Guidelines for O-Glycoside Formation from First Principles

Minimally Protected and Stereoselective O-Glycosylation of Carboxylic Acid Allows Rapid Access to α-1-O- and 2-O-Acyl Glycosides

We herein reported a catalytic, minimally protected, and highly α-stereoselective glycosylation protocol using carboxylic acid as an acceptor and glycosyl 8-alkynyl-1-naphthoate as a donor, enabling efficient access to unprotected α-1-O- and 2-O-acyl glycosides. This method demonstrates excellent functional compatibility and scope generality, allowing for the glycosylation of a wide range of complex carboxylic acids. Notably, we successfully synthesized two natural products, α-penta-O-galloyl-d-glucopyranose and nyctanthesin A, using this protocol. Mechanistic studies highlighted the crucial role of the 1-O ester functionality in ensuring chemoselectivity and the important contribution of the 2-O functionality in facilitating the reaction.

Synthesis of Nucleotide Diphosphate Uronic Acids via the Coupling of Activated Nucleotides with Uronic Acid-1-phosphates

The stereoselective synthesis of nucleotide diphosphate (NDP) uronic acids from simple sugar precursors, including d-gluco-, d-galacto-, and d-mannopyranoside derivatives, is described. Key to this convergent synthesis is the coupling of unprotected uronic acid 1-phosphate with a nucleotide phosphorimidazolide to directly form the NDP-uronic acid, of which 11 derivatives were prepared. The coupling is compatible with the carboxylic acid functionality present in uronic acid-1-phosphates, with conversions of >95% and isolated yields typically above 60%. Key features of this work include (i) stereoselective synthesis of α-d-phosphoglycosides from perbenzylated α- and β-d-thioglycosides, (ii) selective and mild oxidation of galactose-, glucose-, and mannose-1-phosphates to the corresponding uronic acid-1-phosphate, and (iii) mild coupling conditions to directly provide nucleotide diphosphate uronic acids from unprotected uronic acid-1-phosphates and nucleotide phosphorimidazolides. This chemistry is currently in use to develop inhibitors of key enzymes involved in antibiotic resistance.

Carbohydrate Synthesis is Entering the Data-Driven Digital Era

Glycans are vital in biological processes, but their nontemplated, heterogeneous structures complicate structure-function analyses. Glycosylation, the key reaction in synthetic glycochemistry, remains not entirely predictable due to its complex mechanism and the need for protecting groups that impact reaction outcomes. This concept highlights recent advancements in glycochemistry and emphasizes the integration of digital tools, including automation, computational modelling, and data management, to improve carbohydrate synthesis and support further progress in the field.

Isothiourea - catalyzed α-selective glycosylations

Herein, we present a catalytic strategy to efficiently form both α-1,2-cis and α-1,2-trans glycosyl linkages from either glycosyl bromide or chloride donors using the commercially available HyperBTM isothiourea in both good yields and selectivities.

Why Are 5-Thioglycopyranosyl Donors More Axially Selective than their Glycopyranosyl Counterparts? A Low and Variable Temperature NMR Spectroscopy and Computational Study

5-Thioglycopyranosyl donors differ in reactivity and selectivity from simple glycopyranosyl donors. An extensive study has been conducted on the nature and stability of the reactive intermediates generated on the activation of per-O-acetyl and per-O-methyl 5-thioglucopyranosyl donors and the corresponding glucopyranosyl donors. Variable temperature nuclear magnetic resonance (NMR) studies with per-O-methylated or per-O-acetyl glycosyl sulfoxides and trichloroacetimidates on activation with trifluoromethanesulfonic anhydride or trimethylsilyl triflate are reported. These show that following initial adduct formation with the promoter conversion of the 5-thioglucopyranosyl donors to the 5-thioglucopyranosyl triflates requires higher temperatures than that of the glucopyranosyl donors to the glucopyranosyl triflates. It is demonstrated that neighboring group participation is a less important phenomenon for the peracetylated thioglucosyl donors than for the peracetylated glucosyl donors. A simple thiocarbenium ion was generated by protonation of 2,3-dihydro-4H-thiopyran at low temperature and characterized by NMR spectroscopy. However, the corresponding 5-thioglucopyranosyl thenium ions were not observed in any of the NMR studies of the 5-thiopyranosyl donors: the electron-withdrawing C-O bonds around the thiopyranoside core discourage thiocarbenium ion formation, just as they discourage oxocarbenium ion formation. Density functional theory (DFT) calculations reveal the tetrahydrothiopyranyl thiocarbenium ion to be approximately 2.5 kcal/mol lower in energy than the corresponding tetrahydropyranyl oxocarbenium ion relative to the corresponding covalent triflates. However, the computations reveal a 5.8 kcal/mol activation barrier for conversion of the tetrahydrothiopyranyl triflate to the thiocarbenium ion, while formation of the oxocarbenium ion-triflate ion pair from tetrahydropyranyl triflate requires only 2.6 kcal·mol-1. Overall, the greater axial selectivity of 5-thioglycopyranosyl donors compared to analogous glycopyranosyl donors derives from (i) the lower kinetic reactivity necessitating higher reaction temperatures, (ii) the greater stability of the thiocarbenium ion over the oxocarbenium ion facilitating equilibration under thermodynamic conditions, (iii) the greater magnitude of the anomeric effect in the 5-thiosugars, and (iv) decreased neighboring group participation in the per-esterified 5-thiosugars.

Stereoselective 1,1'-glycosylation via reactivity tuning with protecting groups

Chemical 1,1'-glycosylation for the synthesis of non-reducing disaccharides is complicated by the need to simultaneously control the stereochemistry at two anomeric centers. While considerable progress has been made in the synthesis of α,α-disaccharides, the assembly of 1,1'-β,β- and 1,1'-β,α-linked non-reducing sugars has received comparatively less attention. Many naturally occurring non-reducing disaccharides and their biologically active mimetics feature asymmetrically located functional groups at different positions on the two pyranose rings, highlighting the demand for reliable stereoselective methods to synthesize fully orthogonally protected 1,1'-conjugated sugars suitable for targeted functionalisation to create important biomolecules. By exploiting specific electronic and torsional effects imposed by protecting groups on both glycosyl donor and lactol acceptor molecules, we achieved highly stereoselective β,β- and β,α-1,1'-glycosylation and successfully synthesised a library of fully orthogonally protected β,β- and β,α-linked diglucosamines. Our approach is based on the premise that acceptor reactivity can greatly influence the stereochemical outcome of the glycosylation reaction. We show that the tailored choice of orthogonal protecting groups can alter the anomeric preferences in lactol acceptors, stabilising specific anomeric conformations, and that protecting group-driven modulation of lactol nucleophilicity is a useful tool to achieve stereoselective 1,1'-glycosidic bond formation. Structure-activity relationships have been established for a number of fully orthogonally protected glycosyl donor-lactol acceptor pairs, with a focus on optimizing lactol acceptor nucleophilicity to facilitate stereoselective 1,1'-β,β- and 1,1'-β,α-glycosylation on the acceptor side and enhance neighboring group-driven stereoselectivity on the donor side.

The Stereoselectivity of Neighboring Group-Directed Glycosylation Is Concentration-Dependent

The formation of 1,2-trans-glycosides taking advantage of neighboring group participation by stereodirecting esters at the 2-position of glycosyl donors is widely held to be a robust and reliable protocol. Examples abound, however, of cases in which less-than-perfect selectivity is obtained, causing practitioners to survey different esters or resort to alternative strategies in the quest for optimal selectivities and yields. Consideration of the mechanism of neighboring group participation and in particular of the competing process of SN2-like glycosylation with activated covalent donors leads to the hypothesis that in cases of imperfect selectivity, more careful attention to reaction concentration and stoichiometry may be beneficial. Three case studies are presented to demonstrate the concentration dependence of neighboring group-directed glycosylation reactions targeting the formation of both 1,2-trans-equatorial and 1,2-trans-axial glycosides. Higher concentrations, whether achieved through increased acceptor:donor stoichiometry or through increased concentration at a fixed stoichiometry, mostly lead to erosion of 1,2-trans-selectivity as the competing SN2-like reaction with the covalent donors becomes increasingly important. These observations underline the importance of a rational, mechanism-based approach to glycosylation in general and more importantly suggest a simple approach to enhancing 1,2-trans-selectivity in neighboring group-directed glycosylation reactions displaying less-than-perfect 1,2-trans-selectivity, namely, moving to a different concentration regime.

The effect of neighbouring group participation and possible long range remote group participation in O-glycosylation

Stereoselective glycosylations are one of the most challenging tasks of synthetic glycochemists. The protecting building blocks on the glycosides contribute significantly in attaining the required stereochemistry of the resulting glycosides. Strategic installation of suitable protecting groups in the C-2 position, vicinal to the anomeric carbon, renders neighbouring group participation, whereas protecting groups in the distal C-3, C-4, and C-6 positions are often claimed to exhibit remote group participation with the anomeric carbon. Neighbouring group participation and remote group participation are being widely studied to help the glycochemists design the synthetic protocols for multistep synthesis of complex oligosaccharides and in turn, standardise the process of the glycosylation towards a particular stereochemical output. While neighbouring group participation has been quite effective in achieving the required stereochemistry of the produced glycosides, remote participation exhibits comparatively less efficacy in achieving complete stereoselectivity in the glycosylation reactions. Remote participation is a still highly debated topic in the scientific community. However, implementing the participating role of the remote groups in glycosylation reactions is widely practised to achieve better stereocontrol and to facilitate the formation of synthetically challenging glycosidic linkages.

β-Stereoselective Kdo C-glycosylation by a (p-Tol)2SO/Tf2O preactivation strategy

There is very little research on the synthesis of β-3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) C-glycosides, which restricted their widespread application. Herein, a convenient and efficient approach to synthesize β-Kdo C-glycosides was developed based on a Tf2O/(p-Tol)2SO preactivation strategy using bench stable peracetylated Kdo thioglycoside as a donor via a thermodynamic SN1-like mechanism.

Destabilization of Glycosyl Cation by an Electron-Withdrawing Substituent at C-5 Makes Sialylation Reaction More α-Stereoselective

Comparison of the reactivity of sialyl chlorides and bromides based on N-acetylneuraminic acid (Neu5Ac) and its deaminated analogue (KDN) in reactions with MeOH and i-PrOH without a promoter revealed that the acetoxy group at C-5 in a molecule of a sialic acid glycosyl donor can destabilize the corresponding glycosyl cation making the SN1-like reaction pathway unfavorable. A change to the SN2-like reaction pathway ensures preferential formation of the α-glycoside.

Stereoelectronic Effect of Protecting Groups on the Stability of Galactosyl Donor Intermediates

Using methods of DFT, we investigated the effect of electron withdrawing and electron donating groups on the relative stability of tentative glycosyl donor reaction intermediates. The calculation shows that by changing the stereoelectronic properties of the protecting group, we can influence the stability of the dioxolenium type of intermediates by up to 10 kcal mol-1, and that by increasing nucleophillicity of the 4-O-Bz group, the dioxolenium intermediate becomes more stable than a triflate-donor pair. We exploited this mechanism to design galactosyl donors with custom protecting groups on O2 and O4, and investigated the outcome of the reaction with cyclohexanol. The reaction showed no change in the product distribution, which suggests that the neighboring group participation takes precedence over remote group participation due to kinetic barriers.

[Au]/[Ag]-Catalyzed Glycosidation of Ethynylcyclohexyl Glycosyl Carbonates Enables Direct Stereoselective Synthesis of 2-Azido-2-deoxy-β-mannopyranosides

We describe the [Au]/[Ag]-catalyzed activation of ethynylcyclohexyl 2-azido-4,6-O-benzylidene mannosyl carbonates for β-mannosamine linkage preparation in high yield and selectivity in a temperature- and concentration-dependent manner. VT-NMR studies reveal an anomeric triflate intermediate generated in the absence of an acceptor alcohol that is stable up to -10 °C. The generality of the protocol was illustrated by successful application to a series of acceptors and by synthesis of a fully protected Streptococcus pneumoniae type 19F capsular trisaccharide.

Cooperative Catalysis in Stereoselective O- and N-Glycosylations with Glycosyl Trichloroacetimidates Mediated by Singly Protonated Phenanthrolinium Salt and Trichloroacetamide

The development of small-molecule catalysts that can effectively activate both reacting partners simultaneously represents a pivotal pursuit in advancing the field of stereoselective glycosylation reactions. We report herein the development of the singly protonated form of readily available phenanthroline as an effective cooperative catalyst that facilitates the coupling of a wide variety of aliphatic alcohols, phenols, and aromatic amines with α-glycosyl trichloroacetimidate donors. The glycosylation reaction likely proceeds via an SN2-like mechanism, generating β-selective glycoside products. The developed protocol provides access to O- and N-glycosides in good yields with excellent levels of β-selectivity and enables late-stage functionalization of O- and N-glycosides via cross-coupling reactions. Importantly, this method exhibits excellent β-selectivity that is unattainable through a C2-O-acyl neighboring group participation strategy, especially in the case of glycosyl donors already containing a C2 heteroatom or sugar unit. Kinetic studies demonstrate that the byproduct trichloroacetamide group plays a previously undiscovered pivotal role in influencing the reactivity and selectivity of the reaction. A proposed mechanism involving simultaneous activation of the glycosyl donor and acceptor by the singly protonated phenanthrolinium salt catalyst with the assistance of the trichloroacetamide group is supported by kinetic analysis and preliminary computational studies. This cooperative catalysis process involves four consecutive hydrogen bond interactions. The first interaction occurs between the carbonyl oxygen of the trichloroacetamide group and the hydroxyl group of alcohol nucleophile (C═O···HO). The second involves the trichloroacetamide-NH2 forming a hydrogen bond with the nitrogen atom of the phenanthroline (NH···N). The third involves the donor trichloroacetimidate (═NH) engaging in a hydrogen bond interaction with the phenanthrolinium-NH (NH···N═H). Lastly, the protonated trichloroacetimidate-NH2 forms a hydrogen bond with the fluorine atom of the tetrafluoroborate ion.

Glycosylium Ions in Superacid Mimic the Transition State of Enzyme Reactions

The hydrolysis of glycosides is a biochemical transformation that occurs in all living organisms, catalyzed by a broad group of enzymes, including glycoside hydrolases. These enzymes cleave the glycosidic bond via a transition state with substantial oxocarbenium ion character, resulting in short-lived oxocarbenium ion-like species. While such transient species have been inferred through theoretical studies and kinetic isotope effect measurements, their direct spectroscopic characterization remains challenging. In this study, we exploit a superacid environment to generate, accumulate, and fully characterize nonprotected 2-deoxy glycosyl cations in the d-glucopyranose, d-galactopyranose, and l-arabinofuranose series using low-temperature NMR spectroscopy, supported by DFT calculations. Additionally, QM/MM MD simulations reveal that the properties of these glycosyl cations in superacid closely resemble those within the active sites of glycosidase enzymes, particularly in terms of conformation and anomeric charge distribution. These findings highlight a parallel between the stabilizing effect of counterions in superacid media and the network of multidentate noncovalent interactions within glycosidase active sites, which stabilize transition states with pronounced oxocarbenium ion character.

Highly stereoselective synthesis of α-glycosylated carboxylic acids by phenanthroline catalysis

Carbohydrate molecules with an α-glycosylated carboxylic acid motif provide access to biologically relevant chemical space but are difficult to synthesize with high selectivity. To address this challenge, we report a mild and operationally simple protocol to synthesize a wide range of functionally and structurally diverse α-glycosylated carboxylic acids in good yields with high diastereoselectivity. Although there is no apparent correlation between reaction conversion and pK a of carboxylic acids, we found that carboxylic acids with a pK a of 4-5 provide high selectivity while those of a pK a of 2.5 or lower do not. Our strategy utilizes readily available 2,9-dibutyl-1,10-phenanthroline as an effective nucleophilic catalyst to displace a bromide leaving group from an activated sugar electrophile in a nucleophilic substitution reaction, forming phenanthrolinium intermediates. The attack of the carboxylic acid takes place from the α-face of the more reactive intermediate, resulting in the formation of α-glycosylated carboxylic acid. Previous calculations suggested that the hydroxyl group participates in the hydrogen bond interaction with the basic C2-oxygen of a sugar moiety and serves as a nucleophile to attack the C1-anomeric center. In contrast, our computational studies reveal that the carbonyl oxygen of the carboxylic acid serves as a nucleophile, with the carboxylic acid-OH forming a hydrogen bond with the basic C2-oxygen of the sugar moiety. This strong hydrogen bond (1.65 Å) interaction increases the nucleophilicity of the carbonyl oxygen of carboxylic acid and plays a critical role in the selectivity-determining step. In contrast, when alcohol acts as a nucleophile, this scenario is not possible since the -OH group of the alcohol interacts with the C2-oxygen and attacks the C1-anomeric carbon of the sugar moiety. This is also reflected in alcohol-OH's weak hydrogen bond (1.95 Å) interaction with the C2-oxygen. The O(C2)-HO (carboxylic acid) angle was measured to be 171° while the O(C2)-HO (alcohol) angle at 122° deviates from linearity, resulting in weak hydrogen bonding.

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.

Origins of Temperature-Dependent Anomeric Selectivity in Glycosylations with an L-Idose Thioglycoside

L-Idose thioglycosides are useful glycosyl donors for the construction of glycosaminoglycan oligosaccharides. When activated with NIS and catalytic TMSOTf in the presence of methanol, the stereoselectivity of O-glycosylation displays an intriguing dependence on the reaction temperature, with an increased preference for formation of the α-glycoside at higher temperatures. Using a combination of vt-NMR spectroscopy and DFT calculations, we show how a simple mechanistic model, based on competing reactions of the iodinated thioglycoside, can explain the main features of the temperature dependence. In this model, the increased selectivity at high temperature is attributed to differences among the entropy and energy terms of the competing reaction pathways. Neighbouring-group participation (giving an intermediate acyloxonium ion) plays an increasingly dominant role as temperature is raised. The general features of this kinetic regime may also apply more broadly to other glycosylations that likewise favour α-glycoside formation at high temperature.

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.

Synthesis of C-Alkyl Glycosides from Alkyl Bromides and Glycosyl Carboxylic Acids via Ni/Photoredox Dual Catalysis

C-Alkyl glycosides, an important class of C-glycosides, are widely found in various drugs and natural products. The synthesis of C-alkyl glycosides has attracted considerable attention. Herein, we developed a Ni/photoredox catalyzed decarboxylative C(sp3)-C(sp3) coupling reaction of stable glycosylcarboxylic acids with simple aliphatic bromides to generate C-alkyl glycosides. The method successfully linked several functional molecular fragments (natural products or drugs) to a sugar moiety, showing the extensive application prospects of this transformation. Controlled experiments and DFT calculations demonstrated that the reaction pathway contains a free radical process, and a possible mechanism is proposed.

Mechanistic insight into benzylidene-directed glycosylation reactions using cryogenic infrared spectroscopy

The stereoselective formation of 1,2-cis glycosidic linkages is challenging. The currently most widely used strategy for their installation uses 4,6-O-benzylidene-protected building blocks. The stereoselectivity of this reaction is thought to be driven by a covalent intermediate, which reacts via an SN2 mechanism. However, the role of cationic SN1-type intermediates in this reaction is unclear. Here we elucidate the structure of glycosyl cations carrying 4,6-O-benzylidene groups using cryogenic infrared ion spectroscopy and computational methods. The data reveal that the intermediates form anhydro cations, which correlates well with the stereoselective outcome of SN1-type glycosylations. The study highlights how cryogenic infrared spectroscopy can elucidate the role of intermediates in sugar chemistry and how these structural data can be linked to reactions in solution.

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.

Reaction Rate and Stereoselectivity Enhancement in Glycosidations with O-Glycosyl Trihaloacetimidate Donors due to Catalysis by a Lewis Acid-Nitrile Cooperative Effect

Activation of O-glycosyl trihaloacetimidate glycosyl donors with AuCl3 as a catalyst and pivalonitrile (tBuCN) as a ligand led to excellent glycosidation results in terms of yield and anomeric selectivity. In this way, various β-d-gluco- and β-d-galactopyranosides were obtained conveniently and efficiently. Experimental studies and density functional theory (DFT) calculations, in order to elucidate the reaction course, support formation of the tBuCN-AuCl2-OR(H)+ AuCl4- complex as a decisive intermediate in the glycosidation event. Proton transfer from this acceptor complex to the imidate nitrogen leads to donor activation. In this way, guided by the C-2 configuration of the glycosyl donor, the alignment of the acceptor complex enforces the stereoselective β-glycoside formation in an intramolecular fashion, thus promoting also a fast reaction course. The high stereocontrol of this novel 'Lewis acid-nitrile cooperative effect' is independent of the glycosyl donor anomeric configuration and without the support of neighboring group or remote group participation. The power of the methodology is shown by a successful glycoalkaloid solamargine synthesis.

Parametric Analysis of Donor Activation for Glycosylation Reactions

The chemical synthesis of complex oligosaccharides relies on efficient and highly reproducible glycosylation reactions. The outcome of a glycosylation is contingent upon several environmental factors, such as temperature, acidity, the presence of residual moisture, as well as the steric, electronic, and conformational aspects of the reactants. Each glycosylation proceeds rapidly and with a high yield within a rather narrow temperature range. For better control over glycosylations and to ensure fast and reliable reactions, a systematic analysis of 18 glycosyl donors revealed the effect of reagent concentration, water content, protecting groups, and structure of the glycosyl donors on the activation temperature. With these insights, we parametrize the first step of the glycosylation reaction to be executed reliably and efficiently.

Total synthesis and cytotoxicity evaluation of the spirostanol saponin gitonin

The spirostanol saponin gitonin was efficiently synthesized in 12 steps (longest linear sequence) in 18.5% overall yield from the commercially available isopropyl β-D-1-thiogalactopyranoside (IPTG) and tigogenin. A cascade two-step glycosylation and Schmidt's inverse procedure significantly facilitated the synthesis of gitonin and its derivatives. The cytotoxic activities of gitonin and its structural analogues were evaluated against A549, HepG2, and MCF-7, and most of them exhibited moderate to excellent inhibitory activity. Our study demonstrates that the removal of the β-D-galactopyranosyl residue (attached at C-2 of the glucose unit) from gitonin would not decrease the inhibition activities; however, further cleavage of sugar units could seriously reduce the activities. A bioassay on these cancer cell lines also suggested that the presence of 2α-hydroxy on the aglycone weakened the cytotoxicity of the designed saponin.

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.

Comparison of glycosyl donors: a supramer approach

The development of new methods for chemical glycosylation commonly includes comparison of various glycosyl donors. An attempted comparison of chemical properties of two sialic acid-based thioglycoside glycosyl donors, differing only in the substituent at O-9 (trifluoroacetyl vs chloroacetyl), at different concentrations (0.05 and 0.15 mol·L-1) led to mutually excluding conclusions concerning their relative reactivity and selectivity, which prevented us from revealing a possible influence of remote protective groups at O-9 on glycosylation outcome. According to the results of the supramer analysis of the reaction solutions, this issue might be related to the formation of supramers of glycosyl donors differing in structure hence chemical properties. These results seem to imply that comparison of chemical properties of different glycosyl donors may not be as simple and straightforward as it is usually considered.

Glycosylation of n-pentenyl glycosides using bromodiethylsulfonium salt as an activator: interception of the glycosyl intermediate by chloride ion transfer

Utilization of n-pentenyl glycosides (NPGs) in modern carbohydrate synthesis may be hindered by their sluggish activation, which results from reversible halogenation and cyclization processes. Bromodiethylsulfonium bromopentachloroantimonate (BDSB) has been previously shown to be a powerful brominating agent for the cation-π polyene cyclization of less reactive and electron-poor polyenes. This study demonstrates the activation of NPGs using BDSB as a powerful brominating agent. BDSB effectively activates the terminal olefins of NPGs and the reaction proceeds through 5-exo-tet cyclization, offering a rapid and mild approach for glycosylation with a wide range of glycosyl donors, including n-pentenyl mannoside, n-pentenyl galactoside, and n-pentenyl glucoside. The success of this approach derives from the chloride ion transfer from the nonnucleophilic SbCl5Br anion to the glycosyl intermediate, which disrupts the equilibrium and produces a glycosyl chloride intermediate that is smoothly converted to 22 coupling products, with yields ranging from moderate to excellent (49-100%). The β-selective glycosylation is accomplished when employing NPGs equipped with a neighboring participating group. The practicality of the BDSB-activated glycosylation is demonstrated by a gram-scale synthesis. This study showcases BDSB as a potent activator for NPG glycosylation through the interception of a glycosyl intermediate that diminishes the equilibration during halogenation and 5-exo-tet cyclization.

Origins of Selectivity in Glycosylation Reactions with Saccharosamine Donors

A combination of DFT calculations and experiments is used to describe how the selection of a promoter can control the stereochemical outcome of glycosylation reactions with the deoxy sugar saccharosamine. Depending on the promoter, either α- or β-linked reactive intermediates are formed. These studies show that differential modes of activation lead to the formation of distinct intermediates that undergo highly selective reactions through an SN2-like mechanism.

Detection and Characterization of Rapidly Equilibrating Glycosylation Reaction Intermediates Using Exchange NMR

The stereoselective introduction of glycosidic bonds (glycosylation) is one of the main challenges in the chemical synthesis of carbohydrates. Glycosylation reaction mechanisms are difficult to control because, in many cases, the exact reactive species driving product formation cannot be detected and the product outcome cannot be explained by the primary reaction intermediate observed. In these cases, reactions are expected to take place via other low-abundance reaction intermediates that are in rapid equilibrium with the primary reaction intermediate via a Curtin-Hammett scenario. Despite this principle being well-known in organic synthesis, mechanistic studies investigating this model in glycosylation reactions are complicated by the challenge of detecting the extremely short-lived reactive species responsible for product formation. Herein, we report the utilization of the chemical equilibrium between low-abundance reaction intermediates and the stable, readily observed α-glycosyl triflate intermediate in order to infer the structure of the former species by employing exchange NMR. Using this technique, we enabled the detection of reaction intermediates such as β-glycosyl triflates and glycosyl dioxanium ions. This demonstrates the power of exchange NMR to unravel reaction mechanisms as we aim to build a catalog of kinetic parameters, allowing for the understanding and eventual prediction of glycosylation reactions.

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.

Activation of thioglycosides under mild alkylation conditions

Reported herein is the development of a novel method for the activation of thioglycosides and thioimidates using benzyl trichloroacetimidate in the presence of catalytic triflic acid. Excellent yields have been achieved with reactive substrates, whereas efficiency of reactions with unreactive glycosyl donors and/or acceptors was modest.

Formation of Glycosyl Trichloroacetamides from Trichloroacetimidate Donors Occurs through an Intermolecular Aglycon Transfer Reaction

To probe the reaction mechanism, underlying the rearrangement of oft-used trichloroacetimidate glycosyl donors into the corresponding anomeric trichloroacetamides, we have used a combination of 13C- and 15N-labeled glycosyl trichloroacetimidate donors in a series of crossover experiments. These unambiguously show that trichloroacetamides are formed via an intermolecular aglycon transfer mechanism. This insight enables the design of more effective glycosylation protocols, preventing the formation of dead-end side products.

Recent Progress in 1,2-cis glycosylation for Glucan Synthesis

Controlling the stereoselectivity of 1,2-cis glycosylation is one of the most challenging tasks in the chemical synthesis of glycans. There are various 1,2-cis glycosides in nature, such as α-glucoside and β-mannoside in glycoproteins, glycolipids, proteoglycans, microbial polysaccharides, and bioactive natural products. In the structure of polysaccharides such as α-glucan, 1,2-cis α-glucosides were found to be the major linkage between the glucopyranosides. Various regioisomeric linkages, 1→3, 1→4, and 1→6 for the backbone structure, and 1→2/3/4/6 for branching in the polysaccharide as well as in the oligosaccharides were identified. To achieve highly stereoselective 1,2-cis glycosylation, including α-glucosylation, a number of strategies using inter- and intra-molecular methodologies have been explored. Recently, Zn salt-mediated cis glycosylation has been developed and applied to the synthesis of various 1,2-cis linkages, such as α-glucoside and β-mannoside, via the 1,2-cis glycosylation pathway and β-galactoside 1,4/6-cis induction. Furthermore, the synthesis of various structures of α-glucans has been achieved using the recent progressive stereoselective 1,2-cis glycosylation reactions. In this review, recent advances in stereoselective 1,2-cis glycosylation, particularly focused on α-glucosylation, and their applications in the construction of linear and branched α-glucans are summarized.

Stereoselective glycosylation reactions with 2-deoxyglucose: a computational study of some catalysts

2-Deoxy glycosides are important components of many oligosaccharides with antibiotic and anti-cancer activity, but their synthesis can be very challenging. Phenanthrolines and substituted pyridines promote stereoselective glycosylation of 1-bromo sugars via a double SN2 mechanism. Pyridine reacting with α-bromo, 2-deoxyglucose was chosen to model this reaction. The first step involves displacement of bromide by pyridine which can be rate limiting because bromide ion is poorly solvated in the non-polar solvents used for these reactions. We examined a series of small molecules to bind bromide and stabilize this transition state. Geometry optimization and vibrational frequencies were calculated using M06-2X/6-31+G(d,p) and SMD implicit solvation for diethyl ether. More accurate energies were obtained with M06-2X/aug-cc-pVTZ and implicit solvation. Urea, thiourea, guanidine and cyanoguanidine bind bromide more strongly than alkylamines, (NH2CH2CH2)nNH3-n. Compared to the uncatalyzed reaction, urea, thiourea and cyanoguanidine lower the free energy of the transition state by 3 kcal/mol while guanidine lowers the barrier by 2 kcal/mol.

Can Side-Chain Conformation and Glycosylation Selectivity of Hexopyranosyl Donors Be Controlled with a Dummy Ligand?

The use of a phenylthio group (SPh) as a dummy ligand at the 6-position to control the side-chain conformation of a series of hexopyranosyl donors is described. The SPh group limits side-chain conformation in a configuration-specific manner, which parallels that seen in the heptopyranosides, and so influences glycosylation selectivity. With both d- and l-glycero-d-galacto-configured donors, the equatorial products are highly favored as they are with an l-glycero-d-gluco donor. For the d-glycero-d-gluco donor, on the other hand, modest axial selectivity is observed. Selectivity patterns are discussed in terms of the side-chain conformation of the donors in combination with the electron-withdrawing effect of the thioacetal group. After glycosylation, removal of the thiophenyl moiety and hydrogenolytic deprotection is achieved in a single step with Raney nickel.

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.

Phase-Transfer Catalyzed Microfluidic Glycosylation: A Small Change in Concentration Results in a Dramatic Increase in Stereoselectivity

Phase-transfer catalysis (PTC) is widely used in glycochemistry for the preparation of aryl glycosides by the glycosylation reaction. While investigating the possibility of synthesis of 4-(3-chloropropoxy)phenyl sialoside (Neu5Ac-OCPP) from N-acetylsialyl chloride with O-acetyl groups (1), we have recently discovered a strong dependence of the PTC glycosylation outcome on the mixing mode: under batch conditions, only α-anomer of Neu5Ac-OCPP was obtained, albeit in low yield (13%), while under microfluidic conditions the yield of Neu5Ac-OCPP increased to 36%, although stereoselectivity decreased (α/β ≤ 6.2). Here, we report that the outcome of this reaction, performed under microfluidic conditions using a Comet X-01 micromixer (at 2 μL/min flow rate), non-linearly depends on the concentration of N-acetylsialyl chloride 1 (5–200 mmol/L). The target Neu5Ac-OCPP was obtained in a noticeably higher yield (up to 66%) accompanied by enhanced stereoselectivity (α/β = 17:1–32:1) in the high concentration range (C > 50 mmol/L), whereas the yield (10–36%) and especially, stereoselectivity (α/β = 0.9:1–6.2:1) were lower in the low concentration range (C ≤ 50 mmol/L). This dramatic stepwise increase in stereoselectivity above critical concentration (50 mmol/L) is apparently related to the changes in the presentation of molecules on the surface of supramers of glycosyl donor, which exist in different concentration ranges.

Influence of Various Silyl Protecting Groups on Stereoselective 2-Deoxyrhamnosylation

The influence of various silyl protecting groups on 2-deoxyrhamnosylation using 2-deoxyrhamnosyl acetates, thioglycosides, and (p-methoxyphenyl)vinylbenzoate (PMPVB) donors has been presented. C-Glycosylation reactions reveal that tert-butyldimethylsilyl (TBDMS), triisopropylsilyl (TIPS), and tert-butyldiphenylsilyl (TBDPS) silyl protected rhamnosyl oxocarbenium ions have no facial selectivity except for the conformationally (⁴H₃) locked tetraisopropyldisiloxane (TIPDS) protected rhamnose donor, which provides complete α-selectivity. However, TBDPS protected rhamnosyl donors are found to be superior protecting groups for α-stereoselective O-glycosylation reactions with various acceptors. The observed results are found consistent across donors and donor activation conditions. Most importantly, the study was conducted at room temperature unlike the other energy-intensive low-temperature studies and was bound to have more practical utility. The outcomes have been explained using kinetic and thermodynamic analyses.

Phenanthroline Catalysis in Stereoselective 1,2-cis Glycosylations

The National Research Council's report in 2012 recognized glycosidic bond forming (glycosylation) reactions as critical due to the central importance of carbohydrates to the glycosciences. This report emphasized the need for the development of reproducible and broadly applicable glycosylation technologies to facilitate the stereoselective synthesis of biomedically relevant glycan libraries for tool development and for research applications by nonspecialists. In response to this report with NIH Common Fund support, the publications of new catalytic diastereoselective glycosylation protocols, some with broad generality under mild conditions, have been recently reported by our group and others. These recent discoveries have also advanced the understanding of the glycosylation reaction mechanism involving the coupling of a sugar electrophile bearing a leaving group at its C1-anomeric center with an alcohol nucleophile. This glycosidic bond forming reaction can lead to a mixture of two stereoisomers that differ in the configuration of the anomeric center.In our group, we discovered that readily available phenanthroline, a rigid and planar organic compound with two fused pyridine rings, could be utilized as a nucleophilic catalyst to promote highly diastereoselective glycosylation of an alcohol nucleophile with a sugar bromide electrophile. The phenanthroline catalysis process allows access to a myriad of high yielding and diastereoselective 1,2-cis pyranosides and furanosides. This catalyst-controlled approach has been applied to the synthesis of a potential vaccine adjuvant α-glucan octasaccharide. For pyranosyl bromide electrophiles, an extensive mechanistic investigation illustrated that two phenanthrolinium ion intermediates, a ⁴C₁ chair-liked equatorial-conformer and a B_2,5 boat-like axial-conformer, are formed in a ratio of 2:1 (equatorial/axial). To obtain high levels of axial-1,2-cis selectivity, a Curtin-Hammett scenario was proposed wherein interconversion of the ⁴C₁ equatorial-conformer and B_2,5 axial-conformer is more rapid than nucleophilic addition. Hydroxyl attack takes place from the axial-face of the more reactive ⁴C₁ chairlike equatorial intermediate to afford an axial-1,2-cis glycoside product. The phenanthroline catalysis system is applicable to a number of furanosyl bromide electrophiles to provide the challenging 1,2-cis substitution products in good yield and diastereoselectivity. NMR experiments and density-functional theory (DFT) calculations support an associative mechanism in which the rate-determining step takes place from an invertive displacement of the faster reacting furanosyl phenanthrolinium ion intermediate with an alcohol nucleophile. Overall, this work stands at the underdeveloped intersection of operationally simple conditions, catalysis, and stereocontrolled glycosidic bond formation, each of which represents an important theme in the preparation of biologically important oligosaccharides and glycopeptides for applications to human health and medicine.

α-Selective Glucosylation Can Be Achieved with 6-O-para-Nitrobenzoyl Protection

A systematic study of the effect of various 6-O-acyl groups on anomeric selectivity in glucosylations with thioglycoside donors was conducted. All eight different esters were found to induce moderate-to-high α-selectivity in glucosylation with l-menthol with the best being 6-O-p-nitrobenzoyl. The effect appears to be general across various glucosyl acceptors, glucosyl donor types, and modes of activation. No evidence was found in favor of distal participation.

The Influence of the Electron Density in Acyl Protecting Groups on the Selectivity of Galactose Formation

The stereoselective formation of 1,2-cis-glycosidic bonds is a major bottleneck in the synthesis of carbohydrates. We here investigate how the electron density in acyl protecting groups influences the stereoselectivity by fine-tuning the efficiency of remote participation. Electron-rich C4-pivaloylated galactose building blocks show an unprecedented α-selectivity. The trifluoroacetylated counterpart with electron-withdrawing groups, on the other hand, exhibits a lower selectivity. Cryogenic infrared spectroscopy in helium nanodroplets and density functional theory calculations revealed the existence of dioxolenium-type intermediates for this reaction, which suggests that remote participation of the pivaloyl protecting group is the origin of the high α-selectivity of the pivaloylated building blocks. According to these findings, an α-selective galactose building block for glycosynthesis is developed based on rational considerations and is subsequently employed in automated glycan assembly exhibiting complete stereoselectivity. Based on the obtained selectivities in the glycosylation reactions and the results from infrared spectroscopy and density functional theory, we suggest a mechanism by which these reactions could proceed.

VaporSPOT: Parallel Synthesis of Oligosaccharides on Membranes

Automated chemical synthesis has revolutionized synthetic access to biopolymers in terms of simplicity and speed. While automated oligosaccharide synthesis has become faster and more versatile, the parallel synthesis of oligosaccharides is not yet possible. Here, a chemical vapor glycosylation strategy (VaporSPOT) is described that enables the simultaneous synthesis of oligosaccharides on a cellulose membrane solid support. Different linkers allow for flexible and straightforward cleavage, purification, and characterization of the target oligosaccharides. This method is the basis for the development of parallel automated glycan synthesis platforms.

Protecting group principles suited to late stage functionalization and global deprotection in oligosaccharide synthesis

Chemical synthesis is a powerful tool to access homogeneous complex glycans, which relies on protecting group (PG) chemistry. However, the overall efficiency of chemical glycan assembly is still low when compared to oligonucleotide or oligopeptide synthesis. There have been many contributions giving rise to collective improvement in carbohydrate synthesis that includes PG manipulation and stereoselective glycoside formation and some of this chemistry has been transferred to the solid phase or adapted for programmable one pot synthesis approaches. However, after all glycoside bond formation reactions are completed, the global deprotection (GD) required to give the desired target OS can be challenging. Difficulties observed in the removal of permanent PGs to release the desired glycans can be due to the number and diversity of PGs present in the protected OSs, nature and structural complexity of glycans, etc. Here, we have reviewed the difficulties associated with the removal of PGs from densely protected OSs to obtain their free glycans. In particularly, this review focuses on the challenges associated with hydrogenolysis of benzyl groups, saponification of esters and functional group interconversion such as oxidation/reduction that are commonly performed in GD stage. More generally, problems observed in the removal of permanent PGs is reviewed herein, including benzyl, acyl (levulinoyl, acetyl), N-trichloroacetyl, N-2,2,2-trichloroethoxycarbonyl, N-phthaloyl etc. from a number of fully protected OSs to release the free sugar, that have been previously reported in the literature.

Beyond GalNAc! Drug delivery systems comprising complex oligosaccharides for targeted use of nucleic acid therapeutics

Nucleic Acid Therapeutics (NATs) are establishing a leading role for the management and treatment of genetic diseases following FDA approval of nusinersen, patisiran, and givosiran in the last 5 years, the breakthrough of milasen, with more approvals undoubtedly on the way. Givosiran takes advantage of the known interaction between the hepatocyte specific asialoglycoprotein receptor (ASGPR) and N-acetyl galactosamine (GalNAc) ligands to deliver a therapeutic effect, underscoring the value of targeting moieties. In this review, we explore the history of GalNAc as a ligand, and the paradigm it has set for the delivery of NATs through precise targeting to the liver, overcoming common hindrances faced with this type of therapy. We describe various complex oligosaccharides (OSs) and ask what others could be used to target receptors for NAT delivery and the opportunities awaiting exploration of this chemical space.

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.

Stabilization of Glucosyl Dioxolenium Ions by "Dual Participation" of the 2,2-Dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA) Protection Group for 1,2-cis-Glucosylation

The stereoselective introduction of glycosidic bonds is of paramount importance to oligosaccharide synthesis. Among the various chemical strategies to steer stereoselectivity, participation by either neighboring or distal acyl groups is used particularly often. Recently, the use of the 2,2-dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA) protection group was shown to offer enhanced stereoselective steering compared to other acyl groups. Here, we investigate the origin of the stereoselectivity induced by the DMNPA group through systematic glycosylation reactions and infrared ion spectroscopy (IRIS) combined with techniques such as isotopic labeling of the anomeric center and isomer population analysis. Our study indicates that the origin of the DMNPA stereoselectivity does not lie in the direct participation of the nitro moiety but in the formation of a dioxolenium ion that is strongly stabilized by the nitro group.

Like Visiting an Old Friend: Fischer Glycosylation in the Twenty-First Century: Modern Methods and Techniques

Fischer glycosylation is typically the chemical reaction of a monosaccharide and an alcohol in presence of an acidic catalyst to afford glycosides in pyranosidic and furanosidic forms. This reaction is still applied today for the synthesis of specialized glycosides, and optimization and modification of the method have continued since its discovery by Emil Fischer in the 1890s. This review presents advancements in Fischer glycosylation described in literature of the past 15 years and its implementation in modern chemical methods.

Towards a Systematic Understanding of the Influence of Temperature on Glycosylation Reactions

Glycosidic bond formation is a continual challenge for practitioners. Aiming to enhance the reproducibility and efficiency of oligosaccharide synthesis, we studied the relationship between glycosyl donor activation and reaction temperature. A novel semi-automated assay revealed diverse responses of members of a panel of thioglycosides to activation at various temperatures. The patterns of protecting groups and the thiol aglycon combine to cause remarkable differences in temperature sensitivity among glycosyl donor building blocks. We introduce the concept of donor activation temperature to capture experimental insights, reasoning that glycosylations performed below this reference temperature evade deleterious side reactions. Activation temperatures enable a simplified temperature treatment and facilitate optimization of glycosyl donor usage. Isothermal glycosylation below the activation temperature halved the equivalents of building block required in comparison to the standard "ramp" regime used in solution- and solid-phase oligosaccharide synthesis to-date.