Thioglycosides in sequential glycosylation strategies

Synthesis and pharmacological evaluation of nature-inspired phenacyl glycosides

Phenylethanoid glycosides are a well-studied class of bioactive compounds found throughout the plant kingdom. In contrast, research on the synthesis and pharmacological activity of phenacyl glycosides, a specific subgroup of phenylethanoid glycosides with a ketone functionality at the alpha position of the phenol ring, has been limited. In this study, we report the synthesis, cytotoxic, antiviral, and anti-inflammatory evaluation of a series of 18 4'-hydroxyphenacyl glycosides. These compounds consist of six different sugar residues (β-d-glucose, β-d-galactose, α-l-arabinose, β-d-xylose, α-l-rhamnose, and β-d-glucuronic acid) and display three distinct methoxylation patterns at the phenacyl ring, similar to the substitution motifs of anthocyanins. We obtained the target phenacyl glycosides in high yield and stereoselectivity through the coupling of benzoyl-protected trichloroacetimidate glycosyl donors and corresponding acetophenones. Our work represents the first total synthesis of the natural products 4'-hydroxyphenacyl-β-d-glucopyranoside (1) and 4'-hydroxy-3'-methoxyphenacyl-β-d-glucopyranoside (2). None of the phenacyl glycosides showed cytotoxicity against the tested cell lines. Notably, several of the synthesized compounds exhibited antiviral activity, with natural product 2 being the most active against herpes simplex virus type 1, while phenacyl arabinoside 9 and natural product 2 were the most active against human coronavirus OC43. Natural product 2 significantly inhibited the production of interleukin-6 in lipopolysaccharide-stimulated microglia cells. Overall, our findings highlight the importance of the sugar residue and phenacyl ring substitution pattern in modulating the antiviral activity of phenacyl glycosides. Natural product 2 and phenacyl arabinoside 9 emerge as promising leads for the development of antiviral agents.

Semisynthetic Glycoconjugates as Potential Vaccine Candidates Against Haemophilus influenzae Type a

Glycoconjugate vaccines are based on chemical conjugation of pathogen-associated carbohydrates with immunogenic carrier proteins and are considered a very cost-effective way to prevent infections. Most of the licensed glycoconjugate vaccines are composed of saccharide antigens extracted from bacterial sources. However, synthetic oligosaccharide antigens have become a promising alternative to natural polysaccharides with the advantage of being well-defined structures providing homogeneous conjugates. Haemophilus influenzae (Hi) is responsible for a number of severe diseases. In recent years, an increasing rate of invasive infections caused by Hi serotype a (Hia) raised some concern, because no vaccine targeting Hia is currently available. The capsular polysaccharide (CPS) of Hia is constituted by phosphodiester-linked 4-β-d-glucose-(1→4)-d-ribitol-5-(PO4→) repeating units and is the antigen for protein-conjugated polysaccharide vaccines. To investigate the antigenic potential of the CPS from Hia, we synthesized related saccharide fragments containing up to five repeating units. Following the synthetic optimization of the needed disaccharide building blocks, they were assembled using the phosphoramidite approach for the installation of the phosphodiester linkages. The resulting CPS-based Hia oligomers were conjugated to CRM197 carrier protein and evaluated in vivo for their immunogenic potential, showing that all glycoconjugates were capable of raising antibodies recognizing Hia synthetic fragments.

Donor Preactivation-Based Glycan Assembly: from Manual to Automated Synthesis

Carbohydrates are called the third chain of life. Carbohydrates participate in many important biochemical functions in living species, and the biological information carried by them is several orders of magnitude larger than that of nucleic acids and proteins. However, due to the intrinsic complexity and heterogeneity of carbohydrate structures, furnishing pure and structurally well-defined glycans for functional studies is a formidable task, especially for homogeneous large-size glycans. To address this issue, we have developed a donor preactivation-based one-pot glycosylation strategy enabling multiple sequential glycosylations in a single reaction vessel.The donor preactivation-based one-pot glycosylation refers to the strategy in which the glycosyl donor is activated in the absence of a glycosyl acceptor to generate a reactive intermediate. Subsequently, the glycosyl acceptor with the same anomeric leaving group is added, leading to a glycosyl coupling reaction, which is then iterated to rapidly achieve the desired glycan in the same reactor. The advantages of this strategy include the following: (1) unique chemoselectivity is obtained after preactivation; (2) it is independent of the reactivity of glycosyl donors; (3) multiple-step glycosylations are enabled without the need for intermediate purification; (4) only stoichiometric building blocks are required without complex protecting group manipulations. Using this protocol, a range of glycans including tumor-associated carbohydrate antigens, various glycosaminoglycans, complex N-glycans, and diverse bacterial glycans have been synthesized manually. Gratifyingly, the synthesis of mycobacterial arabinogalactan containing 92 monosaccharide units has been achieved, which created a precedent in the field of polysaccharide synthesis. Recently, the synthesis of a highly branched arabinogalactan from traditional Chinese medicine featuring 140 monosaccharide units has been also accomplished to evaluate its anti-pancreatic-cancer activity. In the spirit of green and sustainable chemistry, this strategy can also be applied to light-driven glycosylation reactions, where either UV or visible light can be used for the activation of glycosyl donors.Automated synthesis is an advanced approach to the construction of complex glycans. Based on the two preactivation modes (general promoter activation mode and light-induced activation mode), a universal and highly efficient automated solution-phase synthesizer was further developed to drive glycan assembly from manual to automated synthesis. Using this synthesizer, a library of oligosaccharides covering various glycoforms and glycosidic linkages was assembled rapidly, either in a general promoter-activation mode or in a light-induced-activation mode. The automated synthesis of a fully protected fondaparinux pentasaccharide was realized on a gram scale. Furthermore, the automated synthesis of large-size polysaccharides was performed, allowing the assembly of arabinans up to an astonishing 1080-mer using the automated multiplicative synthesis strategy, taking glycan synthesis to a new height far beyond the synthesis of nucleic acids (up to 200-mer) and proteins (up to 472-mer).

Direct Synthesis of Glycosyl Chlorides from Thioglycosides

Reported herein is a new method for the direct synthesis of glycosyl chlorides from thioglycosides using sulfuryl chloride at rt. A variety of thioglycosides and thioimidates could be used as substrates. Both acid- and base-sensitive protecting groups were found compatible with these reaction conditions. Preliminary investigation of the reaction mechanism indicates chlorination of the leaving group at the anomeric sulfur as the key step of the reaction.

Clinical Effect of Thioglycosides Extracted from White Mustard on Dental Plaque and Gingivitis: Randomized, Single-Blinded Clinical Trial

The antibacterial and anti-inflammatory effect of thioglycosides has already been established. This study investigates the effects of thioglycosides extracted from white mustard, specifically the "Bamberka" variety, in the context of oral hygiene. The aim of the study is to clarify an evidence-based link between the documented antibacterial and anti-inflammatory effects attributed to thioglycosides and their practical application in oral care. A randomized, single-blinded (patient-blinded) clinical study was performed on 66 patients using mustard-based toothpaste for oral hygiene. The patients were examined at baseline and after 6 and 12 months. The values of the Approximal Plaque Index (API), the Plaque Index (PI), and Bleeding on probing (BOP) were taken into consideration. The results show a significant reduction in plaque accumulation, especially after 6 months of using mustard-based toothpaste in all examined parameters. This suggests that thioglycosides from mustard contribute to a considerable decrease in dental plaque accumulation, confirming their potential in natural oral care solutions, which is indicated in the main conclusions or interpretations.

Electrochemical nickel-catalyzed cross-coupling of glycosyl thiols with preactivated phenols and ketones

An efficient electrochemical nickel-catalyzed cross-coupling reaction has been reported here for the synthesis of S-glycosides from preactivated phenols and ketones under mild conditions. Various glycosyl thiols, including unprotected sugar, and a diverse range of aryl/alkenyl triflates, including some complex biorelevant phenols and ketones, were well tolerated in this method.

An Unexpected Lewis Acid-Catalyzed Cascade during the Synthesis of the DEF-Benzoxocin Ring System of Nogalamycin and Menogaril: Mechanistic Elucidation by Intermediate Trapping Experiments and Density Functional Theory Studies

An unexpected Lewis acid-catalyzed carbohydrate rearrangement of a 1,5-bis-glycopyranoside to the product of a formal intramolecular C-aryl glycosylation reaction is reported. Mechanistic studies based mainly on intermediate trapping experiments and density functional theory (DFT) calculations reveal a cascade process involving three transient (a)cyclic oxocarbenium cations, the breaking of three single C(sp3)-O bonds, and the formation of three single bonds (i.e., exo-, endo-, and C-glycosidic bonds), leading to the 2,6-epoxybenzoxocine skeleton of bioactive natural glycoconjugates related to serjanione A and mimocaesalpin E. DFT calculations established that the generation of the pyran moiety embedded in the bridged benzoxocin ring system is likely to proceed through an unusual ring-closure of an ortho-quinone methide intermediate in which the attacking nucleophile is a carbonyl oxygen.

Efficient O- and S-glycosylation with ortho-2,2-dimethoxycarbonylcyclopropylbenzyl thioglycoside donors by catalytic strain-release

We herein present a strain-release glycosylation method employing a rationally designed ortho-2,2-dimethoxycarbonylcyclopropylbenzyl (CCPB) thioglycoside donor. The donor is activated through the nucleophilic ring-opening of a remotely activable donor-acceptor cyclopropane (DAC) catalyzed by mild Sc(OTf)3. Our new glycosylation method efficiently synthesizes O-, N-, and S-glycosides, providing facile chemical access to the challenging S-glycosides. Because the activation conditions of conventional glycosyl donors and our CCPB thioglycoside are orthogonal, our novel donor is amenable to controlled one-pot glycosylation reactions with conventional donors for expeditious access to complex glycans. The strain-release glycosylation is applied to the assembly of a tetrasaccharide of O-polysaccharide of Escherichia coli O-33 in one pot and the synthesis of a 1,1'-S-linked glycoside oral galectin-3 (Gal-3) inhibitor, TD139, to demonstrate the versatility and effectiveness of the novel method for constructing both O- and S-glycosides.

Use of Novel Homochiral Thioureas Camphor Derived as Asymmetric Organocatalysts in the Stereoselective Formation of Glycosidic Bonds

We synthesized six new camphor-derived homochiral thioureas 1-6, from commercially available (1R)-(-)-camphorquinone. These new compounds 1-6 were evaluated as asymmetric organocatalysts in the stereoselective formation of glycosidic bonds, with 2,3,4,6-tetra-O-benzyl-D-glucopyranosyl and 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl trichloroacetimidates as donors, and several alcohols as glycosyl acceptors, such as methanol, ethanol, 1-propanol, 1-butanol, 1-octanol, iso-propanol, tert-butanol, cyclohexanol, phenol, 1-naphtol, and 2-naphtol. Optimization of the asymmetric glycosylation reaction was achieved by modifying reaction conditions such as solvent, additive, loading of catalyst, temperature, and time of reaction. The best result was obtained with 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl trichloroacetimidates, using 15 mol% of organocatalyst 1, in the presence of 2 equiv of MeOH in solvent-free conditions at room temperature for 1.5 h, affording the glycosidic compound in a 99% yield and 1:73 α:β stereoselectivity; under the same reaction conditions, without using a catalyst, the obtained stereoselectivity was 1:35 α:β. Computational calculations prior to the formation of the products were modeled, using density functional theory, M06-2X/6-31G(d,p) and M06-2X/6-311++G(2d,2p) methods. We observed that the preference for β glycoside formation, through a stereoselective inverted substitution, relies on steric effects and the formation of hydrogen bonds between thiourea 1 and methanol in the complex formed.

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.

Photosensitizer-free visible-light-promoted glycosylation enabled by 2-glycosyloxy tropone donors

Photochemical glycosylation has attracted considerable attention in carbohydrate chemistry. However, to the best of our knowledge, visible-light-promoted glycosylation via photoactive glycosyl donor has not been reported. In the study, we report a photosensitizer-free visible-light-mediated glycosylation approach using a photoactive 2-glycosyloxy tropone as the donor. This glycosylation reaction proceeds at ambient temperature to give a wide range of O-glycosides or oligosaccharides with yields up to 99%. This method is further applied in the stereoselective preparation of various functional glycosyl phosphates/phosphosaccharides, the construction of N-glycosides/nucleosides, and the late-stage glycosylation of natural products or pharmaceuticals on gram scales, and the iterative synthesis of hexasaccharide. The protocol features uncomplicated conditions, operational simplicity, wide substrate scope (58 examples), excellent compatibility with functional groups, scalability of products (7 examples), and high yields. It provides an efficient glycosylation method for accessing O/N-glycosides and glycans.

Benzyne-Promoted, 1,2-cis-Selective O-Glycosylation with Benzylchalcogenoglycoside Donors

Here, we show that the reaction of benzylchalcogenoglycosides with benzyne in the presence of alcohols results in highly 1,2-cis-selective O-glycosylation in a solvent-dependent manner. Thioglycosides, selenoglycosides, and alcohols with a range of nucleophilicities lead to a productive reaction, and unusual protecting groups, auxiliary groups, and additives are avoided.

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.

Cooperatively Catalyzed Activation of Thioglycosides That Bypasses Intermediacy of Glycosyl Halides

Reported herein is the development of a novel method for activating thioglycosides without a glycosyl halide intermediate. This has been achieved through the use of a silver salt coupled with an acid additive and molecular iodine. The enhanced stereocontrol was achieved via the H-bond mediated aglycone delivery (HAD) method, and the extended trisaccharide synthesis was achieved via iteration of deprotection and glycosylation steps.

Chemical synthesis of oligosaccharides and their application in new drug research

Oligosaccharides are the ubiquitous molecules of life. In order to translate human bioglycosylation into clinical applications, homogeneous samples of oligosaccharides and glycoconjugates can be obtained by chemical, enzymatic or other biological methods for systematic studies. However, the structural complexity and diversity of glycans and their conjugates present a major challenge for the synthesis of such molecules. This review summarizes the chemical synthesis methods of oligosaccharides, the application of oligosaccharides in the field of medicinal chemistry according to their related biological activities, and shows the great prospect of oligosaccharides in the field of pharmaceutical chemistry.

Atom-Economic Synthesis of Unsymmetrical gem-Diarylmethylthio/Seleno Glycosides via Base Mediated C(O)-S/Se Bond Cleavage and Acyl Transfer Approach of Glycosylthio/Selenoacetates

Herein, we invented the Cs₂CO₃-mediated atom economic method that streamlines the scission of the C(O)-S/Se bond involving the in situ generation of an anomeric thiolate/selenolate anion, which reacted with p-QMs to yield novel unsymmetrical gem-diarylmethylthio/seleno glycosides while retaining the anomeric stereochemistry. Notably, the key features of this protocol involve unprecedented long-range acyl transfer (from S/Se to O), thus affording acylation of the final product which is not yet reported by classical methods. This straightforward protocol offers a mild, short reaction time, synthetically simple approach, and compatibility with 8 types of sugar along with phenylthio/benzylseleno esters.

Copper(I)-Catalyzed Sandmeyer-Type S-Arylation of 1-Thiosugars with Aryldiazonium Salts under Mild Conditions

Preparation of S-aryl thioglycosides from 1-thiosugars via S-arylation was demonstrated under mild reaction conditions. A wide range of protected and unprotected 1-thiosugars derived from glucose, glucosamine, galactose, mannose, ribose, maltose, and lactose underwent cross-coupling reactions with functionalized aryldiazonium salts in the presence of copper(I) chloride and DBU. The desired products were obtained in 55-88% yields within 5 min. Various functional groups, including halogens, were tolerated under standard reaction conditions. Synthesis of the biologically relevant antidiabetic dapagliflozin S-analogue and arbutin S-analogues (tyrosinase inhibitors) was demonstrated.

Concise Synthesis of Tunicamycin V and Discovery of a Cytostatic DPAGT1 Inhibitor

A short total synthesis of tunicamycin V (1), a non-selective phosphotransferase inhibitor, is achieved via a Büchner-Curtius-Schlotterbeck type reaction. Tunicamycin V can be synthesized in 15 chemical steps from D-galactal with 21 % overall yield. The established synthetic scheme is operationally very simple and flexible to introduce building blocks of interest. The inhibitory activity of one of the designed analogues 28 against human dolichyl-phosphate N-acetylglucosaminephosphotransferase 1 (DPAGT1) is 12.5 times greater than 1. While tunicamycins are cytotoxic molecules with a low selectivity, the novel analogue 28 displays selective cytostatic activity against breast cancer cell lines including a triple-negative breast cancer.

Glycosyl 3-Phenyl-4-pentenoates as Versatile Glycosyl Donors: Reactivity and Their Application in One-Pot Oligosaccharide Assemblies

Both glycoconjugates and oligosaccharides are important biomolecules having significant roles in several biological processes, and a new strategy for their synthesis is crucial. Here, we report a versatile N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf) promoted glycosidation approach with shelf-stable 3-phenyl-4-pentenoate glycosyl as a donor for the efficient synthesis of O/C-glycosides with free alcohols, silylated alcohols, and C-type nucleophile acceptors in good to excellent yields. The mild activation conditions and outstanding reactivity of phenyl substituted pentenoate donors analogous to 4-pentenoate glycosyl donors enhance their applicability to various one-pot strategies for the synthesis of oligosaccharides, such as single-catalyst one-pot and acceptor reactivity-controlled one-pot strategies.

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.

Visible-Light-Mediated Stadler-Ziegler Arylation of Thiosugars with Anilines

Here, we report a one-pot Stadler-Ziegler reaction toward the synthesis of 1-thioglycosides in good yield from commercially available anilines and (un)protected 1-glycosyl thiols. This simple and mild approach employs the photoredox catalyst [Ru(bpy)₃](PF₆)₂ under visible light.

Boron-Promoted Umpolung Reaction of Sulfonyl Chlorides for the Stereospecific Synthesis of Thioglycosides via Reductive Deoxygenation Coupling Reactions

S-Glycosides have broad biological activities and serve as stable mimics of natural O-glycoside counterparts and thus are of great therapeutic potential. Herein we disclose an efficient method for the stereospecific synthesis of 1-thioglycosides via a boron-promoted reductive deoxygenation coupling reaction from readily accessible sulfonyl chlorides and glycosyl bromides. Our protocol features mild conditions and excellent functional group tolerance and stereoselectivity. The translational potential of this metal-free approach is demonstrated by the late-stage glycodiversification of natural products and drug molecules.

Photolabile 2-(2-Nitrophenyl)-propyloxycarbonyl (NPPOC) for Stereoselective Glycosylation and Its Application in Consecutive Assembly of Oligosaccharides

A photolabile protecting group (PPG) 2-(2-nitrophenyl)-propyloxycarbonyl (NPPOC) was explored in glycosylation and applied in the consecutive synthesis of oligosaccharides. NPPOC displays a strong neighboring group participation (NGP) effect to facilitate the construction of 1,2-trans glycosides in excellent yield. Notably, NPPOC could be efficiently removed by photolysis, and the deprotection conditions are friendly to typical protecting groups. A branched and asymmetric oligomannose Man6 was rapidly prepared, and the consecutive assembly of oligosaccharides without intermediate purification was further investigated owing to the compatibility conditions between NPPPOC's photolysis and glycosylation.

Hydrogen bond activated glycosylation under mild conditions

Herein, we report a new glycosylation system for the highly efficient and stereoselective formation of glycosidic bonds using glycosyl N-phenyl trifluoroacetimidate (PTFAI) donors and a charged thiourea hydrogen-bond-donor catalyst. The glycosylation protocol features broad substrate scope, controllable stereoselectivity, good to excellent yields and exceptionally mild catalysis conditions. Benefitting from the mild reaction conditions, this new hydrogen bond-mediated glycosylation system in combination with a hydrogen bond-mediated aglycon delivery system provides a reliable method for the synthesis of challenging phenolic glycosides. In addition, a chemoselective glycosylation procedure was developed using different imidate donors (trichloroacetimidates, N-phenyl trifluoroacetimidates, N-4-nitrophenyl trifluoroacetimidates, benzoxazolyl imidates and 6-nitro-benzothiazolyl imidates) and it was applied for a trisaccharide synthesis through a novel one-pot single catalyst strategy.

A mild glycosylation system was developed using glycosyl imidate donors and a charge-enhanced thiourea H-bond donor catalyst. The method can be used for the effective synthesis of O-, C-, S- and N-glycosides and chemoselective one-pot glycosylation.

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.

Silylated Sugars – Synthesis and Properties

Silicon has been used in carbohydrate chemistry for half a century, but mostly as a protective group for sugar alcohols. Recently, the use of silicon has expanded to functionalization via C–H activation, conformational arming of glycosyl donors, and conformational alteration of carbohydrates. Silicon has proven useful as more than a protective group and during the last one and a half decades we have demonstrated how it influences both the reactivity of glycosyl donors and stereochemical outcome of glycosylations. Silicon can also be attached directly to the sugar C-backbone, which has even more pronounced effects on the chemistry and properties of the molecules. In this Account, we will give a tour through our work involving silicon and carbohydrates.

1 Introduction

2 Conformational Arming of Glycosyl Donors with Silyl Groups

3 Silyl Protective Groups for Tethering Glycosyl Donors

4. Si–C Glycosides via C–H Activation

4.1 C–H Activation and Oxidation of Methyl 6-Deoxy-l-glycosides

4.2 Synthesis of All Eight 6-Deoxy-l-sugars

4.3 Synthesis of All Eight l-Sugars by C–H Activation

4.4 Modification of the Oxasilolane Ring

5 C–Si in Glycosyl Donors – Activating or Not?

6 Si–C-Substituted Pyranosides

7 Perspective

Total Synthesis of a Chimeric Glycolipid Bearing the Partially Acetylated Backbone of Sponge-Derived Agminoside E

We describe the total synthesis of a chimeric glycolipid bearing both the partially acetylated backbone of sponge-derived agminoside E and the (R)-3-hydroxydecanoic acid chain of bacterial rhamnolipids. The branched pentaglucolipid skeleton was achieved using a [3 + 2] disconnection approach. The β-(1 → 2) and β-(1 → 4)-glycosidic bonds were synthesized through a combination of NIS/Yb(OTf)₃- and TMSOTf-mediated stereoselective glycosylations of thiotolyl, N-phenyltrifluoroacetimidate, and trichloroacetimidate donors. Late-stage pentaacetylation, Staudinger reduction of a (2-azidomethyl)benzoyl group, followed by continuous-flow microfluidic hydrogenolysis completed the total synthesis of the structurally simplified glycolipid, whose partial acetylation pattern on the glycan part was identical to agminoside E. Our study lays the foundation for the total synthesis of sponge-derived agminosides and the understanding of their biological functions in sponges.

Leveraging Trifluoromethylated Benzyl Groups toward the Highly 1,2-Cis-Selective Glucosylation of Reactive Alcohols

Here, we demonstrate that substitution of the benzyl groups of glucosyl imidate donors with trifluoromethyl results in a substantial increase in 1,2-cis-selectivity when activated with TMS-I in the presence of triphenylphosphine oxide. Stereoselectivity is dependent on the number of trifluoromethyl groups (4-trifluoromethylbenzyl vs 3,5-bis-trifluoromethylbenzyl). Particularly encouraging is that we observe high 1,2-cis-selectivity with reactive alcohol acceptors.

Selective demethylation of O-aryl glycosides by iridium-catalyzed hydrosilylation

The cleavage of alkyl ethers by hydrosilylation is a powerful synthetic tool for the generation of silyl ethers. Previous attempts to apply this transformation to carbohydrate derivatives have been constrained by poor selectivity and preferential reduction of the anomeric position. O-Aryl glycosides are found to be stable under iridium- and borane-catalyzed hydrosilylation conditions, allowing for alkyl ether cleavage without loss of anomeric functionality. A cationic bis(phosphine)iridium complex catalyzes the selective 3-demethylation of a variety of 2,3,4-tri-O-methyl pyranoses, offering a unique approach to 3-hydroxy or 3-acetyl 2,4-di-O-methylpyranoses.

Total synthesis of tricolorin A via interrupted Pummerer reaction-mediated glycosylation and one-pot relay glycosylation

Tricolorin A, a bioactive resin glycoside, was synthesized stepwise or in one pot based on interrupted Pummerer reaction-mediated (IPRm) glycosylation. The stepwise synthesis adopted a [2 + 2] assembly sequence, and all of the glycosidic bonds were constructed efficiently by IPRm glycosylation. The one-pot synthesis employed our recently developed one-pot relay glycosylation strategy, in which two different glycosidic bonds were sequentially connected with only one equivalent of external activator.

NIS/TMSOTf-Promoted Glycosidation of Glycosyl ortho-Hexynylbenzoates for Versatile Synthesis of O-Glycosides and Nucleosides

Glycosidation plays a pivotal role in the synthesis of O-glycosides and nucleosides that mediate a diverse range of biological processes. However, efficient glycosidation approach for the synthesis of both O-glycosides and nucleosides remains challenging in terms of glycosidation yields, mild reaction conditions, readily available glycosyl donors, and cheap promoters. Here, we report a versatile N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf)-promoted glycosidation approach with glycosyl ortho-hexynylbenzoates as donors for the highly efficient synthesis of O-glycosides and nucleosides. The glycosidation approach highlights the merits of mild reaction conditions, cheap promoters, extremely wide substrate scope, and good to excellent yields. Notably, the glycosidation approach performs very well in the construction of a series of challenging O- and N-glycosidic linkages. The glycosidation approach is then applied to the efficient synthesis of oligosaccharides via the one-pot strategy and the stepwise strategy. On the basis of the isolation and characterization of the departure species derived from the leaving group, a plausible mechanism of NIS/TMSOTf-promoted glycosidation of glycosyl ortho-hexynylbenzoates is proposed.

Glycosylation by Alkyne Activation of the 2-O-Substituted Propargyl Group in a β-Phenylthioglucoside with a 5 S 1 Conformation

Generally, glycosylation reactions activate an anomeric substituent in a glycosyl donor to generate an oxocarbenium ion intermediate. Here we report a novel glycosylation reaction triggered by the activation of a 2-O-substituted propargyl group in a 3,6-O-1,1′-[(ethane-1,2-diyl)bibenzene-2,2′-bis(methylene)]-β-thioglucoside. This reaction proceeds through a cationic Au(I)-mediated intramolecular migration of the anomeric substituent onto the alkyne moiety of the propargyl group, followed by α-attack by the hydroxy group in the glycosyl acceptor on the oxocarbenium ion. The migration of the anomeric group occurs selectively through a 6-exo-dig pathway. The 2-(phenylsulfanyl)prop-2-en-1-yl group produced during the glycosylation is removable under conditions similar to those used for removing an allyl group. This reaction will be developed for further applications in orthogonal oligosaccharide synthesis.

Sequential activation of thioglycosides enables one-pot glycosylation

This review describes recent developments in relative reactivity value (RRV) controlled sequential glycosylation, pre-activation based iterative glycosylation, and sulfoxide activation initiated one-pot glycosylation.

Indolylthio Glycosides As Effective Building Blocks for Chemical Glycosylation

The S-indolyl (SIn) anomeric moiety was investigated as a new leaving group that can be activated for chemical glycosylation under a variety of conditions including thiophilic and metal-assisted pathways. Understanding of the reaction pathways for the SIn moiety activation was achieved via the extended mechanistic study. Also reported is how the new SIn donors fit into selective activation strategies for oligosaccharide synthesis.

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

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

A Modular Chemoenzymatic Synthesis of Disialosyl Globopentaosylceramide (DSGb5Cer) Glycan

The total synthesis of the oligosaccharide moiety of disialosyl globopentaosylceramide (DSGb5 Cer), a dominant ganglioside isolated from malignant renal cell carcinoma tissues, is reported. The synthetic strategy relies on a chemical α(2,6)-sialylation at the internal GalNAc unit of a Gb5 pentasaccharide backbone that furnishes a Neu5Acα(2,6)GalNAc-linked hexasaccharide, suitable for an enzymatic α(2,3)-sialylation of the terminal Gal residue to construct a heptasaccharide glycan. Convergent access to this key α(2,6)-sialylated hexasaccharide was also achieved through a [3+3] glycosylation building upon a Galβ(1,3)[Neu5Acα(2,6)]GalNAc-based trisaccharide donor and a Gb3 acceptor. The synthetic DSGb5 glycan bearing a 6-azidohexyl aglycon at the reducing end could undergo further regioselective functionalization. This approach represents a viable chemoenzymatic method for accessing complex ganglioside glycans and should be useful for the synthesis and biological investigation of DSGb5 derivatives.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.