"One-Pot" Protection, Glycosylation, and Protection-Glycosylation Strategies of Carbohydrates

Cyanomethyl Ether as an Orthogonal Participating Group for Stereoselective Synthesis of 1,2-trans-β-O-Glycosides

Stereoselective formation of glycosidic linkages has been the prime focus for contemporary carbohydrate chemistry. Herein, we report cyanomethyl (CNMe) ether as an efficient and effective participating orthogonal protecting group for the stereoselective synthesis of 1,2-trans-β-O-glycosides. The participating group facilitated good to high β-selective glycosylation with a broad range of electron-rich and electron-deficient glycosyl acceptors. Detailed experimental and theoretical studies reveal the involvement of CNMe ether in the formation of a six-membered imine-type cyclic intermediate for the observed stereoselectivity. Rapid incorporation and selective removal of the CNMe ether group in the presence of benzyl ether and isopropylidene acetal protection have also been reported here. The nitrile group provided an opportunity for the glycodiversification through further derivatizations.

Conformational Studies of Oligosaccharides

The conformation of a molecule strongly affects its function, as demonstrated for peptides and nucleic acids. This correlation is much less established for carbohydrates, the most abundant organic materials in nature. Recent advances in synthetic and analytical techniques have enabled the study of carbohydrates at the molecular level. Recurrent structural features were identified as responsible for particular biological activities or material properties. In this Minireview, recent achievements in the structural characterization of carbohydrates, enabled by systematic studies of chemically defined oligosaccharides, are discussed. These findings can guide the development of more potent glycomimetics. Synthetic carbohydrate materials by design can be envisioned.

Novel eco-friendly solution for the regioselective acetylation of per-O-TMS carbohydrates

Regioselective acetylation of per-O-TMS carbohydrates was achieved preferentially at the 6 position or 1,6-position under mild conditions involving the eco-friendly solvent acetonitrile, at room temperature, in ambient atmosphere and in a shorter time. Good or moderate yields were obtained via 4-dimethylaminopyridine, without auxiliary equipment. A single α-O-Acetyl acetylation anomer was exclusively defined for the involved substrates. A 6-O-monoacetate derivative was applied and used as a stable glycosyl donor in the disaccharides construction. The methodology was successful for a range of substrates, which include the following: d-lactose, d-trehalose, d-galactose, methyl α-d-galactose, d-glucose, d-mannose, d-xylose, and l-fucose.

Artificial Glycoproteins as a Scaffold for Targeted Drug Therapy

Akin to a cellular "fingerprint," the glycocalyx is a glycan-enriched cellular coating that plays a crucial role in mediating cell-to-cell interactions. To gain a better understanding of the factors that govern in vivo recognition, artificial glycoproteins were initially created to probe changes made to the accumulation and biodistribution of specific glycan assemblies through biomimicry. As a result, the organ-specific accumulation for a variety of glycoproteins decorated with simple and/or complex glycans was identified. Additionally, binding trends with regard to cancer cell selectivity were also investigated. To exploit the knowledge gained from these studies, numerous groups thus became engaged in developing targeted drug methodologies based on the use of artificial glycoproteins. This has either been done through adopting the glycoprotein scaffold as a drug carrier, or to directly glycosylate therapeutic proteins/enzymes to localize their biological activity. The principle aim of this Review is to present the foundational research that has driven artificial glycoprotein-based targeting and subsequent adaptations with potential therapeutic applications.

Picoloyl protecting group in synthesis: focus on a highly chemoselective catalytic removal

The picoloyl ester (Pico) has proven to be a versatile protecting group in carbohydrate chemistry. It can be used for the purpose of stereocontrolling glycosylations via an H-bond-mediated Aglycone Delivery (HAD) method. It can also be used as a temporary protecting group that can be efficiently introduced and chemoselectively cleaved in the presence of practically all other common protecting groups used in synthesis. Herein, we will describe a new method for rapid, catalytic, and highly chemoselective removal of the picoloyl group using inexpensive copper(ii) or iron(iii) salts.

Recent advances in glycan synthesis

Carbohydrates play important roles in life science, but their synthesis is always hampered by their complicated chemical structures. Scientists have never stopped trying to solve the problem of glycan synthesis from various aspects. Here a brief overview of recent progress in glycan synthesis, including chemical approaches, chemoenzymatic approaches, and automated synthesis, will be discussed, focusing on the efficiency of new glycosylation methods, the stereoselectivity of coupled products, and their applications in the assembly of complex glycan chains.

Sub-stoichiometric reductive etherification of carbohydrate substrates and one-pot protecting group manipulation

In this study, we report a new reductive etherification procedure for protection of carbohydrate substrates and its application for one-pot preparation of glycosyl building blocks. The reported procedure features the use of polymethylhydrosiloxane (PMHS) as a sub-stoichiometric reducing agent, which prevents the transilylation side reaction and improves the efficiency of the reductive etherification method. Application of the PMHS reductive etherification procedure for one-pot protecting group manipulation are described.

Fluoride Migration Catalysis Enables Simple, Stereoselective, and Iterative Glycosylation

Challenges in the assembly of glycosidic bonds in oligosaccharides and glycoconjugates pose a bottleneck in enabling the remarkable promise of advances in the glycosciences. Here, we report a strategy that applies unique features of highly electrophilic boron catalysts, such as tris(pentafluorophenyl)borane, in addressing a number of the current limitations of methods in glycoside synthesis. This approach utilizes glycosyl fluoride donors and silyl ether acceptors while tolerating the Lewis basic environment found in carbohydrates. The method can be carried out at room temperature using air- and moisture-stable forms of the catalyst, with loadings as low as 0.5 mol %. These characteristics enable a wide array of glycosylation patterns to be accessed, including all C1-C2 stereochemical relationships in the glucose, mannose, and rhamnose series. This method allows one-pot, iterative glycosylations to generate oligosaccharides directly from monosaccharide building blocks. These advances enable the rapid and experimentally straightforward preparation of complex oligosaccharide units from simple building blocks.

Modular Synthesis of Nona-Decasaccharide Motif from Psidium guajava Polysaccharides: Orthogonal One-Pot Glycosylation Strategy

The synthesis of long, branched, and complex carbohydrate sequences remains a challenging task in chemical synthesis. Reported here is an efficient and modular one-pot synthesis of a nona-decasaccharide and shorter sequences from Psidium guajava polysaccharides, which have the potent α-glucosidase inhibitory activity. The synthetic strategy features: 1) several one-pot glycosylation reactions on the basis of N-phenyltrifluoroacetimidate (PTFAI) and Yu glycosylation to streamline the chemical synthesis of oligosaccharides, 2) the successful and efficient assembly sequences (first O3', second O5', final O2') toward the challenging 2,3,5-branched Araf motif, 3) the stereoselective 1,2-cis-glucosylation by reagent control, and 4) the convergent [6+6+7] one-pot coupling reaction for the final assembly of the target nona-decasaccharide. This orthogonal one-pot glycosylation strategy can streamline the chemical synthesis of long, branched, and complicated carbohydrate chains.

One-Pot Relay Glycosylation

A novel one-pot relay glycosylation has been established. The protocol is characterized by the construction of two glycosidic bonds with only one equivalent of triflic anhydride. This method capitalizes on the in situ generated cyclic-thiosulfonium ion as the relay activator, which directly activates the newly formed thioglycoside in one pot. A wide range of substrates are well-accommodated to furnish both linear and branched oligosaccharides. The synthetic utility and advantage of this method have been demonstrated by rapid access to naturally occurring phenylethanoid glycoside kankanoside F and resin glycoside merremoside D.

Regioselective Sulfonylation/Acylation of Carbohydrates Catalyzed by FeCl3 Combined with Benzoyltrifluoroacetone and Its Mechanism Study

A catalytic amount of FeCl₃ combined with benzoyl trifluoroacetone (Hbtfa) (FeCl₃/Hbtfa = 1/2) was used to catalyze sulfonylation/acylation of diols and polyols using diisopropylethylamine (DIPEA) or potassium carbonate (K₂CO₃) as a base. The catalytic system exhibited high catalytic activity, leading to excellent isolated yields of sulfonylation/acylation products with high regioselectivities. Mechanism studies indicated that FeCl₃ initially formed [Fe(btfa)₃] (btfa = benzoyl trifluoroacetonate) with twice the amount of Hbtfa under basic conditions in the solvent acetonitrile at room temperature. Then, Fe(btfa)₃ and two hydroxyl groups of the substrates formed a five- or six-membered ring intermediate in the presence of the base. The subsequent reaction between the cyclic intermediate and a sulfonylation reagent led to the selective sulfonylation of the substrate. All key intermediates were captured in the high-resolution mass spectrometry assay, therefore demonstrating this mechanism for the first time.

Analysis of glycan expression on cell surfaces by using a glassy carbon electrode modified with MnO2 nanosheets and DNA-generated electrochemical current

Electrochemical assay for analysis of cell surface glycan expression is reported. Mannose on human breast cancer cells (type MCF-7) is selected as the glycan model. Gold nanoparticles are modified with binding aptamer for MCF-7 cells and act as electrochemical probe. The analysis of cell surface glycan expression follows a traditional sandwich protocol. Concanavalin A that can specifically recognize mannose is immobilized onto MnO₂ nanosheets modified electrode for the capture of MCF-7 cells. Then, the modified gold nanoparticles are immobilized onto the electrode via the binding between MCF-7 cell and aptamer on the gold nanoparticles. The aptamer on the gold nanoparticles reacts with molybdate. More specifically, the reaction of the phosphate backbone of aptamer with molybdate results in the formation of a redox-active molybdophosphate precipitate and generates an electrochemical current. The current intensity at 0.20 V (vs. Ag/AgCl) is recorded to test the linear range of the assay. The assay shows an obvious response to MCF-7 cells with a wide linear range from 1.0 × 10³ to 1.0 × 10⁶ cells mL^-1 and a limit of detection down to 300 cells mL^-1. The assay can be used to selectively monitor the change of mannose expression on cell surfaces upon the treatment with the N-glycan inhibitor. Graphical abstractSchematic of an electrochemical assay for analysis of cell surface glycan expression of MCF-7 cancer cells.

Glycosyl ortho-(1-phenylvinyl)benzoates versatile glycosyl donors for highly efficient synthesis of both O-glycosides and nucleosides

Both of O-glycosides and nucleosides are important biomolecules with crucial rules in numerous biological processes. Chemical synthesis is an efficient and scalable method to produce well-defined and pure carbohydrate-containing molecules for deciphering their functions and developing therapeutic agents. However, the development of glycosylation methods for efficient synthesis of both O-glycosides and nucleosides is one of the long-standing challenges in chemistry. Here, we report a highly efficient and versatile glycosylation method for efficient synthesis of both O-glycosides and nucleosides, which uses glycosyl ortho-(1-phenylvinyl)benzoates as donors. This glycosylation protocol enjoys the various features, including readily prepared and stable donors, cheap and readily available promoters, mild reaction conditions, good to excellent yields, and broad substrate scopes. In particular, the applications of the current glycosylation protocol are demonstrated by one-pot synthesis of several bioactive oligosaccharides and highly efficient synthesis of nucleosides drugs capecitabine, galocitabine and doxifluridine.

Synthetic carbohydrate-based vaccines: challenges and opportunities

Glycoconjugate vaccines based on bacterial capsular polysaccharides (CPS) have been extremely successful in preventing bacterial infections. The glycan antigens for the preparation of CPS based glycoconjugate vaccines are mainly obtained from bacterial fermentation, the quality and length of glycans are always inconsistent. Such kind of situation make the CMC of glycoconjugate vaccines are difficult to well control. Thanks to the advantage of synthetic methods for carbohydrates syntheses. The well controlled glycan antigens are more easily to obtain, and them are conjugated to carrier protein to from the so-call homogeneous fully synthetic glycoconjugate vaccines. Several fully glycoconjugate vaccines are in different phases of clinical trial for bacteria or cancers. The review will introduce the recent development of fully synthetic glycoconjugate vaccine.

(2-Ketulosonyl)onate 2,3-O-thionocarbonate donors for the synthesis of KO and KDO α-glycosides and a one-pot glycosylation method for 2-keto acid donors

Bifunctional (2-ketulosonyl)onate thionocarbonates are effective donors for the synthesis of KO and KDO α-glycosides with perfect control in stereoselectivity.

SnCl4-catalyzed solvent-free acetolysis of 2,7-anhydrosialic acid derivatives

Sialic acid-containing glycans are found in different sialic acid forms and a variety of glycosidic linkages in biologically active glycoconjugates. Hence, the preparation of suitably protected sialyl building blocks requires high attention in order to access glycans in a pure form. In line with this, various C-5-substituted 2,7-anhydrosialic acid derivatives bearing both electron-donating and -withdrawing protecting groups were synthesized and subjected to different Lewis acid-catalyzed solvent-free ring-opening reactions at room temperature in the presence of acetic anhydride. Among the various Lewis acids tested, the desired acetolysis products were obtained in moderate yields under tin(IV) chloride catalysis. Our methodology could be extended to regioselective protecting group installations and manipulations towards a number of thiosialoside and halide donors.

Synthesis of 2-azido-2-deoxy- and 2-acetamido-2-deoxy-D-manno derivatives as versatile building blocks

Reported herein is the synthesis of a number of building blocks of 2-amino-2-deoxy-d-mannose from common d-glucose precursors.

Chemical synthesis of marine saponins

Covering: 1989-2017 Saponins are characteristic metabolites of starfish and sea cucumbers, and occasionally are also found in sponges, soft coral, and small fish. These steroid or triterpenoid glycosides often show remarkable biological and pharmacological activities, such as antifungal, antifouling, shark repellent, antitumor and anti-inflammatory activities. Over one thousand marine saponins have been characterized; the majority of them can be categorized into three major structural types, i.e., asterosaponins, polyhydroxysteroid glycosides, and holostane glycosides. Thus far, only 12 marine saponins have been synthesized; those representing the major types were successfully synthesized recently. The syntheses involve preparation of the aglycones from the terrestrial steroid or triterpene materials, installation of the carbohydrate units, and manipulation of the protecting groups. Herein, we provide a comprehensive review on these syntheses.

Site-Selective and Stereoselective O-Alkylation of Glycosides by Rh(II)-Catalyzed Carbenoid Insertion

Carbohydrates are synthetically challenging molecules with vital biological roles in all living systems. Selective synthesis and functionalization of carbohydrates provide tremendous opportunities to improve our understanding on the biological functions of this fundamentally important class of molecules. However, selective functionalization of seemingly identical hydroxyl groups in carbohydrates remains a long-standing challenge in chemical synthesis. We herein describe a practical and predictable method for the site-selective and stereoselective alkylation of carbohydrate hydroxyl groups via Rh(II)-catalyzed insertion of metal carbenoid intermediates. This represents one of the mildest alkylation methods for the systematic modification of carbohydrates. Density functional theory (DFT) calculations suggest that the site selectivity is determined in the Rh(II)-carbenoid insertion step, which prefers insertion into hydroxyl groups with an adjacent axial substituent. The subsequent intramolecular enolate protonation determines the unexpected high stereoselectivity. The most prevalent trans-1,2-diols in various pyranoses can be systematically and predictably differentiated based on the model derived from DFT calculations. We also demonstrated that the selective O-alkylation method could significantly improve the efficiency and stereoselectivity of glycosylation reactions. The alkyl groups introduced to carbohydrates by OH insertion reaction can serve as functional groups, protecting groups, and directing groups.

Phosphotungstic acid as a novel acidic catalyst for carbohydrate protection and glycosylation

This work demonstrates the utilization of phosphotungstic acid (PTA) as a novel acidic catalyst for carbohydrate reactions, such as per-O-acetylation, regioselective O-4,6 benzylidene acetal formation, regioselective O-4 ring-opening, and glycosylation. These reactions are basic and salient during the synthesis of carbohydrate-based bioactive oligomers. Phosphotungstic acid's high acidity and eco-friendly character make it a tempting alternative to corrosive homogeneous acids. The various homogenous acid catalysts were replaced by the phosphotungstic acid solely for different carbohydrate reactions. It can be widely used as a catalyst for organic reactions as it is thermally stable and easy to handle. In our work, the reactions are operated smoothly under ambient conditions; the temperature varies from 0 °C to room temperature. Good to excellent yields were obtained in all four kinds of reactions.

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.

Triflic acid-mediated synthesis of thioglycosides

An efficient synthesis of thioglycosides from per-acetates in the presence of triflic acid is described. The developed protocol features high reaction rates and product yields. Some reactive sugar series give high efficiency in the presence of sub-stoichiometric trifluoromethanesulfonic acid (TfOH) in contrast to other known protocols that require multiple equivalents of Lewis acids to reach high conversion rates.

Phenylpropanoid glycosides from the fruit of Lycium barbarum L. and their bioactivity

Fifteen phenylpropanoid glycosides, including six undescribed compounds were isolated from the fruit of Lycium barbarum L. (Solanaceae) (goji or wolfberry). Their structures were identified by detailed spectroscopic analyses. Seven known compounds were firstly isolated from the genus Lycium, in which the 1D and 2D NMR data of one compound were reported for the first time. Notably, two undescribed compounds were a pair of rare tautomeric glycoside anomers characterized by the presence of free anomeric hydroxy. Antioxidant and hypoglycemic activities of all these compounds were assessed using DPPH radical scavenging, oxygen radical absorbance capacity (ORAC), and α-glucosidase inhibitory assays, respectively. These compounds showed different levels of oxygen radical absorbance capacity, and some isolates exhibited potent antioxidant activity with greater ORAC values than the positive control (EGCG).

Synthesis and bioactivity of novel C2-glycosyl benzofuranylthiazoles derivatives as acetylcholinesterase inhibitors

A new series of C2-glycosyl benzofuranylthiazole derivatives was synthesised by the further cyclization of glycosyl thiourea and 2-(bromoacetyl)-benzofuran via Hantzsch’s method. The corresponding 2-(bromoacetyl)-benzofuran derivatives were obtained by the reaction from various salicylaldehydes, and the glycosyl thiourea was prepared through a series of steps from D-Glucosamine. The acetylcholinesterase-inhibitory activities of the products were tested by Ellman’s method. The most active compounds were subsequently evaluated for the 50% inhibitory concentration values. N-(1,3,4,6-tetra-O-benzyl-2-deoxy-β-D-glucopyranosyl)-4-(5-methoxy-benzofuran-2-yl)-1,3-thiazole-2-amine possessed the best acetylcholinesterase-inhibition activity with a 50% inhibitory concentration of 2.03 ± 0.26 μM.

An Efficient Modular One-Pot Synthesis of Heparin-Based Anticoagulant Idraparinux

Idraparinux is a fully O-sulfated α-methyl glycoside of heparin pentasaccharide motif known to interact with the antithrombin III domain and act as anticoagulant. The current most effective synthesis of Idraparinux is complicated and nonstereoselective, requiring numerous stepwise procedures with low yields. We report here an efficient modular one-pot synthesis of Idraparinux involving the use of a glycosyl phosphate with 6- O- tert-butyl diphenyl silyl group and a d-glucuronic acid-containing disaccharide thioglycoside with 6- O-acetyl group as donor building blocks for the α-directing one-pot glycosylations with an l-iduronic acid-containing disaccharide acceptor building block. The uronic acid was incorporated in a disaccharide module used in the one-pot synthesis to avoid the complicated late-stage installation of these acidic sugars. The one-pot synthesis of Idraparinux demonstrated here is an effective strategy and should be applicable to the modular assembly of other heparan sulfates with regiodefined sulfation pattern for functional study.

Stable Benzylic (1-Ethynylcyclohexanyl)carbonates Protect Hydroxyl Moieties by the Synergistic Action of [Au]/[Ag] Catalytic System

Chemical syntheses of oligosaccharides and glycosides call utilization of many protecting groups that can be installed or deprotected without affecting other functional groups present. Benzyl ethers are routinely used in the synthesis of glycans as they can be subjected to hydrogenolysis under neutral conditions. However, installation of benzyl ethers is often carried out under strong basic conditions using benzyl halides. Many a times, strongly basic conditions will be detrimental for some of the other sensitive functionalities (e.g., esters). Later introduced reagents such as benzyl trichloroacetimidate and BnOTf are not shelf-stable, and hence, a new method is highly desirable. Taking a cue from the [Au]/[Ag]-catalyzed glycosidations, we have identified a method that enables protection of hydroxyl groups as benzyl, p-methoxybenzyl, or naphthylenemethyl ethers using easily accessible and stable carbonate reagent. A number of saccharide-derived alcohols were subjected to the benzylation successfully using a catalytic amount of gold phosphite and silver triflate. Furthermore, the protocol is suitable for even protecting menthol, cholesterol, serine, disaccharide OH, and furanosyl-derived alcohol easily. The often-utilized olefins and benzoates, as well as benzylidene-, silyl-, Troc-, and Fmoc-protecting groups do not get affected during the newly identified protocol. Regioselective protection and one-pot installation of benzyl and p-methoxybenzyl ethers are demonstrated.

Synthesis of carbohydrate building blocks via regioselective uniform protection/deprotection strategies

Discussed herein is the synthesis of partially protected carbohydrates by manipulating only one type of a protecting group for a given substrate. The first focus of this review is the uniform protection of an unprotected starting material in a way that only one (or two) hydroxyl group remains unprotected. The second focus involves regioselective partial deprotection of uniformly protected compounds in a way that only one (or two) hydroxyl group becomes liberated.

Use of trichloroacetonitrile as a hydrogen chloride generator for ring-opening reactions of aziridines

Regioselective ring-opening reactions of 2-aryl-N-tosylaziridines are described, in which hydrogen chloride is generated by photodegradation of trichloroacetonitrile. HCl adducts are obtained in high yields in 1,4-dioxane, whereas methanol adducts are predominantly obtained in methanol. Trichloroacetonitrile can serve as a photoresponsive molecular storage generator for hydrogen chloride.