Stereocontrolled glycosyl transfer reactions with unprotected glycosyl donors

Stereoselective Synthesis of Glycosides via Tsuji-Trost Type Glycosylation Using 3,4-Carbonate Galactals

Pd-catalyzed stereoselective glycosylations using unsaturated sugar derivatives, glycals, have been successfully achieved in recent years. This review focuses on approaches to control the stereoselectivities of glycosides via π-allyl intermediates that mimic the Tsuji-Trost asymmetric allylic alkylation reactions, enabling stereoselectivity control through rational design. In the reaction process, zwitterionic Pd-π-allyl complexes, formed after the oxidative addition and decarboxylation, play a crucial role in increasing reactivities and enhancing the stereoselectivities of α- and β-glycosides. We summarized recently developed Tsuji-Trost type glycosylations using 3,4-carbonate galactals, featuring high efficiency, exclusive stereoselectivities, and a broad reaction scope including O-, N-, S-, and C-glycosylations.

Stereoselective protecting-group-free synthesis of alkyl glycosides using dibenzyloxy triazine type glycosyl donors

Chemical O-glycosylation is a key step for the synthesis of sugar-containing molecules such as glycolipids. However, traditional carbohydrate chemistry is characterized by extensive use of protective groups, resulting in laborious manipulations and poor atom economy. Here, we present a protecting-group-free glycosylation strategy employing dibenzyloxy-1,3,5-triazin-2-yl glycosides (DBT-glycosides) as glycosyl donors. The DBT-glycosyl donors could be prepared directly through an alkaline nucleophilic substitution from unprotected sugars in aqueous media. The O-glycosylation of alcohols by using DBT-glycosyl donors has been carried out under mild hydrogenolytic conditions, affording the corresponding alkyl glycosides stereo-selectively in good yields.

Mechanistic Study of the Copper(II)-Mediated Site-Selective O-Arylation of Glycosides with Arylboronic Acids

Glycosides having multiple free OH groups have been shown to undergo site-selective O-arylations in the presence of arylboronic acids and copper(II) acetate. Herein, a mechanistic analysis of these Chan-Evans-Lam-type couplings is presented based on reaction kinetics, mass spectrometric analysis of reaction mixtures, and substituent effect studies. The results establish that the formation of a substrate-derived boronic ester accelerates the rate-determining transmetalation step. Intramolecular transfer of the aryl group from the boronic ester is ruled out in favor of a pathway in which the key pre-transmetalation assembly is generated from a boronic ester, a copper complex, and a second equivalent of arylboronic acid.

Protecting-group- and microwave-free synthesis of β-glycosyl esters and aryl β-glycosides of N-acetyl-d-glucosamine

A protecting-group-free method for synthesis of β-glycosyl esters and aryl β-glycosides was developed by using latent chemical reactivity of N-acetyl-d-glucosamine (GlcNAc) oxazoline. The GlcNAc oxazoline was spontaneously reacted with carboxylic acids and phenol derivatives via the oxazoline ring opening without the use of a catalyst or heating conditions (i.e., microwave irradiation), affording the desired products in moderate to excellent yields with β-selectivity. This simple protecting-group-free method exhibits a wide substrate scope and good functional group tolerance, and it allows the efficient production of a novel class of GlcNAc-conjugated biomaterials and prodrug candidates.

Direct and Catalytic C-Glycosylation of Arenes: Expeditious Synthesis of the Remdesivir Nucleoside

Since early 2020, scientists have strived to find an effective solution to fight SARS-CoV-2, in particular by developing reliable vaccines that inhibit the spread of the disease and repurposing drugs for combatting its effects on the human body. The antiviral prodrug Remdesivir is still the most widely used therapeutic during the early stages of the infection. However, the current synthetic routes rely on the use of protecting groups, air-sensitive reagents, and cryogenic conditions, thus impeding a cost-efficient supply to patients. We have, therefore, focused on the development of a straightforward, direct addition of (hetero)arenes to unprotected sugars. Here we report a silylium-catalyzed and completely stereoselective C-glycosylation that initially yields the open-chain polyols, which can be selectively cyclized to provide either the kinetic α-furanose or the thermodynamically favored β-anomer. The method significantly expedites the synthesis of Remdesivir precursor GS-441524 after a subsequent Mn-catalyzed C-H oxidation and deoxycyanation.

Anomeric alkylations and acylations of unprotected mono- and disaccharides mediated by pyridoneimine in aqueous solutions

A site-specific deprotonation followed by alkylations and acylations of sugar hemiacetals to the corresponding alkyl glycosides and acylated sugars in aqueous solutions is disclosed herein. Pyridoneimine as a new base is developed to mediate the deprotonation of readily available sugar hemiacetals and further reactions with alkylation and acylation agents.

Streamlined Iterative Assembly of Thio-Oligosaccharides by Aqueous S-Glycosylation of Diverse Deoxythio Sugars

A streamlined iterative assembly of thio-oligosaccharides was developed by aqueous glycosylation. Facile syntheses of various deoxythio sugars with the sulfur on different positions from commercially available starting materials were described. These syntheses featured efficient chemical methods including our recently reported BTM-catalyzed site-selective acylation. The resulting deoxythio sugars could then be used for the Ca(OH)2 -promoted protecting group-free S-glycosylation in water at room temperature. The aqueous glycosylation reaction proceeded smoothly to afford the corresponding 1,2-trans S-glycosides in good yields with high chemo- and stereoselectivity. An appropriate choice of protecting groups for the thiol in the glycosyl donor was necessary for the development of iterative synthesis of thio-oligosaccharides. The aqueous glycosylation was then applied to the synthesis of a trimannoside moiety of N-linked glycans core region.

Transition-Metal-Mediated Glycosylation with Thioglycosides

Thioglycosides are among the most common glycosyl donors that find broad application in the synthesis of glycans and glycoconjugates. However, the requirement for toxic and/or large access of activators needed for common glycosylations with thioglycosides remains a notable drawback. Due to the increased awareness of the chemical waste impact on the environment, synthetic studies have been driven by the goal of finding non-toxic reagents. The main focus of this review is to highlight recent methods for thioglycoside activation that rely on transition metal catalysis.

Diastereoselective Synthesis of Thioglycosides via Pd-Catalyzed Allylic Rearrangement

Stereoselective glycosylation is challenging in carbohydrate chemistry. Herein, stereoselective thioglycosylation of glycals via palladium-catalyzed allylic rearrangement yields various substituents on α-isomer thioglycosides. Two comprehensive series of aryl and benzyl thioglycosides were obtained via a combination of thiosulfates with glycals derived from glucose, arabinose, galactose, and rhamnose. Furthermore, diosgenyl α-l-rhamnoside and isoquercitrin achieved selectivity via stereospecific [2,3]-sigma rearrangements of α-sulfoxide-rhamnoside and α-sulfoxide-glucoside, respectively.

Biocatalytic Alkylation Chemistry: Building Molecular Complexity with High Selectivity

Biocatalysis has traditionally been viewed as a field that primarily enables access to chiral centers. This includes the synthesis of chiral alcohols, amines and carbonyl compounds, often through functional group interconversion via hydrolytic or oxidation-reduction reactions. This limitation is partly being overcome by the design and evolution of new enzymes. Here, we provide an overview of a recently thriving research field that we summarize as biocatalytic alkylation chemistry. In the past 3-4 years, numerous new enzymes have been developed that catalyze sp3 C-C/N/O/S bond formations. These enzymes utilize different mechanisms to generate molecular complexity by coupling simple fragments with high activity and selectivity. In many cases, the engineered enzymes perform reactions that are difficult or impossible to achieve with current small-molecule catalysts such as organocatalysts and transition-metal complexes. This review further highlights that the design of new enzyme function is particularly successful when off-the-shelf synthetic reagents are utilized to access non-natural reactive intermediates. This underscores how biocatalysis is gradually moving to a field that build molecules through selective bond forming reactions.

Protecting group-free method for synthesis of N-glycosyl carbamates and an assessment of the anomeric effect of nitrogen in the carbamate group

The first protecting group-free synthesis of N-glycosyl carbamates has been developed through reaction of d-glucose with n-butyl carbamate in acidic aqueous media. The structures of the N-glucosyl carbamates were unambiguously determined by comparison with authentic samples, prepared using the isocyanide method. With this protective group-free method for synthesis of N-glycosyl carbamates in hand, an anomeric pair of N-xylopyranosyl carbamates were prepared and used to assess the anomeric effect of nitrogen in the carbamate group.

A Palette of Minimally Tagged Sucrose Analogues for Real-Time Raman Imaging of Intracellular Plant Metabolism

Sucrose is the main saccharide used for long-distance transport in plants and plays an essential role in energy metabolism; however, there are no analogues for real-time imaging in live cells. We have optimised a synthetic approach to prepare sucrose analogues including very small (≈50 Da or less) Raman tags in the fructose moiety. Spectroscopic analysis identified the alkyne-tagged compound 6 as a sucrose analogue recognised by endogenous transporters in live cells and with higher Raman intensity than other sucrose derivatives. Herein, we demonstrate the application of compound 6 as the first optical probe to visualise real-time uptake and intracellular localisation of sucrose in live plant cells using Raman microscopy.

A Palette of Minimally Tagged Sucrose Analogues for Real-Time Raman Imaging of Intracellular Plant Metabolism

Sucrose is the main saccharide used for long-distance transport in plants and plays an essential role in energy metabolism; however, there are no analogues for real-time imaging in live cells. We have optimised a synthetic approach to prepare sucrose analogues including very small (≈50 Da or less) Raman tags in the fructose moiety. Spectroscopic analysis identified the alkyne-tagged compound 6 as a sucrose analogue recognised by endogenous transporters in live cells and with higher Raman intensity than other sucrose derivatives. Herein, we demonstrate the application of compound 6 as the first optical probe to visualise real-time uptake and intracellular localisation of sucrose in live plant cells using Raman microscopy.

Recent advances in the direct O-arylation of carbohydrates

Methods for the O-arylation of hydroxyl and hemiacetal groups in carbohydrates via C(sp2)-O bond formation are discussed. Such methods provide an alternative disconnection to the traditional approach of nucleophilic substitution between a sugar-derived electrophile and a phenol or phenoxide nucleophile. They have led to new opportunities for stereoselectivity, site-selectivity and chemoselectivity in the preparation of O-aryl glycosides and carbohydrate-derived aryl ethers, compounds that are useful for a broad range of applications in medicinal chemistry, glycobiology and organic synthesis.

Mild Cu(OTf)2-Mediated C-Glycosylation with Chelation-Assisted Picolinate as a Leaving Group

C-Glycosylation reactions of glycosyl picolinates with allyltrimethylsilane or silyl enol ethers were developed. Picolinate as a chelation-assisted leaving group could be activated by Cu(OTf)2 and avoided the use of harsh Lewis acids. The glycosylations were operated under mild neutral conditions and gave the corresponding C-glycosides in up to 95% yield with moderate to excellent stereoselectivities.

Synthesis of Enantiopure Triols from Racemic Baylis-Hillman Adducts Using a Diastereoselective Peroxidation Reaction

Using a chiral (-)-menthone auxiliary, enantiopure cyclic derivatives of Baylis-Hillman adducts were synthesized. A diastereoselective peroxidation reaction was used to introduce an oxygen atom and establish another stereocenter. The resulting products could be elaborated by employing a one-flask reduction-acetylation protocol followed by a diastereoselective nucleophilic substitution reaction. Removal of the (-)-menthone auxiliary provided an enantiopure triol with a structure related to naturally occurring polyols.

Synthesis of Glycosyl Chlorides and Bromides by Chelation Assisted Activation of Picolinic Esters under Mild Neutral Conditions

A general method has been developed for the formation of glycosyl chlorides and bromides from picolinic esters under mild and neutral conditions. Benchtop stable picolinic esters are activated by a copper(II) halide species to afford the corresponding products in high yields with a traceless leaving group. Rare β glycosyl chlorides are accessible via this route through neighboring group participation. Additionally, glycosyl chlorides with labile protecting groups previously not easily accessible can be prepared.

Systematic synthesis and characterization of a series of different bromoalkylglycosides by Fischer glycosylation

In order to investigate the possibilities of Fischer glycosylation towards the synthesis of bromoalkylglycosides we performed a variety of different reactions resulting in a small library of 16 different glycosides. Using standardized reaction conditions we could gain a broad range of results from small to higher yields. Finally we randomly selected three reactions and performed them with higher amounts of bromoalcohol resulting in significantly better yields, showing the optimization potential of these basic research work.

Secondary amine salt catalyzed controlled activation of 2-deoxy sugar lactols towards alpha-selective dehydrative glycosylation

A new organocatalytic glycosylation method exploiting the lactol functionality has been disclosed. The catalytic generation of glycosyl oxacarbenium ions from lactols under forcible conditions via weakly Brønsted-acidic, readily available secondary amine salts affects the diastereoselective glycosylation of 2-deoxypyranoses and furanoses. This operationally simple iminium catalyzed activation of 2-deoxy hemi-acetals is a potential alternative to the existing cumbersome methods that need specialized handling. The mechanisms for this unique transformation and kinetic/thermodynamic effects have been discussed based on both experimental evidence and theoretical studies.

Stereo- and regioselective glycosylation with protection-less sugar derivatives: an alluring strategy to access glycans and natural products

This review delivers insights for dedicated chemists into the development of efficient methods in accessing carbohydrates at a lower cost.

Glycosylation reactions mediated by hypervalent iodine: application to the synthesis of nucleosides and carbohydrates

To synthesize nucleoside and oligosaccharide derivatives, we often use a glycosylation reaction to form a glycoside bond. Coupling reactions between a nucleobase and a sugar donor in the former case, and the reaction between an acceptor and a sugar donor of in the latter are carried out in the presence of an appropriate activator. As an activator of the glycosylation, a combination of a Lewis acid catalyst and a hypervalent iodine was developed for synthesizing 4'-thionucleosides, which could be applied for the synthesis of 4'-selenonucleosides as well. The extension of hypervalent iodine-mediated glycosylation allowed us to couple a nucleobase with cyclic allylsilanes and glycal derivatives to yield carbocyclic nucleosides and 2',3'-unsaturated nucleosides, respectively. In addition, the combination of hypervalent iodine and Lewis acid could be used for the glycosylation of glycals and thioglycosides to produce disaccharides. In this paper, we review the use of hypervalent iodine-mediated glycosylation reactions for the synthesis of nucleosides and oligosaccharide derivatives.

Triazine-Based Cationic Leaving Group: Synergistic Driving Forces for Rapid Formation of Carbocation Species

A new triazine-based cationic leaving group has been developed for the acid-catalyzed alkylation of O- and C-nucleophiles. There are two synergistic driving forces, namely, stable C═O bond formation and charge-charge repulsive effects, involved in the rapid generation of the carbocation species in the presence of trifluoromethanesulfonic acid (∼200 mol %). Considerable rate acceleration of benzylation, allylation, and p-nitrobenzylation was observed as compared to the reactions with less than 100 mol % of the acid catalyst. The triazine-based leaving group showed superior p-nitrobenzylation yield and stability in comparison to common leaving groups, trichloroacetimidate and bromide. A plausible reaction mechanism (the cationic leaving group pathway) was proposed on the basis of mechanistic and kinetic studies, NMR experiments, and calculations.

Dehydrative glycosylation with cyclic phosphonium anhydrides

Cyclic phosphonium anhydrides generated from bis-phosphine oxides and trifluoromethanesulfonic anhydride are shown as general coupling reagents in a dehydrative glycosylation reaction of C1-hemiacetals. This reaction protocol is characterized by a broad substrate scope and high yields, including reactions of O-, C-, N-, and S-based nucleophiles with furanose, pyranose, and deoxysugar donors.

Preactivation-based chemoselective glycosylations: A powerful strategy for oligosaccharide assembly

Most glycosylation reactions are performed by mixing the glycosyl donor and acceptor together followed by the addition of a promoter. While many oligosaccharides have been synthesized successfully using this premixed strategy, extensive protective group manipulation and aglycon adjustment often need to be performed on oligosaccharide intermediates, which lower the overall synthetic efficiency. Preactivation-based glycosylation refers to strategies where the glycosyl donor is activated by a promoter in the absence of an acceptor. The subsequent acceptor addition then leads to the formation of the glycoside product. As donor activation and glycosylation are carried out in two distinct steps, unique chemoselectivities can be obtained. Successful glycosylation can be performed independent of anomeric reactivities of the building blocks. In addition, one-pot protocols have been developed that have enabled multiple-step glycosylations in the same reaction flask without the need for intermediate purification. Complex glycans containing both 1,2-cis and 1,2-trans linkages, branched oligosaccharides, uronic acids, sialic acids, modifications such as sulfate esters and deoxy glycosides have been successfully synthesized. The preactivation-based chemoselective glycosylation is a powerful strategy for oligosaccharide assembly complementing the more traditional premixed method.

Applications of Nonenzymatic Catalysts to the Alteration of Natural Products

The application of small molecules as catalysts for the diversification of natural product scaffolds is reviewed. Specifically, principles that relate to the selectivity challenges intrinsic to complex molecular scaffolds are summarized. The synthesis of analogues of natural products by this approach is then described as a quintessential "late-stage functionalization" exercise wherein natural products serve as the lead scaffolds. Given the historical application of enzymatic catalysts to the site-selective alteration of complex molecules, the focus of this Review is on the recent studies of nonenzymatic catalysts. Reactions involving hydroxyl group derivatization with a variety of electrophilic reagents are discussed. C-H bond functionalizations that lead to oxidations, aminations, and halogenations are also presented. Several examples of site-selective olefin functionalizations and C-C bond formations are also included. Numerous classes of natural products have been subjected to these studies of site-selective alteration including polyketides, glycopeptides, terpenoids, macrolides, alkaloids, carbohydrates, and others. What emerges is a platform for chemical remodeling of naturally occurring scaffolds that targets virtually all known chemical functionalities and microenvironments. However, challenges for the design of very broad classes of catalysts, with even broader selectivity demands (e.g., stereoselectivity, functional group selectivity, and site-selectivity) persist. Yet, a significant spectrum of powerful, catalytic alterations of complex natural products now exists such that expansion of scope seems inevitable. Several instances of biological activity assays of remodeled natural product derivatives are also presented. These reports may foreshadow further interdisciplinary impacts for catalytic remodeling of natural products, including contributions to SAR development, mode of action studies, and eventually medicinal chemistry.

Strategies toward protecting group-free glycosylation through selective activation of the anomeric center

Glycosylation is an immensely important biological process and one that is highly controlled and very efficient in nature. However, in a chemical laboratory the process is much more challenging and usually requires the extensive use of protecting groups to squelch reactivity at undesired reactive moieties. Nonetheless, by taking advantage of the differential reactivity of the anomeric center, a selective activation at this position is possible. As a result, protecting group-free strategies to effect glycosylations are available thanks to the tremendous efforts of many research groups. In this review, we showcase the methods available for the selective activation of the anomeric center on the glycosyl donor and the mechanisms by which the glycosylation reactions take place to illustrate the power these techniques.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Solution and Solid-Phase Stereocontrolled Synthesis of 1,2-cis-Glycopyranosides with Minimally Protected Glycopyranosyl Donors Catalyzed by BF3-N,N-Dimethylformamide Complex

Methods are described for the stereoselective synthesis of 1,2-cis glycopyranosides in the d-galacto, d-gluco, and 2-azido-2-deoxy-d-glucopyranoside series utilizing minimally protected (3-bromo-2-pyridyloxy) β-d-glycopyranosyl donors in the presence of BF3-N,N-dimethylformamide (DMF) as a catalyst and a variety of alcohol acceptors relying on the "remote activation concept". Precursors to antifreeze glycopeptide components are synthesized in excellent yields and high α/β ratios. The method is adaptable to one-pot sequential glycosidation as well as to solid-supported synthesis giving access to diverse sets of minimally protected α-d-glycopyranosides as major products.

Aqueous Glycosylation of Unprotected Sucrose Employing Glycosyl Fluorides in the Presence of Calcium Ion and Trimethylamine

We report a synthetic glycosylation reaction between sucrosyl acceptors and glycosyl fluoride donors to yield the derived trisaccharides. This reaction proceeds at room temperature in an aqueous solvent mixture. Calcium salts and a tertiary amine base promote the reaction with high site-selectivity for either the 3'-position or 1'-position of the fructofuranoside unit. Because nonenzymatic aqueous oligosaccharide syntheses are underdeveloped, mechanistic studies were carried out in order to identify the origin of the selectivity, which we hypothesized was related to the structure of the hydroxyl group array in sucrose. The solution conformation of various monodeoxysucrose analogs revealed the co-operative nature of the hydroxyl groups in mediating both this aqueous glycosyl bond-forming reaction and the site-selectivity at the same time.

Advances in C-alkynylation of sugars and its application in organic synthesis

C-Glycosidation plays a significant role in the synthesis of optically active scaffolds.

Regioselective modification of unprotected glycosides

The regioselective modification of unprotected glycosides represents shortcuts in carbohydrate chemistry and enables efficient routes to complex derivatives.

Chemistry of xylopyranosides

Xylose is one of the few monosaccharidic building blocks that are used by mammalian cells. In comparison with other monosaccharides, xylose is rather unusual and, so far, only found in two different mammalian structures, i.e. in the Notch receptor and as the linker between protein and glycosaminoglycan (GAG) chains in proteoglycans. Interestingly, simple soluble xylopyranosides can not only initiate the biosynthesis of soluble GAG chains but also function as inhibitors of important enzymes in the biosynthesis of proteoglycans. Furthermore, xylose is a major constituent of hemicellulosic xylans and thus one of the most abundant carbohydrates on Earth. Altogether, this has spurred a strong interest in xylose chemistry. The scope of this review is to describe synthesis of xylopyranosyl donors, as well as protective group chemistry, modifications, and conformational analysis of xylose.

Engineering the specificity of trehalose phosphorylase as a general strategy for the production of glycosyl phosphates

A two-step process is reported for the anomeric phosphorylation of galactose, using trehalose phosphorylase as biocatalyst. The monosaccharide enters this process as acceptor but can subsequently be released from the donor side, thanks to the non-reducing nature of the disaccharide intermediate. A key development was the creation of an optimized enzyme variant that displays a strict specificity (99%) for β-galactose 1-phosphate as product.

1,2-cis Alkyl glycosides: straightforward glycosylation from unprotected 1-thioglycosyl donors

A 1,2-cis-alkyl glycosidation protocol that makes use of unprotected phenyl 1-thioglycosyl donors is reported. Glycosylation of various functionalized alcohols was accomplished in moderate to high yield and selectivity to give the 1,2-cis-glycosides. In order to quickly develop optimum glycosylation conditions, an FIA (flow injection analysis)-ESI-TOF-MS method was developed that enabled rapid and quantitative evaluation of yield on small scale. This methodology, coupled with NMR spectroscopy, allowed for rapid evaluation of the overall reactions.