Preactivation: An Alternative Strategy in Stereoselective Glycosylation and Oligosaccharide Synthesis

Donor Preactivation-Based Glycan Assembly: from Manual to Automated Synthesis

Carbohydrates are called the third chain of life. Carbohydrates participate in many important biochemical functions in living species, and the biological information carried by them is several orders of magnitude larger than that of nucleic acids and proteins. However, due to the intrinsic complexity and heterogeneity of carbohydrate structures, furnishing pure and structurally well-defined glycans for functional studies is a formidable task, especially for homogeneous large-size glycans. To address this issue, we have developed a donor preactivation-based one-pot glycosylation strategy enabling multiple sequential glycosylations in a single reaction vessel.The donor preactivation-based one-pot glycosylation refers to the strategy in which the glycosyl donor is activated in the absence of a glycosyl acceptor to generate a reactive intermediate. Subsequently, the glycosyl acceptor with the same anomeric leaving group is added, leading to a glycosyl coupling reaction, which is then iterated to rapidly achieve the desired glycan in the same reactor. The advantages of this strategy include the following: (1) unique chemoselectivity is obtained after preactivation; (2) it is independent of the reactivity of glycosyl donors; (3) multiple-step glycosylations are enabled without the need for intermediate purification; (4) only stoichiometric building blocks are required without complex protecting group manipulations. Using this protocol, a range of glycans including tumor-associated carbohydrate antigens, various glycosaminoglycans, complex N-glycans, and diverse bacterial glycans have been synthesized manually. Gratifyingly, the synthesis of mycobacterial arabinogalactan containing 92 monosaccharide units has been achieved, which created a precedent in the field of polysaccharide synthesis. Recently, the synthesis of a highly branched arabinogalactan from traditional Chinese medicine featuring 140 monosaccharide units has been also accomplished to evaluate its anti-pancreatic-cancer activity. In the spirit of green and sustainable chemistry, this strategy can also be applied to light-driven glycosylation reactions, where either UV or visible light can be used for the activation of glycosyl donors.Automated synthesis is an advanced approach to the construction of complex glycans. Based on the two preactivation modes (general promoter activation mode and light-induced activation mode), a universal and highly efficient automated solution-phase synthesizer was further developed to drive glycan assembly from manual to automated synthesis. Using this synthesizer, a library of oligosaccharides covering various glycoforms and glycosidic linkages was assembled rapidly, either in a general promoter-activation mode or in a light-induced-activation mode. The automated synthesis of a fully protected fondaparinux pentasaccharide was realized on a gram scale. Furthermore, the automated synthesis of large-size polysaccharides was performed, allowing the assembly of arabinans up to an astonishing 1080-mer using the automated multiplicative synthesis strategy, taking glycan synthesis to a new height far beyond the synthesis of nucleic acids (up to 200-mer) and proteins (up to 472-mer).

Boron-mediated aglycon delivery (BMAD) for the stereoselective synthesis of 1,2-cis glycosides

1,2-cis Glycosides are frequently found in biologically active natural products, pharmaceutical compounds, and highly functional materials. Therefore, elucidating the role of mechanism of their biological activities will help clarify the structure-activity relationships of these diverse compounds and create new lead compounds for pharmaceuticals by modifying their structures. However, unlike 1,2-trans glycosides, the stereoselective synthesis of 1,2-cis glycosides remains difficult due to the nonavailability of neighboring group participation from the 2-O-acyl functionalities of the glycosyl donors. In this context, we recently developed organoboron-catalyzed 1,2-cis-stereoselecitve glycosylations, called boron-mediated aglycon delivery (BMAD) methods. In this review article, we introduce the BMAD methods and several examples of their application to the synthesis of biologically active glycosides.

Small tools for sweet challenges: advances in microfluidic technologies for glycan synthesis

Glycans, including oligosaccharides and glycoconjugates, play an integral role in modulating the biological functions of macromolecules. Many physiological and pathological processes are mediated by interactions between glycans, which has led to the use of glycans as biosensors for pathogen and biomarker detection. Elucidating the relationship between glycan structure and biological function is critical for advancing our understanding of the impact glycans have on human health and disease and for expanding the repertoire of glycans available for bioanalysis, especially for diagnostics. Such efforts have been limited by the difficulty in obtaining sufficient quantities of homogenous glycan samples needed to resolve the exact relationships between glycan structure and their structural or modulatory functions on a given glycoconjugate. Synthetic strategies offer a viable route for overcoming these technical hurdles. In recent years, microfluidics have emerged as powerful tools for realizing high-throughput and reproducible syntheses of homogenous glycans for the potential use in functional studies. This critical review provides readers with an overview of the microfluidic technologies that have been developed for chemical and enzymatic glycan synthesis. The advantages and limitations associated with using microreactor platforms to improve the scalability, productivity, and selectivity of glycosylation reactions will be discussed, as well as suggested future work that can address certain pitfalls.

Total Synthesis of a Chimeric Glycolipid Bearing the Partially Acetylated Backbone of Sponge-Derived Agminoside E

We describe the total synthesis of a chimeric glycolipid bearing both the partially acetylated backbone of sponge-derived agminoside E and the (R)-3-hydroxydecanoic acid chain of bacterial rhamnolipids. The branched pentaglucolipid skeleton was achieved using a [3 + 2] disconnection approach. The β-(1 → 2) and β-(1 → 4)-glycosidic bonds were synthesized through a combination of NIS/Yb(OTf)₃- and TMSOTf-mediated stereoselective glycosylations of thiotolyl, N-phenyltrifluoroacetimidate, and trichloroacetimidate donors. Late-stage pentaacetylation, Staudinger reduction of a (2-azidomethyl)benzoyl group, followed by continuous-flow microfluidic hydrogenolysis completed the total synthesis of the structurally simplified glycolipid, whose partial acetylation pattern on the glycan part was identical to agminoside E. Our study lays the foundation for the total synthesis of sponge-derived agminosides and the understanding of their biological functions in sponges.

Transition-metal-free synthesis of aryl 1-thioglycosides with arynes at room temperature

A mild, convenient and transition-metal-free protocol for the synthesis of aryl 1-thioglycosides is presented via arynes generated in situ combined with glycosyl thiols in the presence of TBAF(tBuOH)₄. The methodology provides a general and efficient way to prepare a series of functionalized thioglycosides in good to excellent yields with a perfect control of the anomeric configuration at room temperature. In addition, the reaction conditions tolerate a variety of the pentoses and hexoses, and the reaction also performs smoothly on protected monosaccharides and disaccharides.

Generation of glucosylated sn-1-glycerolphosphate teichoic acids: glycerol stereochemistry affects synthesis and antibody interaction

Lipoteichoic acids (LTAs) have been addressed as possible antigen candidates for vaccine development against several opportunistic Gram-positive pathogens. The study of structure-immunogenicity relationship represents a challenge due to the heterogenicity of LTA extracted from native sources. LTAs are built up from glycerol phosphate (GroP) repeating units and they can be substituted at the C-2-OH with carbohydrate appendages or d-alanine residues. The substitution pattern, but also the absolute chirality of the GroP residues can impact the interaction with chiral biomolecules including antibodies and biosynthesis enzymes. We have generated a set of diastereomeric GroP hexamers bearing a glucosyl modification at one of the residues. The chirality of the glycerol building block had an important impact on the stereoselectivity of the glycosylation reaction between the glycosyl donor and the glycerol C-2-OH acceptor. The GroP C-2-chirality also played an important role in the interaction with TA recognizing antibodies. These findings have important implications for the design and synthesis of synthetic TA fragments for diagnostic and therapeutic applications.

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.

Stereoselective synthesis of a branched α-decaglucan

The first and convergent synthesis of a branched Arca subcrenata Lischke α-decaglucan containing all of the α-(1 → 3), α-(1 → 4), and α-(1 → 6) glycosyl linkages was efficiently achieved. The tri- and tetrasaccharide fragments and fully protected decasaccharide were assembled in a one-pot manner with excellent α-stereoselectivity, which was secured by the synergistic α-directing effects of the TolSCl/AgOTf catalysis system and the remote participation effect or steric β-shielding of functionalized groups at the donor 6-O-position. Low substrate concentration was revealed to favor the α-stereochemical outcome of glycosylations between bulkier building blocks. The synthetic approach established here would be very useful for the preparation of more complex α-glucans containing different types of glycosidic linkages and branched architectures.

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.

"One-Pot" Protection, Glycosylation, and Protection-Glycosylation Strategies of Carbohydrates

Carbohydrates, which are ubiquitously distributed throughout the three domains of life, play significant roles in a variety of vital biological processes. Access to unique and homogeneous carbohydrate materials is important to understand their physical properties, biological functions, and disease-related features. It is difficult to isolate carbohydrates in acceptable purity and amounts from natural sources. Therefore, complex saccharides with well-defined structures are often most conviently accessed through chemical syntheses. Two major hurdles, regioselective protection and stereoselective glycosylation, are faced by carbohydrate chemists in synthesizing these highly complicated molecules. Over the past few years, there has been a radical change in tackling these problems and speeding up the synthesis of oligosaccharides. This is largely due to the development of one-pot protection, one-pot glycosylation, and one-pot protection-glycosylation protocols and streamlined approaches to orthogonally protected building blocks, including those from rare sugars, that can be used in glycan coupling. In addition, new automated strategies for oligosaccharide syntheses have been reported not only for program-controlled assembly on solid support but also by the stepwise glycosylation in solution phase. As a result, various sugar molecules with highly complex, large structures could be successfully synthesized. To summarize these recent advances, this review describes the methodologies for one-pot protection and their one-pot glycosylation into the complex glycans and the chronological developments associated with automated syntheses of oligosaccharides.

Reagent Controlled Stereoselective Synthesis of α-Glucans

The development of a general glycosylation method that allows for the stereoselective construction of glycosidic linkages is a tremendous challenge. Because of the differences in steric and electronic properties of the building blocks used, the outcome of a glycosylation reaction can vary greatly when switching form one glycosyl donor-acceptor pair to another. We here report a strategy to install cis-glucosidic linkages in a fully stereoselective fashion that is under direct control of the reagents used to activate a single type of donor building block. The activating reagents are tuned to the intrinsic reactivity of the acceptor alcohol to match the reactivity of the glycosylating agent with the reactivity of the incoming nucleophile. A protecting group strategy is introduced that is based on the sole use of benzyl-ether type protecting groups to circumvent changes in reactivity as a result of the protecting groups. For the stereoselective construction of the α-glucosyl linkages to a secondary alcohol, a per-benzylated glusosyl imidate donor is activated with a combination of trimethylsilyltriflate and DMF, while activation of the same imidate donor with trimethylsilyl iodide in the presence of triphenylphosphine oxide allows for the stereoselective cis-glucosylation of primary alcohols. The effectiveness of the strategy is illustrated in the modular synthesis of a Mycobacterium tuberculosis nonasaccharide, composed of an α-(1-4)-oligoglucose backbone bearing different α-glucosyl branches.

Regenerative Glycosylation

Previously, we communicated 3,3-difluoroxindole (HOFox)-mediated glycosylations wherein 3,3-difluoro-3H-indol-2-yl (OFox) imidates were found to be key intermediates. Both the in situ synthesis from the corresponding glycosyl bromides and activation of the OFox imidates could be conducted in a regenerative fashion. Herein, we extend this study to the synthesis of various glycosidic linkages using different sugar series. The main outcome of this study relates to enhanced yields and/or reduced reaction times of glycosylations. The effect of HOFox-mediated reactions is particularly pronounced in case of unreactive glycosyl donors and/or glycosyl acceptors. A multistep regenerative synthesis of oligosaccharides is also reported.

Intramolecular glycosylation

Carbohydrate oligomers remain challenging targets for chemists due to the requirement for elaborate protecting and leaving group manipulations, functionalization, tedious purification, and sophisticated characterization. Achieving high stereocontrol in glycosylation reactions is arguably the major hurdle that chemists experience. This review article overviews methods for intramolecular glycosylation reactions wherein the facial stereoselectivity is achieved by tethering of the glycosyl donor and acceptor counterparts.

A flexible 1,2-cis α-glycosylation strategy based on in situ adduct transformation

Towards a universal 1,2-cis α-glycosylation strategy using the joined forces of formamide and iodide nucleophile additives.

Glycosylation intermediates studied using low temperature1H- and19F-DOSY NMR: new insight into the activation of trichloroacetimidates

Low temperature¹H- and¹⁹F-DOSY have been used for analyzing reactive intermediates in glycosylation reactions, where a glycosyl trichloroacetimidate donor has been activated using different catalysts.

Light-driven highly efficient glycosylation reactions

A novel and efficient light-driven glycosylation strategy was developed with wide substrate scope/generality.

Assembly of a Complex Branched Oligosaccharide by Combining Fluorous-Supported Synthesis and Stereoselective Glycosylations using Anomeric Sulfonium Ions

There is an urgent need to develop reliable strategies for the rapid assembly of complex oligosaccharides. This paper presents a set of strategically selected orthogonal protecting groups, glycosyl donors modified by a (S)-phenylthiomethylbenzyl ether at C-2, and a glycosyl acceptor containing a fluorous tag, which makes it possible to rapidly prepare complex branched oligosaccharides of biological importance. The C-2 auxiliary controlled the 1,2-cis anomeric selectivity of the various galactosylations. The orthogonal protecting groups, 2-naphthylmethyl ether (Nap) and levulinic ester (Lev), made it possible to generate glycosyl acceptors and allowed the installation of a crowded branching point. After the glycosylations, the chiral auxiliary could be removed using acidic conditions, which was compatible with the presence of the orthogonal protecting groups Lev and Nap, thereby allowing the efficient installation of 1,2-linked glycosides. The light fluorous tag made it possible to purify the compounds by a simple filtration method using silica gel modified by fluorocarbons. The set of building blocks was successfully employed for the preparation of the carbohydrate moiety of the GPI anchor of Trypanosoma brucei, which is a parasite that causes sleeping sickness in humans and similar diseases in domestic animals.

Synthesis of unsymmetrical 3,6-branched Man5 oligosaccharide: a comparison between one-pot sequential glycosylation and stepwise synthesis

An expeditious three-component, one-pot sequential glycosylation protocol has been developed for the preparation of 3,6-branched unsymmetrical mannopentaose (Man5), employing a mannose trisaccharide donor, a mannose monosaccharide donor and a mannose monosaccharide acceptor. The high efficiency of this one-pot procedure was demonstrated by comparison study with a stepwise synthesis using the same three building blocks.

Regioselective one-pot protection, protection-glycosylation and protection-glycosylation-glycosylation of carbohydrates: a case study with D-glucose

Well-defined oligosaccharides are important requirements in evaluating structure-activity relationships to decipher the roles of carbohydrates in various physiological processes. These oligosaccharides are accessed mainly through chemical synthesis, which nonetheless remains a huge undertaking despite the many advances in recent years. A combinatorial and regioselective one-pot protection strategy was previously disclosed by us to reduce the effort and wastes associated with carbohydrate synthesis. With the tetra-trimethylsilylated 4-methylphenyl thioglucoside as the starting material, we herein show the one-pot preparations of diols, triols and fully protected derivatives of thioglucosides, and, more importantly, we generated building blocks in situ that effectively acted as glycosyl donors and glycosyl acceptors for further coupling with other monosaccharide building blocks. Our one-pot protection-glycosylation and protection-glycosylation-glycosylation approaches made use of the perceived reactivity differences between thioglycoside donors to conveniently supply disaccharide and trisaccharide skeletons as well as the backbone of a recently discovered compatible solute from two thermophilic bacteria of the Petrotoga species. The demonstrated protocol is another step in reducing the enormous work in carbohydrate synthesis and efficiently delivering sugar constructs for application in other areas of glycobiology.

Microwave-assisted one-pot synthesis of 1,6-anhydrosugars and orthogonally protected thioglycosides

Living organisms employ glycans as recognition elements because of their large structural information density. Well-defined sugar structures are needed to fully understand and take advantage of glycan functions, but sufficient quantities of these compounds cannot be readily obtained from natural sources and have to be synthesized. Among the bottlenecks in the chemical synthesis of complex glycans is the preparation of suitably protected monosaccharide building blocks. Thus, easy, rapid, and efficient methods for building-block acquisition are desirable. Herein, we describe routes directly starting from the free sugars toward notable monosaccharide derivatives through microwave-assisted one-pot synthesis. The procedure followed the in situ generation of per-O-trimethylsilylated monosaccharide intermediates, which provided 1,6-anhydrosugars or thioglycosides upon treatment with either trimethylsilyl trifluoromethanesulfonate or trimethyl(4-methylphenylthio)silane and ZnI2, respectively, under microwave irradiation. We successfully extended the methodology to regioselective protecting group installation and manipulation toward a number of thioglucosides and the glycosylation of persilylated derivatives, all of which were conducted in a single vessel. These developed approaches open the possibility for generating arrays of suitably protected building blocks for oligosaccharide assembly in a short period with minimal number of purification stages.

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.

Selective synthesis of 1,2-cis-α-glycosides without directing groups. Application to iterative oligosaccharide synthesis

A method for the highly selective synthesis of 1,2-cis-α-linked glycosides that does not require the use of the specialized protecting group patterns normally employed to control diastereoselectivity is described. Thioglycoside acceptors can be used, permitting iterative oligosaccharide synthesis. The approach eliminates the need for lengthy syntheses of monosaccharides possessing highly specialized and unconventional protecting group patterns.