Iterative One-Pot α-Glycosylation Strategy: Application to Oligosaccharide Synthesis

Recent chemical synthesis of plant polysaccharides

Here, chemical syntheses of long, branched and complex glycans over 10-mer from plants are summarized, which highlights amylopectin 20-mer from starch, 17-mer from carthamus tinctorius, α-glucan 30-mer from Longan, 19-mer from psidium guajava and 11-mer from dendrobium huoshanense. The glycans assembly strategies, protecting groups utilization and glycosylation methods discussed here will inspire the efficient synthesis of diverse complex glycans with many 1,2-cis glycosidic linkages.

Directed SN2 Glycosylation Employing an Amide-Functionalized 1-Naphthoate Platform Featuring a Selectivity-Safeguarding Mechanism

This work implements a catalytic SN2 glycosylation by employing an amide-functionalized 1-naphthoate platform as a latent glycosyl leaving group. Upon gold-catalyzed activation, the amide group enables the SN2 process by directing the attack of the glycosyl acceptor via H-bonding interaction, which results in stereoinversion at the anomeric center. Unique in this approach is that the amide group also enables a novel safeguarding mechanism by trapping oxocarbenium intermediates and, hence, minimizing stereorandom SN1 processes. The strategy is applicable to the synthesis of a broad range of glycosides with high to excellent levels of stereoinversion from anomerically pure/enriched glycosyl donors. These reactions are generally high-yielding, and their applications in the synthesis of challenging 1,2-cis-linkage-rich oligosaccharides are demonstrated.

Recent Advances in Stereoselective Chemical O-Glycosylation Reactions

Carbohydrates involving glycoconjugates play a pivotal role in many life processes. Better understanding toward glycobiological events including the structure–function relationship of these biomolecules and for diagnostic and therapeutic purposes including tailor-made vaccine development and synthesis of structurally well-defined oligosaccharides (OS) become important. Efficient chemical glycosylation in high yield and stereoselectivity is however challenging and depends on the fine tuning of a protection profile to get matching glycosyl donor–acceptor reactivity along with proper use of other important external factors like catalyst, solvent, temperature, activator, and additive. So far, many glycosylation methods have been reported including several reviews also. In the present review, we will concentrate our discussion on the recent trend on α- and β-selective glycosylation reactions reported during the past decade.

Recent chemical syntheses of bacteria related oligosaccharides using modern expeditious approaches

Apart from some essential and crucial roles in life processes carbohydrates also are involved in a few detrimental courses of action related to human health, like infections by pathogenic microbes, cancer metastasis, transplanted tissue rejection, etc. Regarding management of pathogenesis by microbes, keeping in mind of multi drug-resistant bacteria and epidemic or endemic incidents, preventive measure by vaccination is the best pathway as also recommended by the WHO; by vaccination, eradication of bacterial diseases is also possible. Although some valid vaccines based on attenuated bacterial cells or isolated pure polysaccharide-antigens or the corresponding conjugates thereof are available in the market for prevention of several bacterial diseases, but these are not devoid of some disadvantages also. In order to develop improved conjugate T-cell dependent vaccines oligosaccharides related to bacterial antigens are synthesized and converted to the corresponding carrier protein conjugates. Marketed Cuban Quimi-Hib is such a vaccine being used since 2004 to resist Haemophilus influenza b infections. During nearly the past two decades research is going on worldwide for improved synthesis of bacteria related oligosaccharides or polysaccharides towards development of such semisynthetic or synthetic glycoconjugate vaccines. The present dissertation is an endeavour to encompass the recent syntheses of several pathogenic bacterial oligosaccharides or polysaccharides, made during the past ten-eleven years with special reference to modern expeditious syntheses.

Sequential activation of thioglycosides enables one-pot glycosylation

This review describes recent developments in relative reactivity value (RRV) controlled sequential glycosylation, pre-activation based iterative glycosylation, and sulfoxide activation initiated one-pot glycosylation.

Reagent controlled stereoselective synthesis of teichoic acid α-(1,2)-glucans

The stereoselective construction of 1,2-cis-glycosidic linkages is key in the assembly of biologically relevant glycans, but remains a synthetic challenge. Reagent-controlled glycosylation methodologies, in which external nucleophiles are employed to modulate the reactivity of the glycosylation system, have become powerful means for the construction of 1,2-cis-glycosidic linkages. Here we establish that nucleophilic additives can support the construction of α-1,2-glucans, and apply our findings in the construction of a d-alanine kojibiose functionalized glycerol phosphate teichoic acid fragment. This latter molecule can be found in the cell wall of the opportunistic Gram-positive bacterium, Enterococcus faecalis and represents a structural element that can possibly be used in the development of therapeutic vaccines and diagnostic tools.

Reagent Controlled Stereoselective Assembly of α-(1,3)-Glucans

Pre-activation based glycosylations have become a very powerful tool in the assembly of oligosaccharides and the use of nucleophilic additives allows for the in situ generation of reactive intermediates with tailored reactivity. We here use a glycosylation strategy that is based on the use of per-benzylated imidate building blocks for the fully stereoselective construction of a spacer equipped Aspergillus fumigatus α-1,3-octaglucan. We have used the trimethylsilyl iodide (TMSI)-triphenylphosphine oxide (Ph3P=O) for the stereoselective installation of an azidopropanol spacer and triflic acid (TfOH)-dimethyl formamide (DMF) enabled glycosylations for the coupling reactions with the secondary glucosyl C-3-alcohols. An operationally simple in situ activation coupling procedure is introduced and used for the final glycosylation events towards the octasaccharide.

Automated Chemical Oligosaccharide Synthesis: Novel Approach to Traditional Challenges

Advances in carbohydrate chemistry have certainly made common oligosaccharides much more accessible. However, many current methods still rely heavily upon specialized knowledge of carbohydrate chemistry. The application of automated technologies to chemical and life science applications such as genomics and proteomics represents a vibrant field. These automated technologies also present opportunities for their application to organic synthesis, including that of the synthesis of oligosaccharides. However, application of automated methods to the synthesis of carbohydrates is an underdeveloped area as compared to other classes of biomolecules. The overarching goal of this review article is to present the advances that have been made at the interface of carbohydrate chemistry and automated technology.

Synthetic avenues towards a tetrasaccharide related to Streptococcus pneumonia of serotype 6A

Streptococcus pneumonia (SPn) is a Gram-positive bacterium which causes life threatening diseases. The bacteria protect themselves against non-specific host defence by an external polysaccharide (PS) capsule which bears a repeating unit, α-D-Galp(1->3)-α-D-Glcp(1->3)-α-L-Rhap(1->3)-D-Rib (SPn 6A). A closer look at the structure reveals the presence of α-linked galactose and glucose residues. The synthesis of these 1,2-cis glycosidic linkages are considered challenging particularly in the context of a one-pot oligosaccharide synthesis. We have synthesized the aforesaid tetrasaccharide (SPn 6A) based on both stepwise and sequential one-pot glycosylation reactions using easily accessible common building blocks; eventually similar overall yields were obtained in both cases.

Pre-activation Based Stereoselective Glycosylations

Due to the wide presence of carbohydrates in nature and their crucial roles in numerous important biological processes, oligosaccharides have attracted a lot of attention in synthetic organic chemistry community. Many innovative synthetic methods have been developed for oligosaccharide synthesis, among which the pre-activation based glycosylation is particularly noteworthy. Traditionally, glycosylation reactions are carried out when the glycosyl donor and the acceptor are both present when the promoter is added. In comparison, the pre-activation based glycosylation is unique, where the glycosyl donor is activated by the promoter in the absence of the acceptor. Upon complete donor activation, the acceptor is added to the reaction mixture enabling glycosylation. The key step in any oligosaccharide synthesis is the stereoselective formation of the glycosidic bond. As donor activation and acceptor glycosylation are temporally separated, pre-activation based glycosylation can bestow unique stereochemical control. This review systematically discusses factors impacting the stereochemical outcome of a pre-activation based glycosylation reaction including substituents on the glycosyl donor, reaction solvent, and additives. Applications of pre-activation based stereoselective glycosylation in assembly of complex oligosaccharides are also discussed.

Controlling the stereoselectivity of glycosylation via solvent effects

This review covers a special topic in carbohydrate chemistry — solvent effects on the stereoselectivity of glycosylation reactions. Obtaining highly stereoselective glycosidic linkages is one of the most challenging tasks in organic synthesis, as it is affected by various controlling factors. One of the least understood factors is the effect of solvents. We have described the known solvent effects while providing both general rules and specific examples. We hope this review will not only help fellow researchers understand the known aspects of solvent effects and use that in their experiments, but moreover, we expect that more studies on this topic will be started and continued to expand our understanding of the mechanistic aspects of solvent effects in glycosylation reactions.

Stereocontrolled 1,2-cis glycosylation as the driving force of progress in synthetic carbohydrate chemistry

Recent developments in stereoselective 1,2-cis glycosylation that have emerged during the past decade are surveyed herein.

Recent developments in stereoselective 1,2-cis glycosylation that have emerged during the past decade are surveyed herein. For detailed coverage of the previous achievements in the field the reader is referred to our earlier reviews: A. V. Demchenko, Curr. Org. Chem. , 2003, 7 , 35–79 and Synlett , 2003, 1225–1240.

Cu(ClO4)2·6H2O catalyzed solvent free per-O-acetylation and sequential one-pot conversions of sugars to thioglycosides

The solvent free per-O-acetylation of various reducing and non-reducing sugars has been carried out using stoichiometric amounts of acetic anhydride and copper(ii) perchlorate hexahydrate as the catalyst.

Modulating glycosylation with exogenous nucleophiles: an overview

The major challenge in carbohydrate synthesis is stereochemical control of glycosidic bond formation. Different glycosylation methods have been developed that are based on the modulation effect of external nucleophiles. This review highlights the development, synthetic application, challenges and outlook of the modulated glycosylation methods.

Hydrogen bond mediated aglycone delivery: synthesis of linear and branched α-glucans

A Hydrogen bond mediated aglycone delivery (HAD) method was applied to the synthesis of α-glucans, which are abundant in nature, but as targets represent a notable challenge to chemists. The synthesis of linear oligosaccharide sequences was accomplished in complete stereoselectivity in all glycosylations. The efficacy of HAD may diminish with the increased bulk of the glycosyl acceptor, and may be an important factor for the syntheses of oligomers beyond pentasaccharides. The synthesis of a branched structure proved more challenging, particularly with bulky trisaccharide acceptors.

Regenerative glycosylation under nucleophilic catalysis

This article describes 3,3-difluoroxindole (HOFox)-mediated glycosylation. The uniqueness of this approach is that both the in situ synthesis of 3,3-difluoro-3H-indol-2-yl (OFox) glycosyl donors and activation thereof can be conducted in a regenerative fashion as is a typical reaction performed under nucleophilic catalysis. Only a catalytic amount of the OFox imidate donor and a Lewis acid activator are present in the reaction medium. The OFox imidate donor is constantly regenerated upon its consumption until glycosyl acceptor has reacted.