Exploring the Mechanism of Neighboring Group Assisted Glycosylation Reactions

Neighboring-Group Participation by a Less Electron-Donating, Participating C-2-Ester Ensures Higher 1,2-trans Stereoselectivity in Nucleophilic Substitution Reactions of Furanosyl Acetals

Nucleophilic substitution reactions of C-2-acyloxy furanosyl acetals can be highly diastereoselective. We here show that the presence of a less electron-donating p-nitrobenzoyloxy group at C-2 of a furanosyl acetal can be of use to control the 1,2-trans stereoselectivity of acetal substitution reactions with higher stereoselectivity than the analogue with the more electron-donating benzoyloxy group, just as what was observed in the pyranosyl system. Computational results support a reaction manifold involving both open oxocarbenium ions and cis-dioxolenium ions to provide the 1,2-cis and 1,2-trans products. Participation by the less electron-donating C-2-(p-nitrobenzoyloxy) group forms a less stabilized cis-dioxolenium ion that reacts with the incoming nucleophile more readily to provide 1,2-trans products. The relative stability of the furanosyl cis-dioxolenium ion versus the open oxocarbenium ion is much higher than the pyranosyl system as a result of the lower energy penalty for forming the cis-fused [5,5]-bicyclic dioxolenium ion.

Investigation of Neighboring Group Participation in 3,4-Diacetylated Glycosyl Donors in the Gas Phase

A key challenge in oligosaccharide synthesis is the stereoselective installation of glycosidic bonds. Each glycosidic linkage has one of two possible stereo-chemical geometries, α/β or 1,2-cis/trans. An established approach to install 1,2-trans glycosidic bonds is neighboring group participation (NGP), mediated by a 2-O-acyl group. Extension of this intramolecular stabilization to nucleophilic groups located at more remote positions has also been suggested, but remains poorly understood. Previously, we employed infrared ion spectroscopy to characterize the molecular ions of monoacetylated sugar donors and showed how the strength of the stabilizing effect depends on the position of the participating ester group on the glycosyl donor ring as well as on its relative stereochemistry. In this work, we investigated glycosyl donors carrying two acyl groups. Using isotope labelling and isomer population analysis we were able to resolving spectra of isomeric mixtures and establish the relative contribution of individual species. We conclude that 3,4-diacetyl mannosyl donors exclusively form a dioxanium ion as a result of C-3 acyl stabilization. In contrast, the glucosyl and galactosyl cations form mixtures of C-3 and C-4 acyl participation products. Hence, the combination of isotope labeling and population analysis allows for the study of increasingly complex glycosyl cations.

Synthesis of propargyl glycosides of Streptococcus pneumoniae serotypes 6A and 6B for glycoconjugate vaccines

We developed a method for making immune responses to bacterial glycans T cell-dependent, which involves attachment of short, synthetic glycans to a virus-like nanoparticle (VLP). This strategy enhances immune responses to glycans by facilitating cognate T cell help of B cells, leading to antibody class switching and affinity maturation yielding high-affinity, anti-glycan antibodies. This method requires synthesis of bacterial glycans as propargyl glycosides for covalent attachment to VLPs, and the resulting short linker between the VLP and glycan is important for optimal T cell receptor recognition. In this work, glycans that are part of the capsular polysaccharides (CPS) produced by Streptococcus pneumoniae serotypes Sp6A and Sp6B were synthesized as disaccharides and trisaccharides. The optimal glycan epitope for antibody binding to the CPS from these serotypes is unknown, and differing "frames" of disaccharides and trisaccharides were prepared to elucidate the optimal antigen for antibody binding.

Mild and General Protocol for Selective Deacetylation of Acetyl/Benzoyl-Protected Carbohydrates

Herein, we report a mild and general protocol for chemoselective deacetylation of mixed acetyl- and benzoyl-protected carbohydrates under mild acidic conditions. The protocol allows quick access to partially protected carbohydrates, which serve as versatile synthetic intermediates during the total synthesis of various mono- and oligosaccharide targets. The applicability of the developed protocol was successfully demonstrated on a range of carbohydrate substrates of various configurations and substitution patterns featuring functionalized aliphatic and aromatic aglycones. The protocol has shown excellent compatibility with the widely used O-anomeric protecting groups, prespacer aglycones, and thioglycoside glycosyl donors.

Design and Synthesis of Thiourea-Conjugating Organic Arsenic D-Glucose with Anticancer Activities

Organic arsenic compounds such as p-aminophenylarsine oxide (p-APAO) are easier for structural optimization to improve drug-like properties such as pharmacokinetic properties, therapeutic efficacy, and target selectivity. In order to strengthen the selectivity of 4-(1,3,2-dithiarsinan-2-yl) aniline 7 to tumor cell, a thiourea moiety was used to strengthen the anticancer activity. To avoid forming a mixture of α/β anomers, the strategy of 2-acetyl's neighboring group participation was used to lock the configuration of 2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanate from 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl bromide. 1-(4-(1,3,2-dithiarsinan-2-yl) aniline)-2-N-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranos-1-yl)-thiourea 2 can increase the selectivity of human colon cancer cells HCT-116 (0.82 ± 0.06 μM vs. 1.82 ± 0.07 μM) to human embryonic kidney 293T cells (1.38 ± 0.01 μM vs. 1.22 ± 0.06 μM) from 0.67 to 1.68, suggesting a feasible approach to improve the therapeutic index of arsenic-containing compounds as chemotherapeutic agents.

Anomeric Triflates versus Dioxanium Ions: Different Product-Forming Intermediates from 3-Acyl Benzylidene Mannosyl and Glucosyl Donors

Minimal structural differences in the structure of glycosyl donors can have a tremendous impact on their reactivity and the stereochemical outcome of their glycosylation reactions. Here, we used a combination of systematic glycosylation reactions, the characterization of potential reactive intermediates, and in-depth computational studies to study the disparate behavior of glycosylation systems involving benzylidene glucosyl and mannosyl donors. While these systems have been studied extensively, no satisfactory explanations are available for the differences observed between the 3-O-benzyl/benzoyl mannose and glucose donor systems. The potential energy surfaces of the different reaction pathways available for these donors provide an explanation for the contrasting behavior of seemingly very similar systems. Evidence has been provided for the intermediacy of benzylidene mannosyl 1,3-dioxanium ions, while the formation of the analogous 1,3-glucosyl dioxanium ions is thwarted by a prohibitively strong flagpole interaction of the C-2-O-benzyl group with the C-5 proton in moving toward the transition state, in which the glucose ring adopts a B2,5-conformation. This study provides an explanation for the intermediacy of 1,3-dioxanium ions in the mannosyl system and an answer to why these do not form from analogous glucosyl donors.

An environmentally benign protocol for the synthesis of sugar 1,2-orthoesters in poly(ethylene glycol) dimethyl ether (DMPE)

An environmentally benign procedure has been developed for the synthesis of sugar orthoesters using anhydrous sodium acetate in poly (ethylene glycol)dimethyl ether (DMPE). Various sugar orthoesers were prepared without using volatile organic solvent and quaternary ammonium salt. The sugar orthoesters were obtained in good to excellent yields.

Neighboring-Group Participation by C-2 Acyloxy Groups: Influence of the Nucleophile and Acyl Group on the Stereochemical Outcome of Acetal Substitution Reactions

A single acyloxy group at C-2 can control the outcome of nucleophilic substitution reactions of pyran-derived acetals, but the extent of the neighboring-group participation depends on a number of factors. We show here that neighboring-group participation does not necessarily control the stereochemical outcome of acetal substitution reactions with weak nucleophiles. The 1,2-trans selectivity increased with increasing reactivity of the incoming nucleophile. This trend suggests the intermediacy of both cis-fused dioxolenium ions and oxocarbenium ions in the stereochemistry-determining step. In addition, as the electron-donating ability of the neighboring group decreased, the preference for the 1,2-trans products increased. Computational studies show how the barriers for the ring-opening reaction on the dioxolenium ions and the transition states to provide the oxocarbenium ions change with the electron-donating capacity of the C-2-acyloxy group and the reactivity of the nucleophile.

Chemical synthesis of oligosaccharide derivatives with partial structure of β1-3/1-6 glucan, using monomeric units for the formation of β1-3 and β1-6 glucosidic linkages

β1-3/1-6 Glucans, known for their diverse structures, comprise a β1-3-linked main chain and β1-6-linked short branches. Laminarin, a β1-3/1-6 glucan extracted from brown seaweed, for instance, includes β1-6 linkages even in the main chain. The diverse structures provide various beneficial functions for the glucan. To investigate the relationship between structure and functionality, and to enable the characterization of β1-3/1-6 glucan-metabolizing enzymes, oligosaccharides containing the exact structures of β1-3/1-6 glucans are required. We synthesized the monomeric units for the synthesis of β1-3/1-6 mixed-linked glucooligosaccharides. 2-(Trimethylsilyl)ethyl 2-O-benzoyl-4,6-O-benzylidene-β-d-glucopyranoside served as an acceptor in the formation of β1-3 linkages. Phenyl 2-O-benzoyl-4,6-O-benzylidene-3-O-(tert-butyldiphenylsilyl)-1-thio-β-d-glucopyranoside and phenyl 2,3-di-O-benzoyl-4,6-di-O-levulinyl-1-thio-β-d-glucopyranoside acted as donors, synthesizing acceptors suitable for the formation of β1-3- and β1-6-linkages, respectively. These were used to synthesize a derivative of Glcβ1-6Glcβ1-3Glcβ1-3Glc, demonstrating that the proposed route can be applied to synthesize the main chain of β-glucan, with the inclusion of both β1-3 and β1-6 linkages.

Archaeal Glycerolipids Are Recognized by C-Type Lectin Receptor Mincle

Recently, various metabolites derived from host microbes have been reported to modulate the immune system, with potential involvement in health or diseases. Archaea, prokaryotic organisms, are present in the human body, but their connection with the host is largely unknown when compared to other microorganisms such as bacteria. This study focused on unique glycerolipids from symbiotic methanogenic archaea and evaluated their activities toward an innate immune receptor. The results revealed that archaeal lipids were recognized by the C-type lectin receptor Mincle and induced immune responses. A concurrent structure-activity relationship study identified the key structural features of archaeal lipids required for recognition by Mincle. Subsequent gene expression profiling suggested qualitative differences between the symbiotic archaeal lipid and the pathogenic bacteria-derived lipid. These findings have broad implications for understanding the function of symbiotic archaea in host health and diseases.

Mutual leveraging of proximity effects and click chemistry in chemical biology

Nature has the remarkable ability to realize reactions under physiological conditions that normally would require high temperature and other forcing conditions. In doing so, often proximity effects such as simultaneous binding of two reactants in the same pocket and/or strategic positioning of catalytic functional groups are used as ways to achieve otherwise kinetically challenging reactions. Though true biomimicry is challenging, there have been many beautiful examples of how to leverage proximity effects in realizing reactions that otherwise would not readily happen under near-physiological conditions. Along this line, click chemistry is often used to endow proximity effects, and proximity effects are also used to further leverage the facile and bioorthogonal nature of click chemistry. This review brings otherwise seemingly unrelated topics in chemical biology and drug discovery under one unifying theme of mutual leveraging of proximity effects and click chemistry and aims to critically analyze the biomimicry use of such leveraging effects as powerful approaches in chemical biology and drug discovery. We hope that this review demonstrates the power of employing mutual leveraging proximity effects and click chemistry and inspires the development of new strategies that will address unmet needs in chemistry and biology.

Convergent chemoenzymatic synthesis of O-GalNAc rare cores 5, 7, 8 and their sialylated forms

All O-GalNAc glycans are derived from 8 cores with 2 or 3 monosaccharides linked via α- or β-glycosidic bonds. While chemical and chemoenzymatic syntheses of β-linked cores 1-4 and 6 and derived glycans have been well developed, the preparation of α-linked rare cores 5, 7, and 8 is challenging due to the presence of this 1,2-cis linkage. Meanwhile, the biosynthesis and functional roles of these structures are poorly understood. Herein, we synthesize 3 α-linked rare cores with exclusive α-configuration from a versatile precursor through multifaceted chemical modulations. Efficient regioselective α2-6sialylion of the rare cores was then achieved by Photobacterium damselae α2-6sialyltransferase-catalyzed reactions. These structures, together with β-linked cores 1-4 and 6, and their sialylated forms, were fabricated into a comprehensive O-GalNAc core microarray to profile the binding of clinically important GalNAc-specific lectins. It is found that only Tn, (sialyl-)core 5, and core 7 are the binders of WFL, VVL, and SBA, while DBA only recognized (sialyl-)core 5, and Jacalin is the only lectin that binds core 8. In addition, activity assays of human α-N-acetylgalactosaminide α2-6sialyltransferases (ST6GalNAcTs) towards the cores suggested that ST6GalNAc1 may be involved in the biosynthesis of previously identified sialyl-core 5 and sialyl-core 8 glycans. In conclusion, we provide efficient routes to access α-linked O-GalNAc rare cores and derived structures, which are valuable tools for functional glycomics studies of mucin O-glycans.

Targeted delivery of Nitric Oxide triggered by α-Glucosidase to Ameliorate NSAIDs-induced Enteropathy

Nonsteroidal anti-inflammatory drugs (NSAIDs) increase risks of severe small intestinal injuries. Development of effective therapeutic strategies to overcome this issue remains challenging. Nitric oxide (NO) as a gaseous mediator plays a protective role in small intestinal injuries. However, small intestine-specific delivery systems for NO have not been reported yet. In this study, we reported a small intestine-targeted polymeric NO donor (CS-NO) which was synthesized by covalent grafting of α-glucosidase-activated NO donor onto chitosan. In vitro and in vivo experiments demonstrated that CS-NO could be activated by intestinal α-glucosidase to release NO in the small intestine. Pre-treatment of mice with CS-NO significantly alleviated small intestinal damage induced by indomethacin, as demonstrated by down-regulation of the levels of pro-inflammatory cytokines and chemokines CXCL1/KC. Moreover, CS-NO also attenuated indomethacin-induced gut barrier dysfunction as evidenced by up-regulation of the levels of tight junction proteins and restoration of the levels of goblet cells and MUC2 production. Meanwhile, CS-NO effectively restored the defense function of Paneth cells against pathogens in small intestine. Our present study paves the way to develop NO-based therapeutic strategy for NSAIDs-induced small intestinal injuries.

Competing C-4 and C-5-Acyl Stabilization of Uronic Acid Glycosyl Cations

Uronic acids are carbohydrates carrying a terminal carboxylic acid and have a unique reactivity in stereoselective glycosylation reactions. Herein, the competing intramolecular stabilization of uronic acid cations by the C-5 carboxylic acid or the C-4 acetyl group was studied with infrared ion spectroscopy (IRIS). IRIS reveals that a mixture of bridged ions is formed, in which the mixture is driven towards the C-1,C-5 dioxolanium ion when the C-5,C-2-relationship is cis, and towards the formation of the C-1,C-4 dioxepanium ion when this relation is trans. Isomer-population analysis and interconversion barrier computations show that the two bridged structures are not in dynamic equilibrium and that their ratio parallels the density functional theory computed stability of the structures. These studies reveal how the intrinsic interplay of the different functional groups influences the formation of the different regioisomeric products.

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

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

Design, Synthesis, and Evaluation of Lung-Retentive Prodrugs for Extending the Lung Tissue Retention of Inhaled Drugs

A major limitation of pulmonary delivery is that drugs can exhibit suboptimal pharmacokinetic profiles resulting from rapid elimination from the pulmonary tissue. This can lead to systemic side effects and a short duration of action. A series of dibasic dipeptides attached to the poorly lung-retentive muscarinic M3 receptor antagonist piperidin-4-yl 2-hydroxy-2,2-diphenylacetate (1) through a pH-sensitive-linking group have been evaluated. Extensive optimization resulted in 1-(((R)-2-((S)-2,6-diaminohexanamido)-3,3-dimethylbutanoyl)oxy)ethyl 4-(2-hydroxy-2,2-diphenylacetoxy)piperidine-1-carboxylate (23), which combined very good in vitro stability and very high rat lung binding. Compound 23 progressed to pharmacokinetic studies in rats, where, at 24 h post dosing in the rat lung, the total lung concentration of 23 was 31.2 μM. In addition, high levels of liberated drug 1 were still detected locally, demonstrating the benefit of this novel prodrug approach for increasing the apparent pharmacokinetic half-life of drugs in the lungs following pulmonary dosing.

Recent Advances in Stereoselective Chemical O-Glycosylation Reactions

Carbohydrates involving glycoconjugates play a pivotal role in many life processes. Better understanding toward glycobiological events including the structure–function relationship of these biomolecules and for diagnostic and therapeutic purposes including tailor-made vaccine development and synthesis of structurally well-defined oligosaccharides (OS) become important. Efficient chemical glycosylation in high yield and stereoselectivity is however challenging and depends on the fine tuning of a protection profile to get matching glycosyl donor–acceptor reactivity along with proper use of other important external factors like catalyst, solvent, temperature, activator, and additive. So far, many glycosylation methods have been reported including several reviews also. In the present review, we will concentrate our discussion on the recent trend on α- and β-selective glycosylation reactions reported during the past decade.

Diastereoselective Substitution Reactions of Acyclic β-Alkoxy Acetals via Electrostatically Stabilized Oxocarbenium Ion Intermediates

Substitution reactions of acyclic β-alkoxy acetals proceeded with generally high diastereoselectivities (>90:10) to form the anti product. Mechanistic experiments supplemented with computational studies suggest that, upon activation of the acetal, the resulting oxocarbenium ion is electrostatically stabilized by the β-alkoxy group. This stabilization defines the conformation of the reactive intermediate, which can be attacked preferentially from the more exposed face, leading to the observed products.

Foldable Detergents for Membrane Protein Study: Importance of Detergent Core Flexibility in Protein Stabilization

Membrane proteins are of biological and pharmaceutical significance. However, their structural study is extremely challenging mainly due to the fact that only a small number of chemical tools are suitable for stabilizing membrane proteins in solution. Detergents are widely used in membrane protein study, but conventional detergents are generally poor at stabilizing challenging membrane proteins such as G protein-coupled receptors and protein complexes. In the current study, we prepared tandem triazine-based maltosides (TZMs) with two amphiphilic triazine units connected by different diamine linkers, hydrazine (TZM-Hs) and 1,2-ethylenediamine (TZM-Es). These TZMs were consistently superior to a gold standard detergent (DDM) in terms of stabilizing a few membrane proteins. In addition, the TZM-Es containing a long linker showed more general protein stabilization efficacy with multiple membrane proteins than the TZM-Hs containing a short linker. This result indicates that introduction of the flexible1,2-ethylenediamine linker between two rigid triazine rings enables the TZM-Es to fold into favourable conformations in order to promote membrane protein stability. The novel concept of detergent foldability introduced in the current study has potential in rational detergent design and membrane protein applications.

ROESY and 13C NMR to distinguish between D- and L-rhamnose in the α-D-Manp-(1 → 4)-β-Rhap-(1 → 3) repeating motif

Many children suffering from autism spectrum disorder (ASD) experience gastrointestinal (GI) conditions. Enterocloster bolteae has been regularly detected in the stool of individuals suffering from GI symptoms and autism. Literature has suggested that E. bolteae strains WAL 16351 and WAL 14578 produce an immunogenic capsular polysaccharide (CPS) comprised of disaccharide repeating units: α-D-Man-(1 → 4)-β-Rha-(1 → 3) that could be used for the development of an immunotherapeutic vaccine. Ambiguity in the configuration of rhamnose led to the synthesis of tri- and disaccharide analogues containing D-rhamnose and L-rhamnose, respectively. ROESY-NMR spectra showed that CH₃-6 of rhamnose and H-2 of mannose in the L-Rha containing disaccharide gave correlation. No such correlation was seen between the CH₃-6 of rhamnose and the H-2 of mannose in the D-Rha containing trisaccharide. Molecular dynamics studies on hexasaccharide containing L-Rha or D-Rha confirmed that these structures adopt conformations resulting in different distances between the C6-rhamnose and the H-2 mannose of the preceding residue. We also demonstrate that assignment of the absolute configuration of the rhamnosyl residue in the β-Rhap-(1 → 3)-D-Man linkage can be determined using the ¹³C chemical shift of C-2 in of D-Mannose. While β-D-Rha will lead to an upfield shift of C-2 due to γ-gauche interaction between H-1 Rha and H-2 Man, β-L-Rha will not. Our results provide insights to distinguish between D- and L-rhamnose in the α-D-Manp-(1 → 4)-β-Rhap-(1 → 3) repeating motif.

Small tools for sweet challenges: advances in microfluidic technologies for glycan synthesis

Glycans, including oligosaccharides and glycoconjugates, play an integral role in modulating the biological functions of macromolecules. Many physiological and pathological processes are mediated by interactions between glycans, which has led to the use of glycans as biosensors for pathogen and biomarker detection. Elucidating the relationship between glycan structure and biological function is critical for advancing our understanding of the impact glycans have on human health and disease and for expanding the repertoire of glycans available for bioanalysis, especially for diagnostics. Such efforts have been limited by the difficulty in obtaining sufficient quantities of homogenous glycan samples needed to resolve the exact relationships between glycan structure and their structural or modulatory functions on a given glycoconjugate. Synthetic strategies offer a viable route for overcoming these technical hurdles. In recent years, microfluidics have emerged as powerful tools for realizing high-throughput and reproducible syntheses of homogenous glycans for the potential use in functional studies. This critical review provides readers with an overview of the microfluidic technologies that have been developed for chemical and enzymatic glycan synthesis. The advantages and limitations associated with using microreactor platforms to improve the scalability, productivity, and selectivity of glycosylation reactions will be discussed, as well as suggested future work that can address certain pitfalls.

Preparation of Tea Aroma Precursor Glycosides: An Efficient and Sustainable Approach via Chemical Glycosidation

Tea aroma precursor glycosides are plant-derived natural products with great economic value. However, the preparation of these glycosides remains largely overlooked in the past decades. Herein, we report a mild, efficient, and sustainable chemocatalytic procedure for the production of tea aroma precursor glycosides. During the study of the glycosidation, the catalysts were found to be decisive in the product formation favoring different reaction pathways; in addition, the influence of molecular sieves was elucidated. With regard to these findings, the serious problem of the competing orthoester formation side reaction was successfully overcome with low catalyst loading (1 mol %) and the use of 5 Å molecular sieves, leading to the preparation of a variety of tea aroma precursor β-d-glucopyranosides and β-primeverosides on a gram scale in high yields in an economical way. Taken together, the current approach features catalytic glycosidation with non-toxic and low-cost catalysts, demonstrates highly favorable greenness and sustainability, and promises industrial production of tea aroma precursor glycosides.

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.

Diastereomeric Cyclopentane-Based Maltosides (CPMs) as Tools for Membrane Protein Study

Amphiphilic agents, called detergents, are invaluable tools for studying membrane proteins. However, membrane proteins encapsulated by conventional head-to-tail detergents tend to denature or aggregate, necessitating the development of structurally distinct molecules with improved efficacy. Here, a novel class of diastereomeric detergents with a cyclopentane core unit, designated cyclopentane-based maltosides (CPMs), were prepared and evaluated for their ability to solubilize and stabilize several model membrane proteins. A couple of CPMs displayed enhanced behavior compared with the benchmark conventional detergent, n-dodecyl-β-d-maltoside (DDM), for all the tested membrane proteins including two G-protein-coupled receptors (GPCRs). Furthermore, CPM-C12 was notable for its ability to confer enhanced membrane protein stability compared with the previously developed conformationally rigid NBMs [J. Am. Chem. Soc. 2017, 139, 3072] and LMNG. The effect of the individual CPMs on protein stability varied depending on both the detergent configuration (cis/trans) and alkyl chain length, allowing us draw conclusions on the detergent structure-property-efficacy relationship. Thus, this study not only provides novel detergent tools useful for membrane protein research but also reports on structural features of the detergents critical for detergent efficacy in stabilizing membrane proteins.

Advances in the Chemical Synthesis of Carbohydrates and Glycoconjugates

Carbohydrates are functional and structural biomolecules with structures ranging from monosaccharides to polysaccharides. They are naturally found as pure glycans or attached to lipids and proteins forming glycoconjugates. The biosynthesis of carbohydrates is not genetically controlled. The regulation takes place by the expression of enzymes that transfer and hydrolyze the glycan units, leading to glycocojugates having complex mixtures of glycan structures. Chemical synthesis emerged as the best strategy to obtain defined glycan and glycoconjugates and overcome the challenging purification processes. Here, we review the recent advances in the synthesis of oligosaccharides using manual and automated methods. The chapter covers the methods for the preparation of building blocks and control of stereoselectivity and regioselectivity during glycosylations. Finally, it also presents the strategies to obtain natural and non-natural glycoconjugates with lipids and proteins.

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

This review is the counterpart of a 2018 Chemical Reviews article (Adero, P. O.; Amarasekara, H.; Wen, P.; Bohé, L.; Crich, D. Chem. Rev. 2018, 118, 8242-8284) that examined the mechanisms of chemical glycosylation in the absence of stereodirecting participation. Attention is now turned to a critical review of the evidence in support of stereodirecting participation in glycosylation reactions by esters from either the vicinal or more remote positions. As participation by esters is often accompanied by ester migration, the mechanism(s) of migration are also reviewed. Esters are central to the entire review, which accordingly opens with an overview of their structure and their influence on the conformations of six-membered rings. Next the structure and relative energetics of dioxacarbeniun ions are covered with emphasis on the influence of ring size. The existing kinetic evidence for participation is then presented followed by an overview of the various intermediates either isolated or characterized spectroscopically. The evidence supporting participation from remote or distal positions is critically examined, and alternative hypotheses for the stereodirecting effect of such esters are presented. The mechanisms of ester migration are first examined from the perspective of glycosylation reactions and then more broadly in the context of partially acylated polyols.

Chiral auxiliaries: Usefullness in stereoselective glycosylation reactions and their synthetic applications

Oligosaccharides play a very important role in biological system and structure-activity relationships that is why it has a lot of application to medicinal chemistry and development of polysaccharide conjugate vaccines. The stereoselective introduction of a glycosidic linkage presents the principal challenge for biological importance oligosaccharide synthesis. The main aim of this review is to described the importance of chiral auxiliary and neibhouring group participation for the stereoselective 1,2-cis glycosidic bonds formation and their application in complex oligosaccharide synthesis.Numerous 1,2-cis-linked oligosaccharides and glyconjugates are naturally found in the compounds of blood group, human milk, antigens of bacterial lipopolysaccharide etc.that predominantly increased it's importance in this field.

Using Neighboring-Group Participation for Acyclic Stereocontrol in Diastereoselective Substitution Reactions of Acetals

Neighboring-group participation of an ester enabled stereocontrol in substitution reactions of acyclic acetals. The ester group formed a trans-fused dioxolenium ion intermediate, which underwent a substitution reaction at the acetal carbon atom to afford the product with high diastereoselectivity. Neighboring-group participation was confirmed by isolating dioxolane products resulting from nucleophilic addition at C-2 of a 1,3-dioxolenium ion intermediate. Using a pivaloate ester as the participating group in combination with strong nucleophiles produced substitution products with diastereoselectivities of ≥90:10.

Total synthesis of tumor-associated KH-1 antigen core nonasaccharideviaphoto-induced glycosylation

KH-1 antigen core nonasaccharide was efficiently assembled by photo-induced glycosylation.

Synthesis and Biological Evaluation of the Novel Growth Inhibitor Streptol Glucoside, Isolated from an Obligate Plant Symbiont

The plant Psychotria kirkii hosts an obligatory bacterial symbiont, Candidatus Burkholderia kirkii, in nodules on their leaves. Recently, a glucosylated derivative of (+)-streptol, (+)-streptol glucoside, was isolated from the nodulated leaves and was found to possess a plant growth inhibitory activity. To establish a structure-activity relationship study, a convergent strategy was developed to obtain several pseudosugars from a single synthetic precursor. Furthermore, the glucosylation of streptol was investigated in detail and conditions affording specifically the α or β glucosidic anomer were identified. Although (+)-streptol was the most active compound, its concentration in P. kirkii plant leaves extract was approximately ten-fold lower than that of (+)-streptol glucoside. These results provide compelling evidence that the glucosylation of (+)-streptol protects the plant host against the growth inhibitory effect of the compound, which might constitute a molecular cornerstone for this successful plant-bacteria symbiosis.

1,2- trans Glycosylation via Neighboring Group Participation of 2- O-Alkoxymethyl Groups: Application to One-Pot Oligosaccharide Synthesis

The use of 2- O-alkoxymethyl groups as effective stereodirecting substituents for the construction of 1,2- trans glycosidic linkages is reported. The observed stereoselectivity arises from the intramolecular formation of a five-membered cyclic architecture between the 2- O-alkoxymethyl substituent and the oxocarbenium ion, which provides the expected facial selectivity. Furthermore, the observed stereocontrol and the extremely high reactivity of 2- O-alkoxymethyl-protected donors allowed development of a one-pot sequential glycosylation strategy that should become a powerful tool for the assembly of oligosaccharides.

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.

2-O-Benzyloxycarbonyl protected glycosyl donors: a revival of carbonate-mediated anchimeric assistance for diastereoselective glycosylation

Benzyloxycarbonyl can be used as participating group for the diastereoselective glycosylation of base-labile products and the synthesis of glycosyl esters.

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.

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

Carbohydrates, which are ubiquitously distributed throughout the three domains of life, play significant roles in a variety of vital biological processes. Access to unique and homogeneous carbohydrate materials is important to understand their physical properties, biological functions, and disease-related features. It is difficult to isolate carbohydrates in acceptable purity and amounts from natural sources. Therefore, complex saccharides with well-defined structures are often most conviently accessed through chemical syntheses. Two major hurdles, regioselective protection and stereoselective glycosylation, are faced by carbohydrate chemists in synthesizing these highly complicated molecules. Over the past few years, there has been a radical change in tackling these problems and speeding up the synthesis of oligosaccharides. This is largely due to the development of one-pot protection, one-pot glycosylation, and one-pot protection-glycosylation protocols and streamlined approaches to orthogonally protected building blocks, including those from rare sugars, that can be used in glycan coupling. In addition, new automated strategies for oligosaccharide syntheses have been reported not only for program-controlled assembly on solid support but also by the stepwise glycosylation in solution phase. As a result, various sugar molecules with highly complex, large structures could be successfully synthesized. To summarize these recent advances, this review describes the methodologies for one-pot protection and their one-pot glycosylation into the complex glycans and the chronological developments associated with automated syntheses of oligosaccharides.

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.

2- O- N-Benzylcarbamoyl as a Protecting Group To Promote β-Selective Glycosylation and Its Applications in the Stereoselective Synthesis of Oligosaccharides

This study examines the utility of the N-benzylcarbamoyl (BnCar) protecting group in glycosylation reactions of the parent O-2 protected carbohydrate donor. It was found that the BnCar group imparted exclusively β-selectivity with primary and secondary alcohols. A mechanistic study revealed the activated intermediate to be the glycosyl triflate in a skew conformation, which results in β-selective glycosylation via an S_N2-like pathway. The BnCar group can be readily cleaved using tetrabutylammonium nitrite, without affecting ester and ether protecting groups. Taken together, these results show BnCar to be useful for the synthesis of complex oligosaccharides, an undertaking that requires delicate chemical differentiation of various protecting groups.

Chemical synthesis of culmorin metabolites and their biologic role in culmorin and acetyl-culmorin treated wheat cells

The Fusarium metabolite culmorin (1) is receiving increased attention as an "emerging mycotoxin". It co-occurs with trichothecene mycotoxins and potentially influences their toxicity. Its ecological role and fate in plants is unknown. We synthesized sulfated and glucosylated culmorin conjugates as potential metabolites, which are expected to be formed in planta, and used them as reference compounds. An efficient procedure for the synthesis of culmorin sulfates was developed. Diastereo- and regioselective glucosylation of culmorin (1) was achieved by exploiting or preventing unexpected acyl transfer when using different glucosyl donors. The treatment of a wheat suspension culture with culmorin (1) revealed an in planta conversion of culmorin into culmorin-8-glucoside (6) and culmorin acetate, but no sulfates or culmorin-11-glucoside (7) was found. The treatment of wheat cells with the fungal metabolite 11-acetylculmorin (2) revealed its rapid deacetylation, but also showed the formation of 11-acetylculmorin-8-glucoside (8). These results show that plants are capable of extensively metabolizing culmorin.

Theoretical Studies of the Glycosidation of 2-O-Substituted 5-Fluorouracil: N-Regioselective Synthesis with the Phase-Transfer-Catalysis Method

The observed N-regioselective glycosidation of 2-O-substituted 5-fluorouracil (5-FU) via the phase-transfer-catalysis (PTC) method was investigated computationally. The Gibbs free energy reaction barrier of the N-reaction between the 5-FU anion and 1-bromo-1-deoxy-2,3,4,6-tetra-O-acetyl-α-d-glucopyranose was computed at the MP2/6-311++G(2d,p)//B3LYP/6-31+G* level. The calculated transition states were, in general, quite "loose", with the ambident reaction sites at the N3- or O4-positions on 5-FU located approximately 2.0 Å from the anomeric carbon. With the S_N2 mechanism, the formation of β-glycosides was explained by the characteristics of transition states, and the N-regioselectivity was explained by three considerations: (1) the conformations of initial complexes and the structural requirement of the reactions; (2) the formation of an ionic pair between nBu₄N⁺ and 2-O-substituted 5-FU anions; and (3) the thermodynamic conversion of O-glycosides to N-glycosides. The reactions between the oxocarbenium ion and the 2-O-substituted 5-FU anions (the fast step of S_N1 mechanism) were also examined at the same level of theory. Because there were no "promoters" to extract Br in the PTC method, the S_N1 mechanism might have an unfavorably high barrier to produce oxocarbenium ion. However, both the formation of β-glycosides and the experimentally observed N-regioselectivity could also be explained by the S_N1 mechanism: The former was explained by the neighboring group participation, and the latter was explained by the formation of ionic pairs between nBu₄N⁺ and 2-O-substituted 5-FU anions. The formation of ionic pairs possibly changed the diffusion-controlled mechanism into an activation-controlled mechanism. Two factors were demonstrated by Marcus theory to play an important role for the experimentally observed N-resioselectivity in the PTC method: (1) the thermodynamic stability of N-products over O-products; (2) the formation of ionic pair between nBu₄N⁺ and 2-O-substituted 5-FU anions.

Rapid assembly of the doubly-branched pentasaccharide domain of the immunoadjuvant jujuboside A via convergent B(C6F5)3-catalyzed glycosylation of sterically-hindered precursors

A convergent synthesis of the complex, doubly-branched pentasaccharide domain of the natural-product immunoadjuvant jujuboside A is described. The key step is a sterically-hindered glycosylation reaction between a branched trisaccharide trichloroacetimidate glycosyl donor and a disaccharide glycosyl acceptor. Conventional Lewis acids (TMSOTf, BF3·Et2O) were ineffective in this glycosylation, but B(C6F5)3 catalyzed the reaction successfully. Inherent complete diastereoselectivity for the undesired α-anomer was overcome by rational optimization with a nitrile solvent system (1 : 5 t-BuCN/CF3Ph) to provide flexible, effective access to the β-linked pentasaccharide.