Dissecting the mechanisms of a class of chemical glycosylation using primary ¹³C kinetic isotope effects

Role of ion pairs in model glycosylation reactions of permethylated glucosyl and xylosyl triflates

Elucidating the molecular mechanisms of chemical O-glycosylation remains a significant challenge in glycochemistry. This study examines the mechanism of the nucleophilic substitution reaction between glycosyl triflates, which are extensively used in studies of glycosylation mechanisms, and several acceptor alcohols. The investigation was conducted through a comparative analysis of permethylated glucosyl triflate GTf and its xylosyl counterpart XTf. The glycosylation reactions, conducted in dichloromethane using GTf and XTf with EtOH, tBuOH, and CF3CH2OH, exhibited diverse α/β selectivities depending on the types of donor and acceptor molecules used. Identifying a unified mechanism to explain this range of selectivities proved challenging. Notably, we observed a distinct trend wherein the addition of excess triflate salt (Bu4NOTf) had a more pronounced effect on the α/β selectivity in glycosylation reactions utilizing XTf compared to those using GTf. Quantum chemical calculations performed at the SCS-MP2//DFT(M06-2X) level, with explicit inclusion of five solvent molecules, showed that contact ion pairs arising from XTf were significantly more stable than those from GTf. These experimental and computational results strongly suggest that ion pairs play a more crucial role in the glycosylation process involving XTf than GTf. Additionally, our quantum chemical analyses clarified that the enhanced stability of the ion pairs from XTf was attributed not to the strength of the C-1-OTf bond within XTf but to the flexibility in the conformational changes of XTf's pyranosyl ring.

Acetal Substitution Reactions: Stereoelectronic Effects, Conformational Analysis, Reactivity vs. Selectivity, and Neighboring-Group Participation

Acetal substitution reactions can proceed by a number of mechanisms, but oxocarbenium ion intermediates are involved in many of these reactions. Our research has focused on understanding the conformational preferences, structures, and reactions of these intermediates. This Account summarizes our observations that electrostatic effects play a significant role in defining the preferred conformations, and that torsional effects determine how those intermediates react. Neighboring-group effects are not as straightforward as they might seem, considering that oxocarbenium ion intermediates are in equilibrium with structures that involve stabilization by a nearby substituent.

Glycosylation with sulfoxide-based glycosyl donors

Sulfoxides have emerged as pivotal constituents in modern carbohydrate chemistry. As anomeric leaving groups, sulfinyl moieties may occupy positions directly at the anomeric position or at a more remote site. This feature article is focused on the evolution and notable advancements of glycosyl sulfoxide donors in glycosylation reactions. Its objective is to elucidate the obstacles and prospects within this evolving research domain, with the aim of enhancing comprehension and progress in the field of carbohydrate chemistry.

Development of a Cryogenic Flow Reactor to Optimize Glycosylation Reactions Based on the Active Donor Intermediate

The development of a continuous flow reactor for stereospecific glycosylation reactions with deoxy sugars is described. This apparatus that permits optimizing the selectivity of glycosylation reactions based on the stability of the activated intermediate is described. By coupling a flow apparatus with HPLC analysis, we can optimize the yield of TsCl-mediated β-linked deoxy sugar construction in a matter of hours. In all cases, results from continuous flow processing translate into improved results in batch-scale reactions, as demonstrated by competition experiments. This is the result of carrying out optimization to identify the ideal temperature for the reaction of the activated intermediate, as opposed to the initial activation conditions. Such an approach allows for the rapid development of highly selective glycosylation reactions in cases in which classical neighboring group participation is not possible.

An Open-Air Palladium-Catalyzed Stereoselective O-Glycosylation of Glycals via in-situ Generation of gem-Disubstituted Methanols from p-Quinone Methides

We devised a palladium-catalyzed α-stereoselective glycosylation that incorporates oxygen via in-situ generation of gem-disubstituted methanols from p-quinone methides to access 2,3-unsaturated gem-diarylmethyl O-glycosides under open-air atmosphere at room temperature. Advantages of this environmentally friendly strategy include the absence of additives and ligands, using water as the green source of oxygen, mildest, operationally simple, exhibiting a wide functional group tolerance, and compatibility with a variety of glycal progenitors in appreciable yields. A mechanistic study has been verified via H2 18 O labeling, which validates that water (moisture) is a sole source of oxygen.

Anomeric Triflates versus Dioxanium Ions: Different Product-Forming Intermediates from 3-Acyl Benzylidene Mannosyl and Glucosyl Donors

Minimal structural differences in the structure of glycosyl donors can have a tremendous impact on their reactivity and the stereochemical outcome of their glycosylation reactions. Here, we used a combination of systematic glycosylation reactions, the characterization of potential reactive intermediates, and in-depth computational studies to study the disparate behavior of glycosylation systems involving benzylidene glucosyl and mannosyl donors. While these systems have been studied extensively, no satisfactory explanations are available for the differences observed between the 3-O-benzyl/benzoyl mannose and glucose donor systems. The potential energy surfaces of the different reaction pathways available for these donors provide an explanation for the contrasting behavior of seemingly very similar systems. Evidence has been provided for the intermediacy of benzylidene mannosyl 1,3-dioxanium ions, while the formation of the analogous 1,3-glucosyl dioxanium ions is thwarted by a prohibitively strong flagpole interaction of the C-2-O-benzyl group with the C-5 proton in moving toward the transition state, in which the glucose ring adopts a B2,5-conformation. This study provides an explanation for the intermediacy of 1,3-dioxanium ions in the mannosyl system and an answer to why these do not form from analogous glucosyl donors.

Synthesis of sialyl halides with various acyl protective groups

Glycosyl halides are historically one of the first glycosyl donors used in glycosylation reactions, and interest in glycosylation reactions involving this class of glycosyl donors is currently increasing. New methods for their activation have been proposed and effective syntheses of oligosaccharides with their participation have been developed. At the same time, the possibilities of using these approaches to the synthesis of sialosides are restricted by the limited diversity of known sialyl halides (previously, mainly sialyl chlorides, less often sialyl bromides and sialyl fluorides, with acetyl (Ac) groups at the oxygen atoms and AcNH, Ac2N and N3 groups at C-5 were used). This work describes the synthesis of six new N-acetyl- and N-trifluoroacetyl-sialyl chlorides and bromides with O-chloroacetyl and O-trifluoroacetyl protective groups. Preparation of N,O-trifluoroacetyl protected derivatives was made possible due to development of the synthesis of sialic acid methyl ester pentaol with N-trifluoroacetyl group.

Unraveling the promoter effect and the roles of counterion exchange in glycosylation reaction

The stereoselectivity of glycosidic bond formation continues to pose a noteworthy hurdle in synthesizing carbohydrates, primarily due to the simultaneous occurrence of SN1 and SN2 processes during the glycosylation reaction. Here, we applied an in-depth analysis of the glycosylation mechanism by using low-temperature nuclear magnetic resonance and statistical approaches. A pathway driven by counterion exchanges and reaction byproducts was first discovered to outline the stereocontributions of intermediates. Moreover, the relative reactivity values, acceptor nucleophilic constants, and Hammett substituent constants (σ values) provided a general index to indicate the mechanistic pathways. These results could allow building block tailoring and reaction condition optimization in carbohydrate synthesis to be greatly facilitated and simplified.

Origin of Stereoselectivity in SE 2' Reactions of Six-membered Ring Oxocarbenium Ions

Oxocarbenium ions are key reactive intermediates in organic chemistry. To generate a series of structure-reactivity-stereoselectivity principles for these species, we herein investigated the bimolecular electrophilic substitution reactions (S_E 2') between allyltrimethylsilane and a series of archetypal six-membered ring oxocarbenium ions using a combined density functional theory (DFT) and coupled-cluster theory approach. These reactions preferentially proceed following a reaction path where the oxocarbenium ion transforms from a half chair (³ H₄ or ⁴ H₃ ) to a chair conformation. The introduction of alkoxy substituents on six-membered ring oxocarbenium ions, dramatically influences the conformational preference of the canonical ³ H₄ and ⁴ H₃ conformers, and thereby the stereochemical outcome of the S_E 2' reaction. In general, we find that the stereoselectivity in the reactions correlates to the "intrinsic preference" of the cations, as dictated by their shape. However, for the C5-CH₂ OMe substituent, steric factors override the "intrinsic preference", showing a more selective reaction than expected based on the shape of the ion. Our S_E 2' energetics correlate well with experimentally observed stereoselectivity, and the use of the activation strain model has enabled us to quantify important interactions and structural features that occur in the transition state of the reactions to precisely understand the relative energy barriers of the diastereotopic addition reactions. The fundamental mechanistic insight provided in this study will aid in understanding the reactivity of more complex glycosyl cations featuring multiple substituents and will facilitate our general understanding of glycosylation reactions.

Stereoselective Synthesis of 2-Deoxy Glycosides via Iron Catalysis

An Fe-catalyzed 2-deoxy glycosylation method was developed from 3,4-O-carbonate glycals directly at room temperature. This novel approach enabled facile access to alkyl and aryl 2-deoxy glycosides in high yields with exclusive α-stereoselectivity, tolerating various alcohols, phenols, and glycals. The synthetic utility and advantage of this strategy have been demonstrated by the modification of six natural products and the construction of a tetrasaccharide.

Conformationally restricted donors for stereoselective glycosylation

In nucleophilic reactions using sugars as electrophiles, i.e., glycosyl donors, their conformation affects the generation rate or stability of the glycosyl cation intermediates and determines at which side of the S_N2-S_N1 borderline and at what rate the reaction occurs. In addition, changes in the conformation create the steric or stereoelectronic effects of the substituents, which also change the reaction rate and stereoselectivity. Bulky silyl protecting groups, uronic acid esters, and transannular structures have been utilized to change the conformation. Consequently, reactions with unique reactivities and stereoselectivities have been developed. In this chapter, a discussion of the reaction mechanisms relating stereoselectivity to conformation is provided.

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.

New chemical processes to streamline carbohydrate synthesis

Carbohydrates hold potential for the future of therapeutic development due to their important role in essential biological processes. However, it is still challenging to produce homogenous materials, especially for non-mammalian sugars that are considered rare. Recent developments in this field have focused on catalytic methods, including organometallic and organocatalytic approaches to regioselective functionalization. Many approaches to glycosylations also utilize catalysts, increasingly in combination with photoredox conditions, to achieve stereoselectivity. Additionally, there have been significant advancements in the automation of glycosylation to synthesize oligosaccharides in less time and with fewer manually conducted steps by the user.

Synthesis and Glycosidation of Anomeric Halides: Evolution from Early Studies to Modern Methods of the 21st Century

Advances in synthetic carbohydrate chemistry have dramatically improved access to common glycans. However, many novel methods still fail to adequately address challenges associated with chemical glycosylation and glycan synthesis. Since a challenge of glycosylation has remained, scientists have been frequently returning to the traditional glycosyl donors. This review is dedicated to glycosyl halides that have played crucial roles in shaping the field of glycosciences and continue to pave the way toward our understanding of chemical glycosylation.

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.

Exhaustive Conformational Search for Sialyl Cation Reveals Possibility of Remote Participation of Acyl Groups

Finding convenient ways for the stereoselective α-sialylation is important due to the high practical significance of α-sialic acid-containing glycans and neoglycoconjugates. It was proposed that sialylation stereoselectivity is determined by the structure of the sialyl cation (also known in biochemistry as "sialosyl cation"), a supposed intermediate in this reaction. Here we design a new approach for studying the conformational space of highly flexible sialyl cation and find 1625 unique conformers including those stabilized by covalent remote participation (also known as long-range participation) of 4-O-acetyl (4-OAc), 5-N-trifluoroacetyl (5-NTFA), as well as 7,8,9-OAc from both α and β sides. The most energetically stable sialyl cation conformers are featured by 4-OAc participation, closely followed by 5-NTFA- and 7-OAc-stabilized conformers; unstabilized sialyl cation conformers are ∼10 kcal mol^-1 less stable than the 4-OAc-stabilized ones. Analysis of all the obtained conformers by means of substituents positions, side chain conformations and ring puckering led us to a new "eight-conformer hypothesis" which describes interconversions among the most important sialyl cation conformers and predicts that stronger remote participation of acyl groups favors β-anomers. Thus, selective synthesis of the desired α-sialosides requires minimization of acyl groups participation.

2,3-Carbamate mannosamine glycosyl donors in glycosylation reactions of diacetone-d-glucose. An experimental and theoretical study

The role of the cyclic 2,3-N,O-carbamate protecting group in directing the selectivity of mannosylation reactions of diacetone-d-glucose, promoted by BSP/Tf2O via α-triflate intermediates, has been investigated through a combined computational and experimental approach. DFT calculations were used to locate the transition states leading to the α or β anomers. These data indicate the preferential formation of the β-adduct with mannosyl donors either equipped with the 4,6-O-benzylidene protection or without it. The synthetic results confirmed this preference, showing in both cases an α/β selectivity of 4:6. This highlights a role for the 2,3-N,O-carbamate in sharp contrast with what described in the case of 2,3-O-carbonate mannosyl donors.

Glycosyl Sulfonates Beyond Triflates

While glycosyl triflates are frequently invoked as intermediates in many chemical glycosylation reactions, the chemistry of other glycosyl sulfonates remains comparatively underexplored. Given the reactivity of sulfonates can span several orders of magnitude, this represents an untapped resource for the development of stereoselective glycosylation reactions. This personal account describes our laboratories efforts to take advantage of this reactivity to develop β-specific glycosylation reactions. Initial investigations led to the development of 2-deoxy-sugar tosylates as highly selective donors for β-glycoside synthesis, an approach which has been used to great success by our group and others for the construction of deoxy-sugar oligosaccharides and natural products. Subsequent studies demonstrate that "matching" the reactivity of the sulfonate to that of the sugar donor leads to highly selective SN 2-glycosylations with a range of substrates.

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.

A Mechanistic Probe into 1,2-cis Glycoside Formation Catalyzed by Phenanthroline and Further Expansion of Scope

Phenanthroline, a rigid and planar compound with two fused pyridine rings, has been used as a powerful ligand for metals and a binding agent for DNA/RNA. We discovered that phenanthroline could be used as a nucleophilic catalyst to efficiently access high yielding and diastereoselective α-1,2-cis glycosides through the coupling of hydroxyl acceptors with α-glycosyl bromide donors. We have conducted an extensive investigation into the reaction mechanism, wherein the two glycosyl phenanthrolinium ion intermediates, a 4C1 chair-liked β-conformer and a B2,5 boat-like α-conformer, have been detected in a ratio of 2:1 (β:α) using variable temperature NMR experiments. Furthermore, NMR studies illustrate that a hydrogen bonding is formed between the second nitrogen atom of phenanthroline and the C1-anomeric hydrogen of sugar moiety to stabilize the phenanthrolinium ion intermediates. To obtain high α-1,2-cis stereoselectivity, a Curtin-Hammett scenario was proposed wherein interconversion of the 4C1 chair-like β-conformer and B2,5 boat-like α-conformer is more rapid than nucleophilic addition. Hydroxyl attack takes place from the α-face of the more reactive 4C1 β-phenanthrolinium intermediate to give an α-anomeric product. The utility of the phenanthroline catalysis is expanded to sterically hindered hydroxyl nucleophiles and chemoselective coupling of an alkyl hydroxyl group in the presence of a free C1-hemiacetal. In addition, the phenanthroline-based catalyst has a pronounced effect on site-selective couplings of triol motifs and orthogonally activates the anomeric bromide leaving group over the anomeric fluoride and sulfide counterparts.

Total Synthesis of the Photorhabdus temperata ssp. Cinereal 3240 Zwitterionic Trisaccharide Repeating Unit

Zwitterionic carbohydrate modifications, such as phosphoethanolamine (PEtN), govern host-pathogen interactions. Whereas it is recognized that these modifications stimulate the host immune system, the purpose of PEtN modification remains largely descriptive. As an enabling step toward studying this carbohydrate modification, we report a synthesis of the P. temperata zwitterionic trisaccharide repeating unit. The 32-step synthesis was enabled by H-phosphonate chemistry to install the PEtN arm on a poorly reactive and sterically hindered C4-alcohol.

Glycosyl Oxocarbenium Ions: Structure, Conformation, Reactivity, and Interactions

Carbohydrates (glycans, saccharides, and sugars) are essential molecules in all domains of life. Research on glycoscience spans from chemistry to biomedicine, including material science and biotechnology. Access to pure and well-defined complex glycans using synthetic methods depends on the success of the employed glycosylation reaction. In most cases, the mechanism of the glycosylation reaction is believed to involve the oxocarbenium ion. Understanding the structure, conformation, reactivity, and interactions of this glycosyl cation is essential to predict the outcome of the reaction. In this Account, building on our contributions on this topic, we discuss the theoretical and experimental approaches that have been employed to decipher the key features of glycosyl cations, from their structures to their interactions and reactivity.We also highlight that, from a chemical perspective, the glycosylation reaction can be described as a continuum, from unimolecular SN1 with naked oxocarbenium cations as intermediates to bimolecular SN2-type mechanisms, which involve the key role of counterions and donors. All these factors should be considered and are discussed herein. The importance of dissociative mechanisms (involving contact ion pairs, solvent-separated ion pairs, solvent-equilibrated ion pairs) with bimolecular features in most reactions is also highlighted.The role of theoretical calculations to predict the conformation, dynamics, and reactivity of the oxocarbenium ion is also discussed, highlighting the advances in this field that now allow access to the conformational preferences of a variety of oxocarbenium ions and their reactivities under SN1-like conditions.Specifically, the ground-breaking use of superacids to generate these cations is emphasized, since it has permitted characterization of the structure and conformation of a variety of glycosyl oxocarbenium ions in superacid solution by NMR spectroscopy.We also pay special attention to the reactivity of these glycosyl ions, which depends on the conditions, including the counterions, the possible intra- or intermolecular participation of functional groups that may stabilize the cation and the chemical nature of the acceptor, either weak or strong nucleophile. We discuss recent investigations from different experimental perspectives, which identified the involved ionic intermediates, estimating their lifetimes and reactivities and studying their interactions with other molecules. In this context, we also emphasize the relationship between the chemical methods that can be employed to modulate the sensitivity of glycosyl cations and the way in which glycosyl modifying enzymes (glycosyl hydrolases and transferases) build and cleave glycosidic linkages in nature. This comparison provides inspiration on the use of molecules that regulate the stability and reactivity of glycosyl cations.

Automated Quantification of Hydroxyl Reactivities: Prediction of Glycosylation Reactions

The stereoselectivity and yield in glycosylation reactions are paramount but unpredictable. We have developed a database of acceptor nucleophilic constants (Aka) to quantify the nucleophilicity of hydroxyl groups in glycosylation influenced by the steric, electronic and structural effects, providing a connection between experiments and computer algorithms. The subtle reactivity differences among the hydroxyl groups on various carbohydrate molecules can be defined by Aka, which is easily accessible by a simple and convenient automation system to assure high reproducibility and accuracy. A diverse range of glycosylation donors and acceptors with well-defined reactivity and promoters were organized and processed by the designed software program "GlycoComputer" for prediction of glycosylation reactions without involving sophisticated computational processing. The importance of Aka was further verified by random forest algorithm, and the applicability was tested by the synthesis of a Lewis A skeleton to show that the stereoselectivity and yield can be accurately estimated.

Computational evidence of glycosyl cations

Glycosyl cations are key intermediates in the glycosylation reactions taking place through a S_N1-type mechanism. To obtain a reliable description of the glycosylation reaction mechanism a combination of computational studies and experimental data such as kinetic isotopic effects is needed. Computational studies have elucidated S_N2-type glycosylation reaction mechanisms, but elucidation of mechanisms in which ion pairs can be formed presents some difficulties because of the recombination of the ions. Recent topological and dynamic studies open the door to the ultimate confirmation of the presence of glycosyl cations in the form of intimate ion pairs during certain glycosylation reactions. This review covers the state-of-the-art tools and applications of computational chemistry mainly developed during the last ten years to understand glycosylation reactions in which an oxocarbenium ion could be involved.

Ab Initio Molecular Dynamics Simulations of the SN1/SN2 Mechanistic Continuum in Glycosylation Reactions

We report a computational approach to evaluate the reaction mechanisms of glycosylation using ab initio molecular dynamics (AIMD) simulations in explicit solvent. The reaction pathways are simulated via free energy calculations based on metadynamics and trajectory simulations using Born-Oppenheimer molecular dynamics. We applied this approach to investigate the mechanisms of the glycosylation of glucosyl α-trichloroacetimidate with three acceptors (EtOH, i-PrOH, and t-BuOH) in three solvents (ACN, DCM, and MTBE). The reactants and the solvents are treated explicitly using density functional theory. We show that the profile of the free energy surface, the synchronicity of the transition state structure, and the time gap between leaving group dissociation and nucleophile association can be used as three complementary indicators to describe the glycosylation mechanism within the SN1/SN2 continuum for a given reaction. This approach provides a reliable means to rationalize and predict reaction mechanisms and to estimate lifetimes of oxocarbenium intermediates and their dependence on the glycosyl donor, acceptor, and solvent environment.

En Route to the Transformation of Glycoscience: A Chemist's Perspective on Internal and External Crossroads in Glycochemistry

Carbohydrate chemistry is an essential component of the glycosciences and is fundamental to their progress. This Perspective takes the position that carbohydrate chemistry, or glycochemistry, has reached three crossroads on the path to the transformation of the glycosciences, and illustrates them with examples from the author's and other laboratories. The first of these potential inflexion points concerns the mechanism of the glycosylation reaction and the role of protecting groups. It is argued that the experimental evidence supports bimolecular SN2-like mechanisms for typical glycosylation reactions over unimolecular ones involving stereoselective attack on naked glycosyl oxocarbenium ions. Similarly, it is argued that the experimental evidence does not support long-range stereodirecting participation of remote esters through bridged bicyclic dioxacarbenium ions in organic solution in the presence of typical counterions. Rational design and improvement of glycosylation reactions must take into account the roles of the counterion and of concentration. A second crossroads is that between mainstream organic chemistry and glycan synthesis. The case is made that the only real difference between glycan and organic synthesis is the formation of C-O rather than C-C bonds, with diastereocontrol, strategy, tactics, and elegance being of critical importance in both areas: mainstream organic chemists should feel comfortable taking this fork in the road, just as carbohydrate chemists should traveling in the opposite direction. A third crossroads is that between carbohydrate chemistry and medicinal chemistry, where there are equally many opportunities for traffic in either direction. The glycosciences have advanced enormously in the past decade or so, but creativity, input, and ingenuity of scientists from all fields is needed to address the many sophisticated challenges that remain, not the least of which is the development of a broader and more general array of stereospecific glycosylation reactions.

Direct N-glycosylation of tosyl and nosyl carbamates with trichloroacetimidate donors

Acidic sulphonamide reactants act as both catalysts and nucleophiles to afford the desired N-glycofuranosyl sulfonamides stereoselectively.

Contributing to the Study of Enzymatic and Chemical Glycosyl Transfer Through the Observation and Mimicry of Glycosyl Cations

This account describes our efforts dedicated to: 1) the design of glycomimetics aimed at targeting therapeutically relevant carbohydrate processing enzymes, and 2) the observation, characterization, and exploitation of glycosyl cations as a tool for studying the glycosylation reaction. These findings have brought important data regarding this key ionic species as well as innovative strategies to access iminosugars of interest.

1 Introduction

2 The Glycosyl Cation, A Central Species in Glycosciences

2.1 A Selection of the Strategies Developed so far to Gain Insights into Glycosyl Cations Structure

2.2 When Superacids Meet Carbohydrates

3 Chemical Probes to Gain Insights into the Pseudorotational Itinerary of Glycosides During Glycosidic Bond Hydrolysis

3.1 Conformationally Locked Glycosides

3.1.1 The Xylopyranose Case

3.1.2 The Mannopyranose Case

3.2 Conformationally Flexible Iminosugars

3.2.1 Nojirimycin Ring Homologues

3.2.2 Noeuromycin Ring Homologues

3.2.3 Seven-Membered Iminosugar C-Glycosides

4 N-Acetyl-d-glucosamine Mimics

5 Ring Contraction: A Useful Tool to Increase Iminosugar’s Structural Diversity

6 Regioselective Deprotection of Iminosugar C-Glycosides to Introduce Diversity at C2 Position

7 Conclusion

Statistical Analysis of Glycosylation Reactions

Chemical synthesis is one of the practical approaches to access carbohydrate-based natural products and their derivatives with high quality and in a large quantity. However, stereoselectivity during the glycosylation reaction is the main challenge because the reaction can generate both α- and β-glycosides. The main focus of the present article is the concept of recent mechanistic studies that have applied statistical analysis and quantitation for defining stereoselective changes during the reaction process. Based on experimental evidence, a detailed discussion associated with the mechanism and degree of influence affecting the stereoselective outcome of glycosylation is included.

Chemical Synthesis Elucidates the Key Antigenic Epitope of the Autism-Related Bacterium Clostridium bolteae Capsular Octadecasaccharide

The gut pathogen Clostridium bolteae has been associated with the onset of autism spectrum disorder (ASD). To create vaccines against C. bolteae, it is important to identify exact protective epitopes of the immunologically active capsular polysaccharide (CPS). Here, a series of C. bolteae CPS glycans, up to an octadecasaccharide, was prepared. Key to achieving the total syntheses is a [2+2] coupling strategy based on a β-d-Rhap-(1→3)-α-d-Manp repeating unit that in turn was accessed by a stereoselective β-d-rhamnosylation. The 4,6-O-benzylidene-induced conformational locking is a powerful strategy for forming a β-d-mannose-type glycoside. An indirect strategy based on C2 epimerization of β-d-quinovoside was efficiently achieved by Swern oxidation and borohydride reduction. Sequential glycosylation, and regioselective and global deprotection produced the disaccharide and tetrasaccharide, up to the octadecasaccharide. Glycan microarray analysis of sera from rabbits immunized with inactivated C. bolteae bacteria revealed a humoral immune response to the di- and tetrasaccharide, but none of the longer sequences. The tetrasaccharide may be a key motif for designing glycoconjugate vaccines against C. bolteae.

Leloir glycosyltransferases of natural product C-glycosylation: structure, mechanism and specificity

A prominent attribute of chemical structure in microbial and plant natural products is aromatic C-glycosylation. In plants, various flavonoid natural products have a β-C-d-glucosyl moiety attached to their core structure. Natural product C-glycosides have attracted significant attention for their own unique bioactivity as well as for representing non-hydrolysable analogs of the canonical O-glycosides. The biosynthesis of natural product C-glycosides is accomplished by sugar nucleotide-dependent (Leloir) glycosyltransferases. Here, we provide an overview on the C-glycosyltransferases of microbial, plant and insect origin that have been biochemically characterized. Despite sharing basic evolutionary relationships, as evidenced by their common membership to glycosyltransferase family GT-1 and conserved GT-B structural fold, the known C-glycosyltransferases are diverse in the structural features that govern their reactivity, selectivity and specificity. Bifunctional glycosyltransferases can form C- and O-glycosides dependent on the structure of the aglycon acceptor. Recent crystal structures of plant C-glycosyltransferases and di-C-glycosyltransferases complement earlier structural studies of bacterial enzymes and provide important molecular insight into the enzymatic discrimination between C- and O-glycosylation. Studies of enzyme structure and mechanism converge on the view of a single displacement (SN2)-like mechanism of enzymatic C-glycosyl transfer, largely analogous to O-glycosyl transfer. The distinction between reactions at the O- or C-acceptor atom is achieved through the precise positioning of the acceptor relative to the donor substrate in the binding pocket. Nonetheless, C-glycosyltransferases may differ in the catalytic strategy applied to induce nucleophilic reactivity at the acceptor carbon. Evidence from the mutagenesis of C-glycosyltransferases may become useful in engineering these enzymes for tailored reactivity.

Highly Selective β-Mannosylations and β-Rhamnosylations Catalyzed by Bis-thiourea

We report highly β-selective bis-thioureas-catalyzed 1,2-cis-O-pyranosylations employing easily accessible acetonide-protected donors. A wide variety of alcohol nucleophiles, including complex natural products, glycosides, and amino acids were β-mannosylated and β-rhamnosylated successfully using an operationally simple protocol under mild and neutral conditions. Less nucleophilic acceptors such as phenols were also glycosylated efficiently in excellent yields and with high β-selectivities.

Total Synthesis of Phospholipomannan of Candida albicans

First, total synthesis of the cell surface phospholipomannan anchor [β-Manp-(1 → 2)-β-Manp]_n-(1 → 2)-β-Manp-(1 → 2)-α-Manp-1 → P-(O → 6)-α-Manp-(1 → 2)-Inositol-1-P-(O → 1)-phytoceramide of Candida albicans is reported. The target phospholipomannan (PLM) anchor poses synthetic challenges such as the unusual kinetically controlled (1 → 2)-β-oligomannan domain, anomeric phosphodiester, and unique phytoceramide lipid tail linked to the glycan through a phosphate group. The synthesis of PLM anchor was accomplished using a convergent block synthetic approach using three main appropriately protected building blocks: (1 → 2)-β-tetramannan repeats, pseudodisaccharide, and phytoceramide-1-H-phosphonate. The most challenging (1 → 2)-β-tetramannan domain was synthesized in one pot using the preactivation method. The phytoceramide-1-H-phosphonate was synthesized through an enantioselective A³ three-component coupling reaction. Finally, the phytoceramide-1-H-phosphonate moiety was coupled with pseudodisaccharide followed by deacetylation to produce the acceptor, which on subsequent coupling with tetramannosyl-H-phosphonate provided the fully protected PLM anchor. Final deprotection was successfully achieved by Pearlman's hydrogenation.

Open-Air Stereoselective Construction of C-Aryl Glycosides

A new methodology of stereoselective C-glycosylation has been developed with 3,4-O-carbonate glycals and boronic acids, catalyzed by 1,2-bis(phenylsulfinyl)ethane palladium(II) acetate under open-air conditions at room temperature. This mild method is simple in operation, wide in substrate range, and tolerant in alcoholic/phenolic hydroxyl and amino groups. High to excellent yields were observed for all substrates tested, with the driving force mainly contributed by decarboxylation. Meanwhile, the high 1,4-trans-selectivity was achieved by steric effects as proposed.

Versatile Glycosyl Sulfonates in β-Selective C-Glycosylation

C-Glycosides are both a common motif in many bioactive natural products and important glycoside mimetics. We demonstrate that activating a hemiacetal with a sulfonyl chloride, followed by treating the resultant glycosyl sulfonate with an enolate results in the stereospecific construction of β-linked C-glycosides. This reaction tolerates a range of acceptors and donors, including disaccharides. The resulting products can be readily derivatized into C-glycoside analogues of β-glycoconjugates, including C-disaccharide mimetics.