Glycosyl donors in “unusual” conformations – influence on reactivity and selectivity

Exploiting π and Chalcogen Interactions for the β-Selective Glycosylation of Indoles through Glycal Conformational Distortion

Harnessing unconventional noncovalent interactions (NCIs) is emerging as a formidable synthetic approach in difficult-to-access glycosidic chemical space. C-Glycosylation, in particular, has gained a flurry of recent attention. However, most reported methods are restricted to the relatively facile access to α-C-glycosides. Herein, we disclose a β-stereoselective glycosylation of indoles by employing a phosphonoselenide catalyst. The robustness of this protocol is exemplified by its amenability for reaction at both the indolyl C- and N- reactivity sites. In contrast to previous reports, in which the chalcogens were solely involved in Lewis acidic activation, our mechanistic investigation unraveled that the often neglected flanking aromatic substituents of phosphonoselenides can substantially contribute to catalysis by engaging in π-interactions. Computations and NMR spectroscopy indicated that the chalcogenic and aromatic components of the catalyst can be collectively exploited to foster conformational distortion of the glycal away from the usual half-chair to the boat conformation, which liberates the convex β-face for nucleophilic attack.

Selective reactivity of glycosyl cation stereoisomers: the role of intramolecular hydrogen bonding

The impact of the stereochemistry of the glycosyl cation species upon its dynamic properties is examined together with their vibrational spectra in order to gain insight into the effects of configurational isomerism on conformer dynamics and proton mobility. Ab initio molecular dynamics (AIMD) simulations and infrared multiple photon dissociation (IRMPD) spectroscopy explore the conformational and reactive dynamics of two pairs of glycosyl cation isomers: (1) protonated α- and β- anomers of methyl-D-galactopyranoside and (2) the oxocarbenium ions of the D-aldohexose C2 epimers galactose and talose. Analysis of these simulations together with experimental spectroscopy, interpreted by anharmonic calculations, points to the key role played by the intramolecular hydrogen bonds which are present in a unique pattern and extent in each isomer. We find that the reactivity of galactoside stereoisomers toward acid-catalyzed nucleophilic substitution, as gauged by the ability to form free oxocarbenium ions, differs markedly in a way that agrees with experimental measurements in the condensed phase. Other properties such as conformer stability and vibrational transitions were also found to reflect the characteristic hydrogen bonding interactions present in each isomer. In both systems, the stereochemistry is shown to determine the strength of intramolecular hydrogen bonding as well as between which substituents proton transfer is possible. We expect that the critical impact of non-covalent interactions on stereoisomer selectivity may be a widely found phenomenon whose effects should be further investigated.

Protecting carbohydrates with ethers, acetals and orthoesters under basic conditions

Chlorinated ethyl and vinyl ethers are introduced at various positions of carbohydrates. Depending on the relative stereochemistry, vinylethers, acetals or orthoesters are formed under basic conditions. The products are stable, but are easily deprotected after dechlorination. The scope of the intramolecular protection is studied using common pentoses and hexoses.

GH47 and Other Glycoside Hydrolases Catalyze Glycosidic Bond Cleavage with the Assistance of Substrate Super-arming at the Transition State

Super-armed glycosyl donors, whose substituents are predominantly held in pseudoaxial positions, exhibit strongly increased reactivity in glycosylation through significant stabilization of oxocarbenium-like transition states. Examination of X-ray crystal structures reveals that the GH47 family of glycoside hydrolases have evolved so as to distort their substrates away from the ground state conformation in such a manner as to present multiple C-O bonds in pseudoaxial positions and so benefit from conformational super-arming of their substrates, thereby enhancing catalysis. Through analysis of literature mutagenic studies, we show that a suitably placed aromatic residue in GHs 6 and 47 sterically enforces super-armed conformations on their substrates. GH families 45, 81, and 134 on the other hand impose conformational super-arming on their substrates, by maintaining the more active ring conformation through hydrogen bonding rather than steric interactions. The recognition of substrate super-arming by select GH families provides a further parallel with synthetic carbohydrate chemistry and nature and opens further avenues for the design of improved glycosidase inhibitors.

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.

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.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Conformationally Switchable Glycosyl Donors

Glycosyl donors functionalized with 2,2'-bipyridine moieties on the 3-OH and 6-OH or the 2-OH and 4-OH undergo a conformational change when forming 1:1 complexes with Zn²⁺ ions. The pyranoside ring of the zinc complexes adopted axial-rich skew boat conformations. The reactivities of the two glycosyl donors were investigated by performing a series of glycosylations in the presence or absence of Zn²⁺ ions. These glycosylations suggested a decrease in reactivity when binding Zn²⁺. The conformational effect of binding Zn²⁺ was therefore studied using a third glycosyl donor, unable to undergo conformational changes when binding Zn²⁺. From competition experiments, it was observed that the binding-induced conformational change increased the reactivity slightly compared to the glycosyl donor unable to undergo a conformational change.

Bromine-Promoted Glycosidation of Conformationally Superarmed Thioglycosides

Presented herein is a study of the conformation and reactivity of highly reactive thioglycoside donors. The structural studies have been conducted using NMR spectroscopy and computational methods. The reactivity of these donors has been investigated in bromine-promoted glycosylations of aliphatic and sugar alcohols. Swift reaction times, high yields, and respectable 1,2-cis stereoselectivity were observed in a majority of these glycosylations.

Allylic Strain as a Stereocontrol Element in the Hydrogenation of 3-Hydroxymethyl-cyclohex-3-en-1,2,5-triol Derivatives. Synthesis of the Carbasugar Pseudo-2-deoxy-α-D-glucopyranose

By adaptation of a literature method developed in the glucose series and standard protecting group manipulations a 2-deoxy-D-glucose derivative is converted to a series of 1R,2R,5R-3-hydroxymethyl-cyclohexen-1,2,5-triol derivatives whose reduction to the corresponding diastereomeric D-gluco and L-ido-2-deoxy carbasugars is studied. When the hydroxymethyl group is tied up in a cyclic isopropylidene acetal with the adjacent 6-hydroxy group the cis-fused products with the ido-configuration are formed preferentially whereas protection of the hydroxymethyl group as a methoxymethyl ether results in the formation of the D-gluco isomer on hydrogenation over Raney nickel. This result is rationalized in terms of the minimization of allylic strain in the course of the reduction, enables the preparation of the carbasugar pseudo-2-deoxy-α-D-glucopyranose, and suggests that the conformation of the side chain is an important control element in the chemistry of the pseudosugars as it is in the sugars themselves.

Automated Chemical Oligosaccharide Synthesis: Novel Approach to Traditional Challenges

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

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.

Immobilization of glycans on solid surfaces for application in glycomics

Carbohydrates are an important class of biomolecules which are involved in a multitude of cellular functions. In the field of glycomics, the structure and function of various carbohydrates, oligosaccharides, glycans and their conjugates are constantly under investigation. In the continuing quest to understand the roles of carbohydrates in their interactions with proteins, immunogens, and other cell-surface carbohydrates, scientists have developed methods for observing the effects of specific saccharide sequences on various cellular components. Carbohydrate immobilization has allowed researchers to study the impact of specific sequences, leading to a deeper understanding of many cellular processes. The goal of this review is to highlight the chemical reactions and interactions that have been used for glycan immobilization.

Synthesis of Conformationally-Locked cis- and trans-Bicyclo[4.4.0] Mono-, Di-, and Trioxadecane Modifications of Galacto- and Glucopyranose; Experimental Limiting 3JH,H Coupling Constants for the Estimation of Carbohydrate Side Chain Populations and Beyond

Hexopyranose side chains populate three staggered conformations, whose proportions can be determined from the three sets of ideal limiting 3JH5,H6R and 3JH5,H6S coupling constants in combination with the time-averaged experimental coupling constants. Literature values for the limiting coupling constants, obtained by the study of model compounds, the use of the Haasnoot-Altona and related equations, or quantum mechanical computations, can result in computed negative populations of one of the three ideal conformations. Such values arise from errors in the limiting coupling constants and/or from the population of nonideal conformers. We describe the synthesis and analysis of a series of cis- and trans-fused mono-, di-, and trioxabicyclo[4.4.0]octane-like compounds. Correction factors for the application of data from internal models (-CH(OR)-CH(OR)-) to terminal systems (-CH(OR)-CH2(OR)) are deduced from comparison of further models, and applied where necessary. Limiting coupling constants so-derived are applied to the side chain conformations of three model hexopyranosides, resulting in calculated conformer populations without negative values. Although, developed primarily for hexopyranose side chains, the limiting coupling constants are suitable, with the correction factors presented, for application to the side chains of higher carbon sugars and to conformation analysis of acyclic diols and their derivatives in a more general sense.

Palladium-Catalyzed Direct Stereoselective Synthesis of Deoxyglycosides from Glycals

Palladium(II) in combination with a monodentate phosphine ligand enables the unprecedented direct and α-stereoselective catalytic synthesis of deoxyglycosides from glycals. Initial mechanistic studies suggest that in the presence of N-phenyl-2-(di-tert-butylphosphino)pyrrole as the ligand, the reaction proceeds via an alkoxy palladium intermediate that increases the proton acidity and oxygen nucleophilicity of the alcohol. The method is demonstrated with a wide range of glycal donors and acceptors, including substrates bearing alkene functionalities.

Conformationally superarmed S-ethyl glycosyl donors as effective building blocks for chemoselective oligosaccharide synthesis in one pot

A new series of superarmed glycosyl donors has been investigated. It was demonstrated that the S-ethyl leaving group allows for high reactivity, which is much higher than that of equally equipped S-phenyl glycosyl donors that were previously investigated by our groups. The superarmed S-ethyl glycosyl donors equipped with a 2-O-benzoyl group gave complete β-stereoselectivity. Utility of the new glycosyl donors has been demonstrated in a one-pot one-addition oligosaccharide synthesis with all of the reaction components present from the beginning.

Stereoselective Synthesis of 5-epi-α-Sialosides Related to the Pseudaminic Acid Glycosides. Reassessment of the Stereoselectivity of the 5-Azido-5-deacetamidosialyl Thioglycosides and Use of Triflate as Nucleophile in the Zbiral Deamination of Sialic Acids

With a view to the eventual synthesis of glycosyl donors for the stereocontrolled synthesis of pseudaminic acid glycosides, the stereocontrolled synthesis of a d-glycero-d-gulo sialic acid adamantanylthioglycoside carrying an axial azide at the 5-position is described. The synthesis employs levulinic acid as nucleophile in the oxidative deamination of an N-acetylneuraminic acid thioglycoside leading to the formation of a 3-deoxy-d-glycero-d-galacto-2-nonulosonic acid (KDN) derivative selectively protected as 5-O-levulinate. Replacement of the levulinate by triflate enables introduction of the axial azide and hence formation of the glycosyl donor. A shorter synthesis uses trifluoromethanesulfonate as nucleophile in the oxidative deamination step when the 5-O-triflyl KDN derivative is obtained directly. Glycosylation reactions conducted with the 5-azido-d-glycero-d-gulo-configured sialyl adamantanylthioglycoside at -78 °C are selective for the formation of the equatorial glycosides, suggesting that the synthesis of equatorial pseudaminic acid glycosides will be possible as suitable donors become available. A comparable N-acetylneuraminic acid adamantanylthioglycoside carrying an equatorial azide at the 5-position was also found to be selective for equatorial glycoside formation under the same conditions, suggesting that reinvestigation of other azide-protected NeuAc donors is merited. Glycosylation stereoselectivity in the d-glycero-d-gulo series is discussed in terms of the side-chain conformation of the donor.

Bimodal concentration-dependent reactivity pattern of a glycosyl donor: Is the solution structure involved?

Changes in concentration (0.001-0.1 M) of an arabinofuranosyl donor (1) have been shown to modulate the temperature T at which activation of 1 occurs (from -23 °C to +7 °C), the reaction time (from 1.5 h to 3 days) and the yield of the disaccharide formed (from 14% to 82%). At concentrations exceeding 0.01 M, these parameters, as well as the specific optical rotation of the solution of 1, virtually do not depend on concentration suggesting formation of reacting species (supramers) of glycosyl donor with similar structures, hence reactivities, but considerably different from those formed in more dilute solutions. The found critical concentration (0.01 M) separates two concentration ranges of reaction solutions corresponding to two types of solution structure that are featured by the presence of fundamentally different supramers of glycosyl donor, which have distinct chemical properties. These results allow a fresh look at the problems of reactivity of chemical compounds and selectivity of the reactions in which they participate.

2,3-Di-O-picolinyl building blocks as glycosyl donors with switchable stereoselectivity

Coordination of 2,3-di-O-picolinyl protected glycosyl donors with PdBr₂ allows to “switch” the stereoselectivity of glycosylations from β- to α-.

Absence of Stereodirecting Participation by 2-O-Alkoxycarbonylmethyl Ethers in 4,6-O-Benzylidene-Directed Mannosylation

The preparation of a series of mannopyranosyl donors carrying 2-O-(2-oxoalkyl) ethers and their use in glycosylation reactions are described. The formation of cyclic products with the simple 2-O-phenacyl ether and with the 2-O-(t-butoxycarbonylmethyl) ether establishes the stereoelectronic feasibility of participation in such systems. The high β-selectivities observed with the bis-trifluoromethyl phenacyl ether indicate that participation can be suppressed through the introduction of electron-withdrawing substituents. The high β-selectivities and absence of cyclic products observed with the 2-O-(methoxycarbonylmethyl) ether exclude the effective participation of esters through six-membered cyclic intermediates in this series. The results are discussed in terms of the conformation of cyclic dioxenium ions (E,E-, E,Z-, or Z,Z-) and in the context of "neighboring group" participation by nonvicinal esters in glycosylation. Methods for the deprotection of the 2-O-phenacyl and 2-O-(methoxycarbonylmethyl) ethers are described.

Anomericity of T-2 toxin-glucoside: masked mycotoxin in cereal crops

T-2 toxin is a trichothecene mycotoxin produced when Fusarium fungi infect grains, especially oats and wheat. Ingestion of T-2 toxin contaminated grain can cause diarrhea, hemorrhaging, and feed refusal in livestock. Cereal crops infected with mycotoxin-producing fungi form toxin glycosides, sometimes called masked mycotoxins, which are a potential food safety concern because they are not detectable by standard approaches and may be converted back to the parent toxin during digestion or food processing. The work reported here addresses four aspects of T-2 toxin-glucosides: phytotoxicity, stability after ingestion, antibody detection, and the anomericity of the naturally occurring T-2 toxin-glucoside found in cereal plants. T-2 toxin-β-glucoside was chemically synthesized and compared to T-2 toxin-α-glucoside prepared with Blastobotrys muscicola cultures and the T-2 toxin-glucoside found in naturally contaminated oats and wheat. The anomeric forms were separated chromatographically and differ in both NMR and mass spectrometry. Both anomers were significantly degraded to T-2 toxin and HT-2 toxin under conditions that mimic human digestion, but with different kinetics and metabolic end products. The naturally occurring T-2 toxin-glucoside from plants was found to be identical to T-2 toxin-α-glucoside prepared with B. muscicola. An antibody test for the detection of T-2 toxin was not effective for the detection of T-2 toxin-α-glucoside. This anomer was produced in sufficient quantity to assess its animal toxicity.

A 3,4-trans-fused cyclic protecting group facilitates α-selective catalytic synthesis of 2-deoxyglycosides

A practical approach has been developed to convert glucals and rhamnals into disaccharides or glycoconjugates with high α-selectivity and yields (77-97%) using a trans-fused cyclic 3,4-O-disiloxane protecting group and TsOH⋅H2O (1 mol%) as a catalyst. Control of the anomeric selectivity arises from conformational locking of the intermediate oxacarbenium cation. Glucals outperform rhamnals because the C6 side-chain conformation augments the selectivity.

Glycosyl alkoxythioimidates as building blocks for glycosylation: a reactivity study

Structural modifications of the leaving group of S-glycosyl O-methyl phenylcarbamothioates (SNea) involving change of substituents that express different electronic effects led to a better understanding of how the reactivity of these glycosyl donors can be modified by changing the structure of their leaving groups. Mechanistic studies involving the isolation of departed aglycones were indicative of the direct activation of both p-methoxy-SNea and p-nitro-SNea leaving groups via the anomeric sulfur rather than the remote nitrogen atom. The presence of an electron donating substituent (p-methoxy) has a strong effect on the nucleophilicity of the sulfur atom that becomes more susceptible toward the attack of thiophilic reagents, in particular. This key observation allowed to differentiate the reactivity levels of p-methoxy-SNea versus p-nitro-SNea and even unmodified SNea leaving groups. The reactivity difference observed in the series of SNea leaving groups is sufficient to be exploited in expeditious oligosaccharide synthesis via selective activation strategies.

The use of O-trifluoroacetyl protection and profound influence of the nature of glycosyl acceptor in benzyl-free arabinofuranosylation

The influence of O-trifluoroacetyl (TFA) groups at different positions of thioglycoside glycosyl donors on stereoselectivity of α-arabinofuranosylation leading to corresponding disaccharides was studied. It was shown that TFA group in thioglycoside glycosyl donors, when combined with 2-O-(triisopropylsilyl) (TIPS) non-participating group, may be regarded as an electron-withdrawing protecting group that may enhance 1,2-cis-selectivity in arabinofuranosylation, the results strongly depending on the nature of glycosyl acceptor. The reactivities of the glycosyl donors were compared with those of a similar thioglycoside with O-pentafluoropropionyl groups and the known phenyl 3,5-O-(di-tert-butylsilylene)-1-thio-α-d-arabinofuranosides with 2-O-TIPS and 2-O-benzyl groups. The 'matching' in the donor-acceptor combination was found to be critical for achieving both high reactivity of glycosyl donor and β-stereoselectivity of arabinofuranosylation. The use of glycosyl donors with TFA and silyl protection may be useful in the realization of the benzyl-free approach to oligoarabinofuranosides with azido group in aglycon-convenient building blocks for the preparation of neoglycoconjugates.

Scope and limitations of 2-deoxy- and 2,6-dideoxyglycosyl bromides as donors for the synthesis of β-2-deoxy- and β-2,6-dideoxyglycosides

It is shown that 2-deoxy- and 2,6-dideoxyglycosyl bromides can be prepared in high yield (72-94%) and engaged in glycosylation reactions with β:α selectivities ≥6:1. Yields of product are 44-90%. Fully armed 2-deoxyglycoside donors are viable, while 2,6-dideoxyglycosides require one electron-withdrawing substituent for high efficiency and β-selectivity. Equatorial C-3 ester protecting groups decrease β-selectivity, and donors bearing an axial C-3 substituent are not suitable. The method is compatible with azide-containing donors and acid-sensitive functional groups.

Design of chemical glycosyl donors: does changing ring conformation influence selectivity/reactivity?

This tutorial review focuses on the design of glycosyl donors, especially on attempts to control selectivity/reactivity by employing bulky substituents, cyclic protecting groups, or bridged structures. These structural modifications are performed to change the conformational distributions of pyranoside/furanoside rings. We also briefly discuss this issue with regard to studies on furanosides and enzymatic glycosylation reactions. Readers will find that some of the designed glycosyl donors have been used to achieve total syntheses of natural products.