Oligosaccharide assembly by one-pot multi-step strategy

Lewis acids as α-directing additives in glycosylations by using 2,3-O-carbonate-protected glucose and galactose thioglycoside donors based on preactivation protocol

Catalytic or stoichiometric amounts of Lewis acids were found to be very effective α-directing additives in the stereoselective glycosylations of diverse 2,3-O-carbonate-protected glucose and galactose thioglycoside donors by preactivation protocol. The poor stereoselectivities of 4,6-di-O-acetyl-2,3-O-carbonate protected thioglycoside donors in glycosyl coupling reactions were greatly improved, and excellent α-stereoselectivities were achieved by the addition of 0.2 equiv of BF(3)·OEt(2). On the other hand, the β-selectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thioglucoside donor toward glycosylations were reversed completely to the α-selectivities by the use of 1 equiv of SnCl(4), making the stereoselectivity controllable. Furthermore, the poor stereoselectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thiogalactoside donor in glycosylations were also improved by using SnCl(4) as additive.

Regioselective glycosylation method using partially protected arabino- and galactofuranosyl thioglycosides as key glycosylating substrates and its application to one-pot synthesis of oligofuranoses

We describe in this paper the development of a novel regioselective furanosylation methodology using partially protected furanosyl thioglycosides as central glycosylating building blocks and its application in the efficient one-pot synthesis of a series of linear and branched-type arabino- and galactofuranoside fragments structurally related to the cell wall polysaccharides of Mycobacterium tuberculosis , Streptococcus pneumoniae serostype 35A, and sugar beet.

Toward automated oligosaccharide synthesis

Carbohydrates have been shown to play important roles in biological processes. The pace of development in carbohydrate research is, however, relatively slow due to the problems associated with the complexity of carbohydrate structures and the lack of general synthetic methods and tools available for the study of this class of biomolecules. Recent advances in synthesis have demonstrated that many of these problems can be circumvented. In this Review, we describe the methods developed to tackle the problems of carbohydrate-mediated biological processes, with particular focus on the issue related to the development of the automated synthesis of oligosaccharides. Further applications of carbohydrate microarrays and vaccines to human diseases are also highlighted.

A highly α-stereoselective synthesis of oligosaccharide fragments of the Vi antigen from Salmonella typhi and their antigenic activities

In this paper, a convenient approach to the synthesis of the repeating α-(1→4)-linked N-acetyl galactosaminuronic acid units from the capsular polysaccharide of Salmonella typhi is reported. The exclusively α-stereoselective glycosylation reactions were achieved by using oxazolidinone-protected glycosides as building blocks based on a pre-activation protocol. Di-, tri-, and tetrasaccharides were prepared by this short and efficient approach in high yields. The enzyme-linked immunosorbent assay experiments show that our synthetic tri- and tetrasaccharide had much higher antigenic activities than previously reported ones in the inhibition of antibody binding by the native polysaccharide. The results demonstrate that the antigenic activities of saccharides can be strengthened greatly by increasing the number of acetyl groups present.

Expeditious oligosaccharide synthesis via selective, semi-orthogonal, and orthogonal activation

Traditional strategies for oligosaccharide synthesis often require extensive protecting and/or leaving group manipulations between each glycosylation step, thereby increasing the total number of synthetic steps while decreasing the efficiency of the synthesis. In contrast, expeditious strategies allow for the rapid chemical synthesis of complex carbohydrates by minimizing extraneous chemical manipulations. Oligosaccharide synthesis by selective activation of one leaving group over another is one such expeditious strategy. Herein, the significant improvements that have recently emerged in the area of the selective activation are discussed. The development of orthogonal strategy further expands the scope of the selective activation methodology. Surveyed in this article, are representative examples wherein these excellent innovations have been applied to the synthesis of various oligosaccharide sequences.

Mannopyranosyl uronic acid donor reactivity

The reactivity of a variety of mannopyranosyl uronic acid donors was assessed in a set of competition experiments, in which two (S)-tolyl mannosyl donors were made to compete for a limited amount of promoter (NIS/TfOH). These experiments revealed that the reactivity of mannuronic acid donors is significantly higher than expected based on the electron-withdrawing capacity of the C-5 carboxylic acid ester function. A 4-O-acetyl-β-(S)-tolyl mannuronic acid donor was found to have similar reactivity as per-O-benzyl-α-(S)-tolyl mannose.

Protecting groups in carbohydrate chemistry: influence on stereoselectivity of glycosylations

Saccharides are polyhydroxy compounds, and their synthesis requires complex protecting group manipulations. Protecting groups are usually used to temporarily mask a functional group which may interfere with a certain reaction, but protecting groups in carbohydrate chemistry do more than protecting groups usually do. Particularly, protecting groups can participate in reactions directly or indirectly, thus affecting the stereochemical outcomes, which is important for synthesis of oligosaccharides. Herein we present an overview of recent advances in protecting groups influencing stereoselectivity in glycosylation reactions, including participating protecting groups, and conformation-constraining protecting groups in general.

Rapid assembly of oligosaccharides: a highly convergent strategy for the assembly of a glycosylated amino acid derived from PSGL-1

P-Selectin and P-selectin glycoprotein ligand 1 (PSGL-1) are vascular adhesion molecules that play an important role in the recruitment of leukocytes to inflamed tissue by establishing leukocyte-endothelial and leukocyte-platelet interaction. P-Selectin binds to the amino-terminus of PSGL-1 through recognition of a sialyl Lewis(x) (SLe(x)) moiety linked to a properly positioned core-2 O-glycan and three tyrosine sulfate residues. We have developed a highly convergent synthesis of the PSGL-1 oligosaccharide linked to threonine based on the use of trichoroacetimidate donors and thioglycosyl acceptors that give products that can immediately be employed in a subsequent glycosylation step without the need for protecting group manipulations. Furthermore, by employing one-pot multistep glycosylation sequences the number of purification steps could be minimized. The process of oligosaccharide assembly was further streamlined by combining protecting group manipulations and glycosylations as a one-pot multistep synthetic procedure. The resulting PSGL-1 oligosaccharide is properly protected for glycopeptide assembly. It is to be expected that the strategic principles employed for the synthesis of the target compound can be applied for the preparation of other complex oligosaccharides of biological and medical importance.

Chemoenzymatic synthesis and enzymatic analysis of 8-modified cytidine monophosphate-sialic acid and sialyl lactose derivatives

The sialic acids found on eukaryotic glycans have remarkably diverse core structures, with a range of modifications at C5, C7, C8 and C9. These carbohydrates have been found to play key roles in cell-cell interactions within eukaryotes and often serve as the initial site of attachment for viruses and bacteria. Consequently simple changes to the structures of the sialic acids can result in profoundly different and often opposing biological effects. Of particular importance are modifications at the 8-position. These include O-acetylation, carried out by an acetyl transferase, and particularly polysialylation, catalyzed by a polysialyltransferase. As part of a structural and mechanistic study of sialyltransferases and polysialyltransferases, access was needed to sialic acid-containing oligosaccharides that are modified at the 8-position of the sialic acid to render this center non-nucleophilic. The free 8-modified sialic acid analogues were synthesized using a concise, divergent chemical synthetic approach, and each was converted to its cytidine monophosphate (CMP) sugar donor form using a bacterial CMP-sialic acid synthetase. The transfer of each of the modified donors to lactose by each of two sialyltransferases was investigated, and kinetic parameters were determined. These yielded insights into the roles of interactions occurring at that position during enzymatic sialyl transfer. A transferase from Campylobacter jejuni was identified as the most suitable for the enzymatic coupling and utilized to synthesize the 8''-modified sialyl lactose trisaccharides in multimilligram amounts.

Carbohydrate vaccines: developing sweet solutions to sticky situations?

Recent technological advances in glycobiology and glycochemistry are paving the way for a new era in carbohydrate vaccine design. This is enabling greater efficiency in the identification, synthesis and evaluation of unique glycan epitopes found on a plethora of pathogens and malignant cells. Here, we review the progress being made in addressing challenges posed by targeting the surface carbohydrates of bacteria, protozoa, helminths, viruses, fungi and cancer cells for vaccine purposes.

A highly alpha-selective glycosylation for the convenient synthesis of repeating alpha-(1-->4)-linked N-acetyl-galactosamine units

The repeating GalpNAc-alpha-(1-->4)-GalpNAc unit is part of a series of essential structures that can be found in many important biomolecules such as the glycoproteins and the O-antigenic polysaccharides of clinically important bacterial strains. In this paper, we describe an exclusive alpha-selective glycosylation reaction, using a 4,6-di-O-tert-butyldimethylsilyl-N-acetyloxazolidinone-protected thioglycoside as the glycosyl donor, under pre-activation conditions, with only half amount of the promoter, providing the product GalpNAc-alpha-(1-->4)-GalpNAc in high isolated yield. This reaction can be also applied to increasing the length of the repeating structure, which is of significant use in further synthesis of branched or linear oligosaccharides.

Chiral-auxiliary-mediated 1,2-cis-glycosylations for the solid-supported synthesis of a biologically important branched alpha-glucan

Solid-phase oligosaccharide synthesis offers the promise of providing libraries of oligosaccharides for glycomics research. A major stumbling block to solid-phase oligosaccharide synthesis has been a lack of general methods for the stereoselective installation of 1,2-cis-glycosides, and intractable mixtures of compounds are obtained if several such glycosides need to be installed. We have prepared on-resin a biologically important glucoside containing multiple 1,2-cis-glycosidic linkages with complete anomeric control by using glycosyl donors having a participating (S)-(phenylthiomethyl)benzyl chiral auxiliary at C2. A branching point could be installed by using 9-fluorenylmethyloxycarbonyl (Fmoc) and allyloxycarbonyl (Alloc) as a versatile set of orthogonal protecting groups. The synthetic strategy made it possible to achieve partial on-resin deprotection of the completed oligosaccharide, thereby increasing the overall efficiency of the synthesis. The combination of classical and auxiliary-mediated neighbouring-group participation for controlling anomeric selectivity is bringing the promise of routine automated solid-supported oligosaccharide synthesis closer.

Fluorous-Assisted One-Pot Oligosaccharide Synthesis

A new method for oligosaccharide assembly that combines the advantages of one-pot synthesis and fluorous separation is described. After one-pot glycosylations are completed, a fluorous tag is introduced into the reaction mixture to selectively "catch" the desired oligosaccharide, which is rapidly separated from non-fluorous impurities by fluorous solid-phase extraction (F-SPE). Subsequent "release" of the fluo rous tag and F-SPE achieved the purification of the desired oligosaccharide without the use of time- and solvent-consuming silica gel chromatography. Linear and branched oligosaccharides have been synthesized with this approach in just a few hours (for the overall oligosaccharide assembly and purification process).

Uronic acids in oligosaccharide and glycoconjugate synthesis

This chapter describes the assembly of uronic acid containing oligosaccharides and glycoconjugates. Two strategies are available to access these target molecules, namely a pre-glycosylation oxidation approach, in which uronic acid building blocks are used, and a post-glycosylation oxidation strategy, which employs an oxidation step after the assembly of the oligosaccharide chain. Because uronic acid building blocks are generally considered to be less reactive than their non-oxidized counterparts, the latter approach has found most application in carbohydrate synthesis. With the aid of selected examples of recent syntheses of biologically relevant oligosaccharides and glycoconjugates, the reactivity of different uronic acid building blocks is evaluated. From these examples it is apparent that the generally assumed low reactivity of uronic acids does not a priori rule out an efficient assembly of these target compounds. Besides influencing the reactivity of a given pyranoside, the C-5 carboxylic acid function can also have a profound effect on the stereochemical course of a glycosylation reaction, which can be exploited in the stereoselective formation of glycosidic bonds.

Superarmed and superdisarmed building blocks in expeditious oligosaccharide synthesis

: Traditional strategies for oligosaccharide synthesis often require extensive protecting and/or leaving group manipulations between each glycosylation step, thereby increasing the total number of synthetic steps while decreasing both the efficiency and yield. In contrast, expeditious strategies allow for the rapid chemical synthesis of complex carbohydrates by minimizing extraneous chemical manipulations. The armed-disarmed approach for chemoselective oligosaccharide synthesis is one such strategy that addresses these challenges. Herein, the significant improvements that have recently emerged in the area of chemoselective activation are discussed. These advancements have expanded the scope of the armed-disarmed methodology so that it can now be applied to a wider range of oligosaccharide sequences, in comparison to the original concept. Surveyed in this chapter are representative examples wherein these excellent innovations have already been applied to the synthesis of various oligosaccharides and glycoconjugates.