding to the Cell Wall O-Polysaccharide of Salmonella enterica O53

Conventional glycoconjugate vaccines are prepared with polysaccharides isolated from bacterial fermentation, an approach with some significant drawbacks such as handling of live bacterial strains, the presence of biological impurities, and inter-batch variations in oligosaccharide epitope structure. However, it has been shown in many cases that a synthetic fragment of appropriate structure conjugated to a protein can be an effective vaccine that circumvents the shortcomings of using full-length oligosaccharides. The development of synthetic strategies to prepare glycoconjugate derivatives against pathogenic bacterial strains is therefore of great interest. Oligosaccharide fragments corresponding to the repeat unit of the cell wall O-antigen of Salmonella enterica strain O53 were synthesized in good yield. Sequential and block glycosylation strategies were used for the synthesis of the target compounds. A number of recently developed reaction conditions were used in the synthetic strategy. A one-pot reaction scheme was also developed for the multiple glycosylation steps. The stereoselective outcomes of all glycosylation reactions were very good.

Expedient synthesis of a pentasaccharide related to the O-specific polysaccharide of Escherichia coli O117:K98:H4 strain

A convenient synthetic strategy has been developed for the synthesis of a pentasaccharide, related to the O-specific polysaccharide ofEscherichia coliO117:K98:H4 strain, using sequential glycosylations of functionalized monosaccharide moieties.

Convergent synthesis of the tetrasaccharide repeating unit of the cell wall lipopolysaccharide of Escherichia coli O40

A tetrasaccharide repeating unit corresponding to the cell-wall lipopolysaccharide of E. coli O40 was synthesized by using a convergent block glycosylation strategy. A disaccharide donor was coupled to a disaccharide acceptor by a stereoselective glycosylation. A 2-aminoethyl linker was chosen as the anomeric protecting group at the reducing end of the tetrasaccharide. All glycosylation steps are significantly high yielding and stereoselective.

Convergent synthesis of the tetrasaccharide repeating unit corresponding to the O-antigen of the verotoxin-producing Escherichia coli O176

A convergent synthesis of the tetrasaccharide repeating unit of the O-antigen of the verotoxin producing E. coli O176 has been achieved in excellent yield adopting a [2+2] block glycosylation strategy. The β-D-mannosidic moiety of the tetrasaccharide was prepared from β-D-glucoside and α-D-galactosamine moiety was derived from D-galactal. The tetrasaccharide was synthesized as its 2-trimethylsilylethyl glycoside in excellent yield. All intermediate steps are high yielding.

Convergent synthesis of the tetrasaccharide repeating unit of the O-antigen of Shigella boydii type 9

A convenient synthesis of the tetrasaccharide repeating unit of the O-antigen of Shigella boydii type 9 has been achieved in excellent yield using a [2 + 2] block glycosylation strategy. TEMPO-mediated selective oxidation of the primary alcohol of the tetrasaccharide derivative 8 to the carboxylic group followed by deprotection of the functional groups furnished target tetrasaccharide 1 as its 4-methoxyphenyl glycoside in high yield.

Synthesis of apiose-containing oligosaccharide fragments of the plant cell wall: fragments of rhamnogalacturonan-II side chains A and B, and apiogalacturonan

Fragments of pectic polysaccharides rhamnogalacturonan-II (RG-II) and apiogalacturonan were synthesised using p-tolylthio apiofuranoside derivatives as key building blocks. Apiofuranose thioglycosides can be conveniently prepared by cyclization of the corresponding dithioacetals possessing a 2,3-O-isopropylidene group, which is required for preservation of the correct (3R) configuration of the apiofuranose ring. The remarkable stability of this protecting group in apiofuranose derivatives requires its replacement with a more reactive protecting group, such as a benzylidene acetal which was used in the synthesis of trisaccharide β-Rhap-(1→3')-β-Apif-(1→2)-α-GalAp-OMe. The X-ray crystal structure of the protected precursor of this trisaccharide has been elucidated.

Zeolites and other silicon-based promoters in carbohydrate chemistry

Silicon-based materials, namely zeolites, clays, and silica gel have been widely used in organic synthesis, allowing mild reaction conditions and environmentally friendly methodologies. These heterogeneous catalysts are easy to handle, possess nontoxic and noncorrosive character and offer the possibility of recovery and reuse, thus contributing to clean and sustainable organic transformations. Moreover, they present shape-selective properties and provide stereo- and regiocontrol in chemical reactions. Herein, we survey the most significant applications of silicon-based materials as catalysts in carbohydrate chemistry, to mediate important transformations such as glycosylation, sugar protection and deprotection, and hydrolysis and dehydration. Emphasis is placed on their promising synthetic potential in comparison with conventional catalysts.

Direct chemical glycosylation with pentenyl- and thioglycoside donors of N-acetylglucosamine

The use of pentenyl and thiophenyl glycosides of N-acetylglucosamine (GlcNAc) as glycosyl donors for the direct preparation of O-glycosides of GlcNAc promoted by N-iodosuccinimide (NIS) and metal triflates in dichloromethane has been investigated. Both glycosyl acceptors 1-octanol and (-)-menthol resulted in good glycosylation yields for both types of donors with pentenyl glycosides being somewhat superior in terms of yield. Carbohydrate-based acceptors were reacted with a benzylated GlcNAc-pentenyl donor but only provided disaccharides in poor to moderate yields. The results show that a variety of metal triflates are capable of acting as an activator for both NIS and the intermediate oxazoline.

MILD AND SELECTIVE O-GLYCOSYLATIONS OF PRIMARY ALCOHOLS WITH THE THIOGLUCOSAMINIDE DERIVATIVE PROMOTED BY N-IODOSUCCINIMIDE AND HBF(4)-ADOSROBED ON SILICA GEL

Selective glycosylations of primary alcohols with the thioglucosaminide 1 are achieved by using NIS and HBF(4)-SiO(2). HBF(4)-SiO(2) is a mild Brønsted acid which requires primary alcohols or phenols to effectively activate 1 with NIS at rt. A wide range of functional groups are tolerated under these conditions.

Synthesis of a tetrasaccharide related to the repeating unit of the O-antigen from Escherichia coli K-12

Synthesis of a tetrasaccharide related to the repeating unit of the O-antigen from Escherichia coli K-12 is reported in the form of its octyl glycoside. Syntheses of the 1,2-cis glycosidic linkages have been accomplished by using NIS in conjunction with H(2)SO(4)-silica, and it was found to be stereoselective and productive. The synthesized tetrasaccharide will be utilized as the substrate for galactofuranosyltransferase, WbbI.

NIS/H2SO4–Silica: a mild and efficient reagent system for the hydrolysis of thioglycosides

Chemoselective hydrolysis of a variety of thioglycosides in the presence of a wide range of protecting groups has been achieved by using N-iodosuccinimide and H(2)SO(4) immobilized on silica in good to excellent yields.

Synthesis of a di- and a trisaccharide related to the O-antigen of Escherichia coli O83:K24:H31

Di- and trisaccharide fragments related to the repeating unit of the O-antigen of Escherichia coli O83:K24:H31 have been synthesized as their methyl glycoside analogs starting from readily available monosaccharides.

Convergent synthesis of a trisaccharide as its 2-(trimethylsilyl)ethyl glycoside related to the flavonoid triglycoside from Gymnema sylvestre

The glycone part of the flavonoid triglycoside, kaempferol 3-O-beta-D-glucopyranosyl-(1-->4)-alpha-L-rhamnopyranosyl-(1-->6)-beta-D-galactopyranoside, has been synthesized in good yield and stereoselectivity using N-iodosuccinimide and HClO4-silica promoted glycosylations of thioglycoside donors.

From Solution Phase to “On-Column” Chemistry: Trichloroacetimidate-Based Glycosylation Promoted by Perchloric Acid−Silica

[reaction: see text] Activation of ester-protected glycosyl trichloroacetimidate donors by perchloric acid immobilized on silica afforded 1,2-trans disaccharides in 60-90% yields. Applying this approach to one-pot sequential glycosylation resulted in efficient syntheses of the N-linked glycan trimannoside and Le(X) and Le(A) trisaccharides in very good yield (76%, 62%, and 59% yields, respectively). Solution phase reactions were also translated to a solid phase format; priming the top of a standard silica chromatography column with perchloric acid immobilized on silica facilitated "on-column" glycosylation with subsequent "in situ" purification of products. Coupling yields from this approach were comparable to those obtained from the corresponding solution-phase disaccharide couplings. A series of glycosylated amino acids were also synthesized in high yield with use of the on-column approach.