Stereoselectivity of Conformationally Restricted Glucosazide Donors

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.

β-Glycosylations with 2-Deoxy-2-(2,4-dinitrobenzenesulfonyl)-amino-glucosyl/galactosyl Selenoglycosides: Assembly of Partially N-Acetylated β-(1 → 6)-Oligoglucosaminosides

An efficient protocol has been established for β-glycosylations with 2-deoxy-2-(2,4-dinitrobenzenesulfonyl)amino (2dDNsNH)-glucopyranosyl/galactopyranosyl selenoglycosides using PhSeCl/AgOTf as an activating system. The reaction features highly β-selective glycosylation with a wide range of alcohol acceptors that are either sterically hindered or poorly nucleophilic. Thioglycoside- and selenoglycoside-based alcohols prove to be viable nucleophiles, opening up new opportunities for one-pot construction of oligosaccharides. The power of this approach is highlighted by the efficient assembly of tri-, hexa-, and nonasaccharides composed of β-(1 → 6)-glucosaminosyl residues based on one-pot preparation of a triglucosaminosyl thioglycoside with DNs, phthaloyl, and 2,2,2-trichloroethoxycarbonyl as the protecting groups of amino groups. These glycans are potential antigens for developing glycoconjugate vaccines against microbial infections.

Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

Mapping the effect of configuration and protecting group pattern on glycosyl acceptor reactivity

The reactivity of the acceptor alcohol can have a tremendous influence on the outcome of a glycosylation reaction, both in terms of yield and stereoselectivity. Through a systematic survey of 67 acceptor alcohols in glycosylation reactions with two glucosyl donors we here reveal how the reactivity of a carbohydrate acceptor depends on its configuration and substitution pattern. The study shows how the functional groups flanking the acceptor alcohol influence the reactivity of the alcohol and show that both the nature and relative orientation play an essential role. The empiric acceptor reactivity guidelines revealed here will aid in the rational optimization of glycosylation reactions and be an important tool in the assembly of oligosaccharides.

Chiral Anthranyl Trifluoromethyl Alcohols: Structures, Oxidative Dearomatization and Chiroptical Properties

Chiral trifluoromethyl alcohol groups were introduced at the hindered ortho positions of 9,10-diphenylanthracenes to investigate their effects on the physical properties and reactivity towards oxidative dearomatization. In such compact structures, the position in different quadrants and the preferred orientation of the -CH(OH)CF₃ groups were determined by the relative and absolute configurations of each stereoisomer, respectively. As a consequence, the stereochemistry governs the organization of the H-bonded molecules in single crystals (homochiral dimers vs ribbon), whereas in chlorinated solvents, they all behave as discrete compounds. Concerning their reactivity, the stereospecific dearomative oxidation of these molecules leads to 9,10-bis-spiro-isobenzofuran-anthracenes, when using organic single-electron transfer oxidants. The chiroptical properties of the alcohols and the corresponding dearomatized products were compared and showed an important modulation of the intensity.

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.

Halogen Atom Participation in Guiding the Stereochemical Outcomes of Acetal Substitution Reactions

Acetal substitution reactions of α-halogenated five- and six-membered rings can be highly stereoselective. Erosion of stereoselectivity occurs as nucleophilicity increases, which is consistent with additions to a halogen-stabilized oxocarbenium ion, not a three-membered-ring halonium ion. Computational investigations confirmed that the open-form oxocarbenium ions are the reactive intermediates involved. Kinetic studies suggest that hyperconjugative effects and through-space electrostatic interactions can both contribute to the stabilization of halogen-substituted oxocarbenium ions.

Hyperconjugative Interactions of the Carbon-Halogen Bond that Influence the Geometry of Cyclic α-Haloacetals

The analysis of the structures of low-energy conformers of different α-haloacetals reveals changes in bond lengths and geometries that correspond to stabilizing orbital interactions that contribute to the ground-state structures of these systems. Several factors, including the electron-donating and electron-accepting abilities of the substituents on the ring, affect the degree of the electronic interactions in these carbohydrate-like systems. The presence of an α-halogen atom that can participate in hyperconjugation has been shown to contribute to the structural characteristics of the low-energy conformer. The experimental evidence is supported by natural bond order (NBO) analysis to identify the types of interactions and to assess their relative importance.

Synthesis of Azido-Globo H Analogs for Immunogenicity Evaluation

Globo H (GH) is a tumor-associated carbohydrate antigen (TACA), and GH conjugations have been evaluated as potential cancer vaccines. However, like all carbohydrate-based vaccines, low immunogenicity is a major issue. Modifications of the TACA increase its immunogenicity, but the systemic modification on GH is challenging and the synthesis is cumbersome. In this study, we synthesized several azido-GH analogs for evaluation, using galactose oxidase to selectively oxidize C6-OH of the terminal galactose or N-acetylgalactosamine on lactose, Gb3, Gb4, and SSEA3 into C6 aldehyde, which was then transformed chemically to the azido group. The azido-derivatives were further glycosylated to azido-GH analogs by glycosyltransferases coupled with sugar nucleotide regeneration. These azido-GH analogs and native GH were conjugated to diphtheria toxoid cross-reactive material CRM197 for vaccination with C34 adjuvant in mice. Glycan array analysis of antisera indicated that the azido-GH glycoconjugate with azide at Gal-C6 of Lac (1-CRM197) elicited the highest antibody response not only to GH, SSEA3, and SSEA4, which share the common SSEA3 epitope, but also to MCF-7 cancer cells, which express these Globo-series glycans.

Assembly of a Library of Pel-Oligosaccharides Featuring α-Glucosamine and α-Galactosamine Linkages

Pseudomonas aeruginosa, a pathogenic Gram-negative bacterium for which currently antibiotic resistance is posing a significant problem and for which no vaccines are available, protects itself by the formation of a biofilm. The Pel polysaccharide, a cationic polymer composed of cis-linked galactosamine (GalN), N-acetyl galactosamine (GalNAc), glucosamine (GlcN) and N-acetyl glucosamine (GlcNAc) monosaccharides, is an important constituent of the biofilm. Well-defined Pel oligosaccharides will be valuable tools to probe the biosynthesis machinery of this polysaccharide and may serve as diagnostic tools or be used as components of glycoconjugate vaccines. We here, report on the development of synthetic chemistry to access well-defined Pel-oligosaccharides. The chemistry hinges on the use of di-tert-butylsilylidene protected GalN and GlcN building blocks, which allow for completely cis-selective glycosylation reactions. We show the applicability of the chemistry by the assembly of a matrix of 3 × 6 Pel heptasaccharides, which has been generated from a single set of suitably protected Pel heptasaccharides, in which a single glucosamine residue is incorporated and positioned at different places along the Pel oligo-galactosamine chain.

Direct Experimental Characterization of a Bridged Bicyclic Glycosyl Dioxacarbenium Ion by 1 H and 13 C NMR Spectroscopy: Importance of Conformation on Participation by Distal Esters

Low-temperature NMR studies with a 4-C-methyl-4-O-benzoyl galactopyranosyl donor enable the observation and characterization of a bridged bicyclic dioxacarbenium ion arising from participation by a distal ester. Variable-temperature NMR studies reveal this bridged ion to decompose at temperatures above ≈-30 °C. In the absence of the methyl group, the formation of a bicyclic ion is not observed. It is concluded that participation by typical secondary distal esters in glycosylation reactions is disfavored in the ground state conformation of the ester from which it is stereoelectronically impossible. Methylation converts the secondary ester to a conformationally more labile tertiary ester, removes this barrier, and renders participation more favorable. Nevertheless, the minor changes in selectivity in model glycosylation reactions on going from the secondary to the tertiary esters at both low and room temperature argue against distal group participation being a major stereodirecting factor even for the tertiary system.

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.

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.

4,6-Di-O-Benzylidenyl group-directed preparation of 2-deoxy-2-azido-α-d-galactopyranosides promoted by 3-O-TBDPS

In this study, we designed a method to prepare 2-deoxy-2-azido-α-d-galactopyranosidic bonds using 4,6-di-O-benzylidenyl-3-O-t-butyldiphenylsilyl protected 2-deoxy-2-azido-1-thio-d-galactopyranoside 5 as donors. The donor 5 gives a good to excellent α-selectivity in the glycosylation with secondary alcohols, which was found to be associated with the benzylidenyl on 4,6-di-O and TBDPS on 3-O of the donor 5. Compared with results of the donor 6 and 7, the 3-O-TBDPS could increase the activity of the thioglycoside, and the lone pairs on 4,6-di-O-benzylidenyl group enhanced the gg-cofnormation, which plays a role in improving the stereoselectivity. Finally, this method was demonstrated through the synthesis of a α-galactosamine -containing pentasaccharide 26.

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.

Reactivity-Stereoselectivity Mapping for the Assembly of Mycobacterium marinum Lipooligosaccharides

The assembly of complex bacterial glycans presenting rare structural motifs and cis-glycosidic linkages is significantly obstructed by the lack of knowledge of the reactivity of the constituting building blocks and the stereoselectivity of the reactions in which they partake. We here report a strategy to map the reactivity of carbohydrate building blocks and apply it to understand the reactivity of the bacterial sugar, caryophyllose, a rare C12-monosaccharide, containing a characteristic tetrasubstituted stereocenter. We mapped reactivity-stereoselectivity relationships for caryophyllose donor and acceptor glycosides by a systematic series of glycosylations in combination with the detection and characterization of different reactive intermediates using experimental and computational techniques. The insights garnered from these studies enabled the rational design of building blocks with the required properties to assemble mycobacterial lipooligosaccharide fragments of M. marinum.

Reagent Controlled Glycosylations for the Assembly of Well-Defined Pel Oligosaccharides

A new additive, methyl(phenyl)formamide (MPF), is introduced for the glycosylation of 2-azido-2-deoxyglucose building blocks. A linear α-(1,4)-glucosamine tetrasaccharide was assembled to prove the utility of MPF. Next, a hexasaccharide fragment of the Pseudomonas aeruginosa exopolysaccharide Pel was assembled using a [2 + 2 + 2] strategy modulated by MPF. The used [galactosazide-α-(1,4)-glucosazide] disaccharide building blocks were synthesized using a 4,6-O-DTBS protected galactosyl azide donor.

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.

Applying the Bent Bond/Antiperiplanar Hypothesis to the Stereoselective Glycosylation of Bicyclic Furanosides

The glycosylation stereoselectivities for a series of bicyclic furanoside models have been carried out in the presence of weak nucleophiles. These results were analyzed through the bent bond/antiperiplanar hypothesis (BBAH) orbital model to test its validity. According to the BBAH, incoming nucleophiles displace one of the two bent bonds of bicyclic oxocarbenium ion intermediates in an antiperiplanar fashion. The glycosylation stereoselectivity is then governed by the displacement of the weaker bent bond as determined by the presence of electron-withdrawing or -donating substituents at C₂. Overall, the BBAH analysis expands Woerpel's "inside/outside attack" glycosylation model by considering the stereoelectronic influence of neighboring electron-withdrawing and -donating groups on the nucleophilic addition to oxocarbenium ion intermediates.

Use of remote acyl groups for stereoselective 1,2-cis-glycosylation with fluorinated glucosazide thiodonors

Introducing remote O-acyl protecting groups enabled 1,2-cis stereoselective glycosylation with fluorinated glucosazide glycosyl donors.

Defining the SN1 Side of Glycosylation Reactions: Stereoselectivity of Glycopyranosyl Cations

The broad application of well-defined synthetic oligosaccharides in glycobiology and glycobiotechnology is largely hampered by the lack of sufficient amounts of synthetic carbohydrate specimens. Insufficient knowledge of the glycosylation reaction mechanism thwarts the routine assembly of these materials. Glycosyl cations are key reactive intermediates in the glycosylation reaction, but their high reactivity and fleeting nature have precluded the determination of clear structure-reactivity-stereoselectivity principles for these species. We report a combined experimental and computational method that connects the stereoselectivity of oxocarbenium ions to the full ensemble of conformations these species can adopt, mapped in conformational energy landscapes (CEL), in a quantitative manner. The detailed description of stereoselective S_N1-type glycosylation reactions firmly establishes glycosyl cations as true reaction intermediates and will enable the generation of new stereoselective glycosylation methodology.

2,4-Dinitrobenzenesulfonamide-Directed SN2-Type Displacement Reaction Enables Synthesis of β-d-Glycosaminosides

An efficient protocol to construct β-d-gluco-/galactosaminosyl linkages was established using nonparticipating and strong electron-withdrawing C-2-2,4-dinitrobenzenesulfonamide (DNsNH)-directed S_N2-like glycosylation of glycosyl ortho-hexynylbenzoates. The reaction is applicable to a wide range of O-, N-, and C-nucleophiles and features convenient conversion of DNsNH into AcNH in high yield under mild conditions. Oligomerization-ready trisaccharide, composed of β-d-(1→3)-glucosamino residues, has been achieved, setting a solid foundation for the synthesis of oligosaccharides associated with Neisseria meningitidis capsular polysaccharide.

Are Brønsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2-cis-Aminoglycoside Formation by Nickel Triflate

Metal triflates have been utilized to catalytically facilitate numerous glycosylation reactions under mild conditions. In some methods, the metal triflate system provides stereocontrol during the glycosylation, rather than the nature of protecting groups on the substrate. Despite these advances, the true activating nature of metal triflates remains unclear. Our findings indicated that the in situ generation of trace amounts of triflic acid from metal triflates can be the active catalyst species in the glycosylation. This fact has been mentioned previously in metal triflate-catalyzed glycosylation reactions; however, a thorough study on the subject and its implications on stereoselectivity has yet to be performed. Experimental evidence from control reactions and 19F NMR spectroscopy have been obtained to confirm and quantify the triflic acid released from nickel triflate, for which it is of paramount importance in achieving a stereoselective 1,2-cis-2-amino glycosidic bond formation via a transient anomeric triflate. A putative intermediate resembling that of a glycosyl triflate has been detected using variable temperature NMR (1H and 13C) experiments. These observations, together with density functional theory calculations and a kinetic study, corroborate a mechanism involving triflic acid-catalyzed stereoselective glycosylation with N-substituted trifluoromethylbenzylideneamino protected electrophiles. Specifically, triflic acid facilitates formation of a glycosyl triflate intermediate which then undergoes isomerization from the stable α-anomer to the more reactive β-anomer. Subsequent SN2-like displacement of the reactive anomer by a nucleophile is highly favorable for the production of 1,2-cis-2-aminoglycosides. Although there is a previously reported work regarding glycosyl triflates, none of these reports have been confirmed to come from the counter ion of the metal center. Our work provides supporting evidence for the induction of a glycosyl triflate through the role of triflic acid in metal triflate-catalyzed glycosylation reactions.

An Empirical Understanding of the Glycosylation Reaction

Reliable glycosylation reactions that allow for the stereo- and regioselective installation of glycosidic linkages are paramount to the chemical synthesis of glycan chains. The stereoselectivity of glycosylations is exceedingly difficult to control due to the reaction's high degree of sensitivity and its shifting, simultaneous mechanistic pathways that are controlled by variables of unknown degree of influence, dominance, or interdependency. An automated platform was devised to quickly, reproducibly, and systematically screen glycosylations and thereby address this fundamental problem. Thirteen variables were investigated in as isolated a manner as possible, to identify and quantify inherent preferences of electrophilic glycosylating agents (glycosyl donors) and nucleophiles (glycosyl acceptors). Ways to enhance, suppress, or even override these preferences using judicious environmental conditions were discovered. Glycosylations involving two specific partners can be tuned to produce either 11:1 selectivity of one stereoisomer or 9:1 of the other by merely changing the reaction conditions.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.

Spiro-epoxyglycosides as Activity-Based Probes for Glycoside Hydrolase Family 99 Endomannosidase/Endomannanase

N-Glycans direct protein function, stability, folding and targeting, and influence immunogenicity. While most glycosidases that process N-glycans cleave a single sugar residue at a time, enzymes from glycoside hydrolase family 99 are endo-acting enzymes that cleave within complex N-glycans. Eukaryotic Golgi endo-1,2-α-mannosidase cleaves glucose-substituted mannose within immature glucosylated high-mannose N-glycans in the secretory pathway. Certain bacteria within the human gut microbiota produce endo-1,2-α-mannanase, which cleaves related structures within fungal mannan, as part of nutrient acquisition. An unconventional mechanism of catalysis was proposed for enzymes of this family, hinted at by crystal structures of imino/azasugars complexed within the active site. Based on this mechanism, we developed the synthesis of two glycosides bearing a spiro-epoxide at C-2 as electrophilic trap, to covalently bind a mechanistically important, conserved GH99 catalytic residue. The spiro-epoxyglycosides are equipped with a fluorescent tag, and following incubation with recombinant enzyme, allow concentration, time and pH dependent visualization of the bound enzyme using gel electrophoresis.

An Improved Approach to the Direct Construction of 2-Deoxy-β-Linked Sugars: Applications to Oligosaccharide Synthesis

A next-generation reagent-controlled approach for the synthesis of 2,6-dideoxy and 2,3,6-trideoxy sugar donors in good yield and high β-selectivity is reported. The use of p-toluenesulfonyl chloride and potassium hexamethyldisilazide (KHMDS) greatly simplifies deoxy-sugar glycoside construction, and can be used for gram-scale glycosylation reactions. The development of this approach and its application to the construction of β-linked deoxy-sugar oligosaccharides are described.

Mapping the Relationship between Glycosyl Acceptor Reactivity and Glycosylation Stereoselectivity

The reactivity of both coupling partners-the glycosyl donor and acceptor-is decisive for the outcome of a glycosylation reaction, in terms of both yield and stereoselectivity. Where the reactivity of glycosyl donors is well understood and can be controlled through manipulation of the functional/protecting-group pattern, the reactivity of glycosyl acceptor alcohols is poorly understood. We here present an operationally simple system to gauge glycosyl acceptor reactivity, which employs two conformationally locked donors with stereoselectivity that critically depends on the reactivity of the nucleophile. A wide array of acceptors was screened and their structure-reactivity/stereoselectivity relationships established. By systematically varying the protecting groups, the reactivity of glycosyl acceptors can be adjusted to attain stereoselective cis-glucosylations.