Recent advances in the synthesis of sialic acid derivatives and sialylmimetics as biological probes

Immunometabolism of Liver Xenotransplantation and Prospective Solutions

End-stage liver diseases, such as hepatocellular carcinoma or acute liver failure, critically necessitate liver transplantation. However, the shortage of available organ donors fails to meet the rapidly growing transplantation demand. Due to the high similarity of liver tissue structure and metabolism between miniature pigs and humans, xenotransplantation of pig livers is considered as a potentially viable solution to organ scarcity. In the 2024, teams from China first time have successfully transplanted a genetically modified Bama miniature pig liver into a clinically brain-dead man lasting for 10 days. This milestone in human xenotransplantation research not only confirms the feasibility of clinical application of xenotransplantation, but also underscores the daunting and protracted nature of this pathway. Despite advanced gene-editing technologies theoretically circumventing the occurrence of most transplant rejection reactions, patients still face challenges such as chronic immune rejection, coagulation disorders, and thrombotic microangiopathy after receiving xenografts. Moreover, prolonged use of immunosuppressive drugs may induce irreversible immune dysfunction, leading to opportunistic infections and metabolic disorders. This article compares the similarities and differences in livers between humans and pigs, summarizes the immunometabolism of xenotransplantation based on current findings, and provides research perspectives on pre-transplantation and post-transplantation strategies for prolonging the survival time of xenografts.

Detection Strategies for Sialic Acid and Sialoglycoconjugates

Glycoconjugates are a vast class of biomolecules implicated in biological processes important for human health and disease. The structural complexity of glycoconjugates remains a challenge to deciphering their precise biological roles and for their development as biomarkers and therapeutics. Human glycoconjugates on the outside of the cell are modified with sialic (neuraminic) acid residues at their termini. The enzymes that install sialic acids are sialyltransferases (SiaTs), a family of 20 different isoenzymes. The removal and degradation of sialic acids is mediated by neuraminidase (NEU; sialidase) enzymes, of which there are four isoenzymes. In this review, we discuss chemical and biochemical approaches for the detection and analysis of sialoglycoconjugate (SGC) structures and their enzymatic products. The most common methods include affinity probes and synthetic substrates. Fluorogenic and radiolabelled substrates are also important tools for many applications, including screening for enzyme inhibitors. Strategies that give insight into the native substrate-specificity of enzymes that regulate SGCs (SiaT & NEU) are necessary to improve our understanding of the role of sialic acid metabolism in health and disease.

Direct Experimental Characterization of a Sialyl Cation

Sialic acids are monosaccharide residues involved in several biological processes. Controlling the stereoselectivity of sialylation reactions is challenging and mechanistic studies on the structure of its intermediate, the sialyl cation, are scarce. Here it is shown that a sialyl cation can be generated and isolated from an ionized sialic acid precursor. This short-lived species is structurally characterized for the first time using cryogenic infrared spectroscopy. In combination with quantum chemical calculations, the results reveal that the positive charge at the anomeric carbon of the sialyl cation is stabilized by remote participation of the C5-NHAc group leading to the formation of a bridged structure. In this structure, the β-side is shielded from nucleophilic attack, potentially explaining the α-selectivity of this building block in SN1-type sialylation reactions. Other modes of participation are energetically unfavored and cannot be observed experimentally.

Synthesis and Thermodynamic Evaluation of Sialyl-Tn MUC1 Glycopeptides Binding to Macrophage Galactose-Type Lectin

Interactions between the tumor-associated carbohydrate antigens of Mucin 1 (MUC1) and the carbohydrate-binding proteins, lectins, often lead to the creation of a pro-tumor microenvironment favoring tumor initiation, progression, metastasis, and immune evasion. Macrophage galactose binding lectin (MGL) is a C-type lectin receptor found on antigen-presenting cells that facilitates the uptake of carbohydrate antigens for antigen presentation, modulating the immune response homeostasis, autoimmunity, and cancer. Considering the crucial role of tumor-associated forms of MUC1 and MGL in tumor immunology, a thorough understanding of their binding interaction is essential for it to be exploited for cancer vaccine strategies. The synthesis of MUC1 glycopeptide models carrying a single or multiple Tn and/or sialyl-Tn antigen(s) is described. A novel approach for the sialyl-Tn threonine building block suitable for the solid phase peptide synthesis was developed. The thermodynamic profile of the binding interaction between the human MGL and MUC1 glycopeptide models was analyzed using isothermal titration calorimetry. The measured dissociation constants for the sialyl-Tn-bearing peptide epitopes were consistently lower compared to the Tn antigen and ranged from 10 μM for mono- to 1 μM for triglycosylated MUC1 peptide, respectively. All studied interactions, regardless of the glycan's site of attachment or density, exhibited enthalpy-driven thermodynamics.

Kinetic characterization of two neuraminic acid synthases and evaluation of their application potential

Neuraminic acid synthases are an important yet underexplored group of enzymes. Thus, in this research, we performed a detailed kinetic and stability analysis and a comparison of previously known neuraminic acid synthase from Neisseria meningitidis, and a novel enzyme, PNH5, obtained from a metagenomic library. A systematic analysis revealed a high level of similarity of PNH5 to other known neuraminic acid synthases, except for its pH optimum, which was found to be at 5.5 for the novel enzyme. This is the first reported enzyme from this family that prefers an acidic pH value. The effect of different metal cofactors on enzyme activity, i.e. Co2+, Mn2+ and Mg2+, was studied systematically. The kinetics of neuraminic acid synthesis was completely elucidated, and an appropriate kinetic model was proposed. Enzyme stability study revealed that the purified enzyme exhibits changes in its structure during time as observed by differential light scattering, which cause a drop in its activity and protein concentration. The operational enzyme stability for the neuraminic acid synthase from N. meningitidis is excellent, where no activity drop was observed during the batch reactor experiments. In the case of PNH5, some activity drop was observed at higher concentration of substrates. The obtained results present a solid platform for the future application of these enzymes in the synthesis of sialic acids. KEY POINTS: • A novel neuraminic acid synthase was characterized. • The effect of cofactors on NeuS activity was elucidated. • Kinetic and stability characterization of two neuraminic acid synthases was performed.

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.

Amphiphilic Sialic Acid Derivatives as Potential Dual-Specific Inhibitors of Influenza Hemagglutinin and Neuraminidase

In the shadow of SARS-CoV-2, influenza seems to be an innocent virus, although new zoonotic influenza viruses evolved by mutations may lead to severe pandemics. According to WHO, there is an urgent need for better antiviral drugs. Blocking viral hemagglutinin with multivalent N-acetylneuraminic acid derivatives is a promising approach to prevent influenza infection. Moreover, dual inhibition of both hemagglutinin and neuraminidase may result in a more powerful effect. Since both viral glycoproteins can bind to neuraminic acid, we have prepared three series of amphiphilic self-assembling 2-thio-neuraminic acid derivatives constituting aggregates in aqueous medium to take advantage of their multivalent effect. One of the series was prepared by the azide-alkyne click reaction, and the other two by the thio-click reaction to yield neuraminic acid derivatives containing lipophilic tails of different sizes and an enzymatically stable thioglycosidic bond. Two of the three bis-octyl derivatives produced proved to be active against influenza viruses, while all three octyl derivatives bound to hemagglutinin and neuraminidase from H1N1 and H3N2 influenza types.

Growing impact of sialic acid-containing glycans in future drug discovery

In nature, almost all cells are covered with a complex array of glycan chain namely sialic acids or nuraminic acids, a negatively charged nine carbon sugars which is considered for their great therapeutic importance since long back. Owing to its presence at the terminal end of lipid bilayer (commonly known as terminal sugars), the well-defined sialosides or sialoconjugates have served pivotal role on the cell surfaces and thus, the sialic acid-containing glycans can modulate and mediate a number of imperative cellular interactions. Understanding of the sialo-protein interaction and their roles in vertebrates in regard of normal physiology, pathological variance, and evolution has indeed a noteworthy journey in medicine. In this tutorial review, we present a concise overview about the structure, linkages in chemical diversity, biological significance followed by chemical and enzymatic modification/synthesis of sialic acid containing glycans. A more focus is attempted about the recent advances, opportunity, and more over growing impact of sialosides and sialoconjugates in future drug discovery and development.

Anti-Wrinkle Efficacy of Edible Bird's Nest Extract: A Randomized, Double-Blind, Placebo-Controlled, Comparative Study

This study aimed to evaluate skin health's functional improvement, such as wrinkles, elasticity, moisture, and whitening, and safety following the consumption of "edible bird's nest extract" for 12 weeks by women. This single-center, double-blinded, parallel-group, placebo-controlled study included women aged 40-60 years. Our primary purpose was to assess improvement in skin wrinkles, elasticity, and moisture after 12 weeks using an SV700, cutometer, and corneometer, respectively, compared to baseline measurements. Our secondary purpose was to evaluate skin wrinkle, elasticity, and moisture changes at 4 and 8 weeks from baseline using the aforementioned equipment, and measure transdermal water loss and melanin and erythema indexes using a tewameter and mexameter, respectively. Experts performed the visual evaluation of skin wrinkles at 4, 8, and 12 weeks from baseline. The participants were randomly allocated in a 1:1 ratio into the edible bird's nest extract or the placebo group with 43 participants each, where they consumed 100 mg of the extract or placebo, respectively, daily for 12 weeks. The outcomes were measured at every visit. In this study, upon comparing changes in the skin elasticity value between the two intake groups at 12 weeks of ingestion, skin elasticity in the edible bird's nest extract group decreased significantly compared with that in the placebo group. Adverse reactions were absent in both groups. In the case of laboratory test results, changes before and after the ingestion of the extract were within the normal range, thus indicating no clinically significant difference. The edible bird's nest extract was effective in improving skin wrinkles. Moreover, it is beneficial for skin health and can be used as a skin nutritional supplement. Compared with the placebo, the edible bird's nest extract was identified as safe. Clinical Trial Registration: https://cris.nih.go.kr/cris/search/detailSearch.do?search_lang=E&search_page=M&pageSize=10&page=undefined&seq=21007&status=5&seq_group=20330, identifier KCT0006558.

Synthetic O-Acetylated Sialosides and their Acetamido-deoxy Analogues as Probes for Coronaviral Hemagglutinin-esterase Recognition

O-Acetylation is a common modification of sialic acids that can occur at carbons 4-, 7-, 8-, and/or 9. Acetylated sialosides are employed as receptors by several betacoronaviruses and toroviruses, and by influenza C and D viruses. The molecular basis by which these viruses recognize specific O-acetylated sialosides is poorly understood, and it is unknown how viruses have evolved to recognize specific O-acetylated sialosides expressed by their host. Here, we describe a chemoenzymatic approach that can readily provide sialoglycan analogues in which acetyl esters at C4 and/or C7 are replaced by stabilizing acetamide moieties. The analogues and their natural counterparts were used to examine the ligand requirements of the lectin domain of coronaviral hemagglutinin-esterases (HEs). It revealed that HEs from viruses targeting different host species exhibit different requirements for O-acetylation. It also showed that ester-to-amide perturbation results in decreased or loss of binding. STD NMR and molecular modeling of the complexes of the HE of BCoV with the acetamido analogues and natural counterparts revealed that binding is governed by the complementarity between the acetyl moieties of the sialosides and the hydrophobic patches of the lectin. The precise spatial arrangement of these elements is important, and an ester-to-amide perturbation results in substantial loss of binding. Molecular Dynamics simulations with HEs from coronaviruses infecting other species indicate that these viruses have adapted their HE specificity by the incorporation of hydrophobic or hydrophilic elements to modulate acetyl ester recognition.

En Route to the Transformation of Glycoscience: A Chemist's Perspective on Internal and External Crossroads in Glycochemistry

Carbohydrate chemistry is an essential component of the glycosciences and is fundamental to their progress. This Perspective takes the position that carbohydrate chemistry, or glycochemistry, has reached three crossroads on the path to the transformation of the glycosciences, and illustrates them with examples from the author's and other laboratories. The first of these potential inflexion points concerns the mechanism of the glycosylation reaction and the role of protecting groups. It is argued that the experimental evidence supports bimolecular SN2-like mechanisms for typical glycosylation reactions over unimolecular ones involving stereoselective attack on naked glycosyl oxocarbenium ions. Similarly, it is argued that the experimental evidence does not support long-range stereodirecting participation of remote esters through bridged bicyclic dioxacarbenium ions in organic solution in the presence of typical counterions. Rational design and improvement of glycosylation reactions must take into account the roles of the counterion and of concentration. A second crossroads is that between mainstream organic chemistry and glycan synthesis. The case is made that the only real difference between glycan and organic synthesis is the formation of C-O rather than C-C bonds, with diastereocontrol, strategy, tactics, and elegance being of critical importance in both areas: mainstream organic chemists should feel comfortable taking this fork in the road, just as carbohydrate chemists should traveling in the opposite direction. A third crossroads is that between carbohydrate chemistry and medicinal chemistry, where there are equally many opportunities for traffic in either direction. The glycosciences have advanced enormously in the past decade or so, but creativity, input, and ingenuity of scientists from all fields is needed to address the many sophisticated challenges that remain, not the least of which is the development of a broader and more general array of stereospecific glycosylation reactions.

An overview and future prospects of sialic acids

Sialic acids (Sias) are negatively charged functional monosaccharides present in a wide variety of natural sources (plants, animals and microorganisms). Sias play an important role in many life processes, which are widely applied in the medical and food industries as intestinal antibacterials, antivirals, anti-oxidative agents, food ingredients, and detoxification agents. Most Sias are composed of N-acetylneuraminic acid (Neu5Ac, >99%), and Sia is its most commonly used name. In this article, we review Sias in terms of their structures, applications, determination methods, metabolism, and production strategies. In particular, we summarise and compare different production strategies, including extraction from natural sources, chemical synthesis, polymer decomposition, enzymatic synthesis, whole-cell catalysis, and de novo biosynthesis via microorganism fermentation. We also discuss research on their physiological functions and applications, barriers to efficient production, and strategies for overcoming these challenges. We focus on efficient de novo biosynthesis strategies for Neu5Ac via microbial fermentation using novel synthetic biology tools and methods that may be applied in future. This work provides a comprehensive overview of recent advances on Sias, and addresses future challenges regarding their functions, applications, and production.

Synthesis of N-Glycolylneuraminic Acid (Neu5Gc) and Its Glycosides

Sialic acids constitute a family of negatively charged structurally diverse monosaccharides that are commonly presented on the termini of glycans in higher animals and some microorganisms. In addition to N-acetylneuraminic acid (Neu5Ac), N-glycolyl neuraminic acid (Neu5Gc) is among the most common sialic acid forms in nature. Nevertheless, unlike most animals, human cells loss the ability to synthesize Neu5Gc although Neu5Gc-containing glycoconjugates have been found on human cancer cells and in various human tissues due to dietary incorporation of Neu5Gc. Some pathogenic bacteria also produce Neu5Ac and the corresponding glycoconjugates but Neu5Gc-producing bacteria have yet to be found. In addition to Neu5Gc, more than 20 Neu5Gc derivatives have been found in non-human vertebrates. To explore the biological roles of Neu5Gc and its naturally occurring derivatives as well as the corresponding glycans and glycoconjugates, various chemical and enzymatic synthetic methods have been developed to obtain a vast array of glycosides containing Neu5Gc and/or its derivatives. Here we provide an overview on various synthetic methods that have been developed. Among these, the application of highly efficient one-pot multienzyme (OPME) sialylation systems in synthesizing compounds containing Neu5Gc and derivatives has been proven as a powerful strategy.

Trypanosoma cruzi trans-sialidase. A tool for the synthesis of sialylated oligosaccharides

Cells are covered by a complex array of carbohydrates. Among them, sialosides are of key importance in intracellular adhesion, recognition and signaling. The need for structurally diverse sialosides impelled the search for efficient synthetic methods since their isolation from natural sources is a difficult task. The enzymatic approach obviates the need of a chemical synthesis for protecting or participating groups in the substrates. The trans-sialidase of Trypanosoma cruzi (TcTS) is highly stereospecific for the transfer of sialic acid from an α-sialylglycoside donor to a terminal β-galactopyranosyl unit in the acceptor substrate to form the α-Neu5Ac-(2 → 3)-β-D-Galp motif. The enzyme was cloned and easily available glycoproteins, e.g. fetuin, may be used as donors of sialic acid, constituting strong points for the scalability of TcTS-catalyzed reactions. This review outlines the preparative use of TcTS for the sialylation of oligosaccharides. A detailed description of the substrates used as sialic acid donors, the acceptor substrates and the methods employed to monitor the reaction is included.

Molybdenum-Catalyzed Diastereoselective anti-Dihydroxylation of Secondary Allylic Alcohols

In this protocol, we report a Mo-catalyzed anti-dihydroxylation of secondary allylic alcohols, providing a general method for the preparation of 1,2,3-triols bearing up to three continuous stereocenters with excellent diastereocontrol. The mechanistic studies reveal that this dihydroxylation reaction consists of two steps and up to excellent diastereomeric ratios of the final triol products can be achieved due to the high level of both diastereocontrol in the initial epoxidation and regiocontrol in the following hydrolysis in situ.

Step-economy synthesis of β-steryl sialosides using a sialyl iodide donor

Steryl glycosides are prevalent in nature and have unique biological activities dictated by sterol structure, sugar composition, and the stereochemical attachment of the aglycone. A single configurational switch can have profound biological consequences meriting the systematic study of structure and function relationships. Steryl congeners of N-acetyl neuraminic acid (NANA) impact neurobiological processes and may also mediate host/microbe interactions. In order to study these processes, a platform for the synthesis of β-steryl sialosides has been established. Promoter-free glycosidations using a novel α-linked sialyl iodide donor efficiently provide unique amphiphilic sialoglycoconjugates for examining bioactivities in various systems.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Catalytic mechanism of type C sialidase from Streptococcus pneumoniae: from covalent intermediate to final product

Streptococcus pneumoniae is a Gram-positive human pathogenic bacterium, which is the main cause of pneumonia and meningitis in children and the elderly. Three sialidases (or neuraminidases) encoded from Streptococcus pneumoniae could catalyze the cleavage of sialic acid linkages. This mechanism is directly connected with infection, apoptosis, and signaling, and usually considered to be one of the critical virulence factors. Type C neuraminidase (NanC) is unique because its primary product of Neu5Ac2en is considered to be an inhibitor to the other two sialidases. Experimentally, there are two different pathways for the formation mechanism of Neu5Ac2en catalyzed by NanC. In this work, a combined quantum mechanical and molecular mechanical approach was employed in all calculations. Starting from the covalent sialylated intermediate, we first examined the reaction to Neu5Ac2en and found the reaction prefers a direct proton abstraction mechanism rather than the water mediated proton abstraction mechanism. Free energy profiles can confirm that Neu5Ac2en is the major product of NanC. Functional roles of some important residues were also investigated, e.g., D315 acts as the proton acceptor during the formation of Neu5Ac2en, while the general base for the hydrolytic reaction to Neu5Ac. This study can facilitate the understanding of the catalytic mechanism of NanC and has the potential to aid in future inhibitor design studies.

Sialidase-catalyzed one-pot multienzyme (OPME) synthesis of sialidase transition-state analogue inhibitors

Sialidase transition state analog inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (Neu5Ac2en, DANA) has played a leading role in developing clinically used anti-influenza virus drugs. Taking advantage of the Neu5Ac2en-forming catalytic property of Streptococcus pneumoniae sialidase SpNanC, an effective one-pot multienzyme (OPME) strategy has been developed to directly access Neu5Ac2en and its C-5, C-9, and C-7-analogs from N-acetylmannosamine (ManNAc) and analogs. The obtained Neu5Ac2en analogs can be further derivatized at various positions to generate a larger inhibitor library. Inhibition studies demonstrated improved selectivity of several C-5- or C-9-modified Neu5Ac2en derivatives against several bacterial sialidases. The study provides an efficient enzymatic method to access sialidase inhibitors with improved selectivity.

Chemical Synthesis of Glycosides of N-Acetylneuraminic Acid

Investigations of methodologies aimed on improving the stereoselective synthesis of sialosides and the efficient assembly of sialic acid glycoconjugates has been the mission of dedicated research groups from the late 1960s. This review presents major accomplishments in the field, with the emphasis on significant breakthroughs and influential synthetic strategies of the last decade.

Assistance of the C-7,8-Picoloyl Moiety for Directing the Glycosyl Acceptors into the α-Orientation for the Glycosylation of Sialyl Donors

An efficient α-sialylation method for many primary hydroxyl acceptors that include 6-OH glycosides has been developed. 7,8-Di-O-picoloyl sialyl glycoside was used as the glycosyl donor, and α-glycoconjugation was controlled by using the 7,8-di-O-picoloyl moiety in CH₂Cl₂. The methodology was successfully applied to the total synthesis of ganglioside Hp-s1 possessing neuritogenic activity.

Synthesis and In Vitro Anti-Influenza Virus Evaluation of Novel Sialic Acid (C-5 and C-9)-Pentacyclic Triterpene Derivatives

The emergence of drug resistant variants of the influenza virus has led to a great need to identify novel and effective antiviral agents. In our previous study, a series of sialic acid (C-2 and C-4)-pentacyclic triterpene conjugates have been synthesized, and a five-fold more potent antiviral activity was observed when sialic acid was conjugated with pentacyclic triterpene via C-4 than C-2. It was here that we further reported the synthesis and anti-influenza activity of novel sialic acid (C-5 and C-9)-pentacyclic triterpene conjugates. Their structures were confirmed by ESI-HRMS, ¹H-NMR, and ¹³C-NMR spectroscopic analyses. Two conjugates (26 and 42) showed strong cytotoxicity to MDCK cells in the CellTiter-Glo assay at a concentration of 100 μM. However, they showed no significant cytotoxicity to HL-60, Hela, and A549 cell lines in MTT assay under the concentration of 10 μM (except compound 42 showed weak cytotoxicity to HL-60 cell line (10 μM, ~53%)). Compounds 20, 28, 36, and 44 displayed weak potency to influenza A/WSN/33 (H1N1) virus (100 μM, ~20–30%), and no significant anti-influenza activity was found for the other conjugates. The data suggested that both the C-5 acetylamide and C-9 hydroxy of sialic acid were important for its binding with hemagglutinin during viral entry into host cells, while C-4 and C-2 hydroxy were not critical for the binding process and could be replaced with hydrophobic moieties. The research presented herein had significant implications for the design of novel antiviral inhibitors based on a sialic acid scaffold.

Synthesis of Complex Tertiary Glycolates by Enantioconvergent Arylation of Stereochemically Labile α-Keto Esters

Enantioconvergent arylation reactions of boronic acids and racemic β-stereogenic α-keto esters have been developed. The reactions are catalyzed by a chiral (diene)Rh(I) complex and provide a wide array of β-stereogenic tertiary aryl glycolate derivatives with high levels of diastereo- and enantioselectivity. Racemization studies employing a series of sterically differentiated tertiary amines suggest that the steric nature of the amine base additive exerts a significant influence on the rate of substrate racemization.

Synthesis of sialic acid derivatives based on chiral substrate-controlled stereoselective aldol reactions using pyruvic acid oxabicyclo[2.2.2]octyl orthoester

The synthesis of sialic acids and their analogs was accomplished based on substrate-controlled asymmetric aldol reactions using pyruvic acid oxabicyclo[2.2.2]octyl orthoester.

Oxidative Deamination of N-Acetyl Neuraminic Acid: Substituent Effects and Mechanism

A study of the mechanism of the oxidative deamination of the N-nitroso-N-acetyl sialyl glycosides leading with overall retention of configuration to the corresponding 2-keto-3-deoxy-D-glycero-D-galacto-nonulopyranosidonic acid (KDN) glycosides is described, making use of a series of differentially O-protected N-nitroso-N-acetyl sialyl glycosides and of isotopic labeling studies. No evidence is found for stereodirecting participation by ester groups at the 4- and 7-positions. Comparisons are drawn with oxidative deamination reactions of 4-amino-4-deoxy and 2-amino-2-deoxy hexopyranosides and a common mechanism is formulated involving the intermediacy of 1-oxabicyclo[3.1.0]hexyl oxonium ions following participation by the pyranoside ring oxygen. A minor reaction pathway has been uncovered by labeling studies in the β-thiosialosides that results in the exchange of the 4-O-acetyl group by the glacial acetic acid that serves as external nucleophile in the general oxidative deamination process. A mechanism is proposed for this exchange involving participation by the thioglycoside at the level of an intermediate diazoalkane.

Molecular dynamics of sialic acid analogues complex with cholera toxin and DFT optimization of ethylene glycol-mediated zinc nanocluster conjugation

Cholera is an infectious disease caused by cholera toxin (CT) protein of bacterium Vibrio cholerae. A sequence of sialic acid (N-acetylneuraminic acid, NeuNAc or Neu5Ac) analogues modified in its C-5 position is modelled using molecular modelling techniques and docked against the CT followed by molecular dynamics simulations. Docking results suggest better binding affinity of NeuNAc analogue towards the binding site of CT. The NeuNAc analogues interact with the active site residues GLU:11, TYR:12, HIS:13, GLY:33, LYS:34, GLU:51, GLN:56, HIE:57, ILE:58, GLN:61, TRP:88, ASN:90 and LYS:91 through intermolecular hydrogen bonding. Analogues N-glycolyl-NeuNAc, N-Pentanoyl-NeuNAc and N-Propanoyl-NeuNAc show the least XPGscore (docking score) of -9.90, -9.16, and -8.91, respectively, and glide energy of -45.99, -42.14 and -41.66 kcal/mol, respectively. Stable nature of CT-N-glycolyl-NeuNAc, CT-N-Pentanoyl-NeuNAc and CT-N-Propanoyl-NeuNAc complexes was verified through molecular dynamics simulations, each for 40 ns using the software Desmond. All the nine NeuNAc analogues show better score for drug-like properties, so could be considered as suitable candidates for drug development for cholera infection. To improve the enhanced binding mode of NeuNAc analogues towards CT, the nine NeuNAc analogues are conjugated with Zn nanoclusters through ethylene glycol (EG) as carriers. The NeuNAc analogues conjugated with EG-Zn nanoclusters show better binding energy towards CT than the unconjugated nine NeuNAc analogues. The electronic structural optimization of EG-Zn nanoclusters was carried out for optimizing their performance as better delivery vehicles for NeuNAc analogues through density functional theory calculations. These sialic acid analogues may be considered as novel leads for the design of drug against cholera and the EG-Zn nanocluster may be a suitable carrier for sialic acid analogues.

Elucidation of several neglected reactions in the GC-MS identification of sialic acids as heptafluorobutyrates calls for an urgent reassessment of previous claims

The current analytical protocol used for the GC-MS determination of free or 1,7-lactonized natural sialic acids (Sias), as heptafluorobutyrates, overlooks several transformations. Using authentic reference standards and by combining GC-MS and NMR analyses, flaws in the analytical protocol were pinpointed and elucidated, thus establishing the scope and limitations of the method. It was demonstrated that (a) Sias 1,7-lactones, even if present in biological samples, decompose under the acidic hydrolysis conditions used for their release; (b) Sias 1,7-lactones are unpredicted artifacts, accidentally generated from their parent acids; (c) the N-acetyl group is quantitatively exchanged with that of the derivatizing perfluorinated anhydride; (d) the partial or complete failure of the Sias esterification-step with diazomethane leads to the incorrect quantification and structure attribution of all free Sias. While these findings prompt an urgent correction and improvement of the current analytical protocol, they could be instrumental for a critical revision of many incorrect claims reported in the literature.

Combination of automated solid-phase and enzymatic oligosaccharide synthesis provides access to α(2,3)-sialylated glycans

A synthetic strategy combining automated solid-phase chemical synthesis and enzymatic sialylation was developed to access α(2,3)-sialylated glycans.

Benzylidene Acetal Protecting Group as Carboxylic Acid Surrogate: Synthesis of Functionalized Uronic Acids and Sugar Amino Acids

Direct oxidation of the 4,6-O-benzylidene acetal protecting group to C-6 carboxylic acid has been developed that provides an easy access to a wide range of biologically important and synthetically challenging uronic acid and sugar amino acid derivatives in good yields. The RuCl3 -NaIO4 -mediated oxidative cleavage method eliminates protection and deprotection steps and the reaction takes place under mild conditions. The dual role of the benzylidene acetal, as a protecting group and source of carboxylic acid, was exploited in the efficient synthesis of six-carbon sialic acid analogues and disaccharides bearing uronic acids, including glycosaminoglycan analogues.

CMP-Sialic Acid Synthetase: The Point of Constriction in the Sialylation Pathway

Sialoglycoconjugates form the outermost layer of animal cells and play a crucial role in cellular communication processes. An essential step in the biosynthesis of sialylated glycoconjugates is the activation of sialic acid to the monophosphate diester CMP-sialic acid. Only the activated sugar is transported into the Golgi apparatus and serves as a substrate for the linkage-specific sialyltransferases. Interference with sugar activation abolishes sialylation and is embryonic lethal in mammals. In this chapter we focus on the enzyme catalyzing the activation of sialic acid, the CMP-sialic acid synthetase (CMAS), and compare the enzymatic properties of CMASs isolated from different species. Information concerning the reaction mechanism and active site architecture is included. Moreover, the unusual nuclear localization of vertebrate CMASs as well as the biotechnological application of bacterial CMAS enzymes is addressed.

Stereoselective reaction of 2-carboxythioesters-1,3-dithiane with nitroalkenes: an organocatalytic strategy for the asymmetric addition of a glyoxylate anion equivalent

Under mild reaction conditions γ-nitro-β-aryl-α-keto esters with up to 92% ee were obtained, realizing a formal catalytic stereoselective conjugate addition of the glyoxylate anion synthon.

Total synthesis of trifluorobutyryl-modified, globally protected sialyl Lewis x by a convergent [2+2] approach

Structural and quantitative changes in the expression of sialic acid residues on the surface of eukaryotic cells profoundly influence a broad range of biological processes including inflammation, antigen recognition, microbial attachment, and tumor metastasis. Uptake and incorporation of sialic acid analogues in mammalian cells enable structure-function studies and perturbation of specific recognition events. Our group has recently shown that a trifluorobutyryl-modified sialic acid metabolite diminishes the adhesion of mammalian cells to E and P-selectin, presumably by leading to the expression of fluorinated sLex epitopes on cell surfaces, and interfering with the sLex-selectin interactions that are well known in mediating tumor cell migration.1 For studies directed towards understanding the molecular basis of this reduced adhesion, chemical synthesis of trifluorobutyrylated sialyl Lewis x (C4F3--sLex) was crucial. We have developed a highly efficient [2+2] approach for the assembly of C4F3-sLex on a preparative scale that contains versatile protective groups allowing the glycan to be surface immobilized or solubilized as needed for biophysical studies to investigate selectin interactions. This strategy can, in principle, be used for preparation of other N-modified sLex analogues.

Synthesis of partially O-acetylated N-acetylneuraminic acid using regioselective silyl exchange technology

Postglycosylation acetylation of sialic acid imparts unique roles to sialoglycoconjugates in mammalian immune response making structural and functional understanding of these analogues important. Five partially O-acetylated Neu5Ac analogues have been synthesized. Reaction of per-O-silylated Neu5Ac ester with AcOH and Ac₂O in pyridine promotes regioselective silyl ether/acetate exchange in the following order: C₄ (2°) > C₉ (1°) > C₈ (2°) > C₂ (anomeric). Subsequent hydrogenolysis affords the corresponding sialic acid analogues as useful chemical biology tools.

Synthesis and elimination of C-3-labeled thiosialosides

The synthesis of C-3-labeled phenylthio sialic acid derivatives and an investigation of stereoselectivity in elimination reactions for the synthesis of 2,3-dehydro derivatives (glycals) is described. The experimental results are consistent with the existence of a conformational change and may be indicative of the intermediacy of an all-axial oxacarbenium ion.

Glycans in immune recognition and response

Glycans at the forefront of cells facilitate immune recognition processes. Cancer cells commonly present altered cell surface glycosylation, especially manifested in the expression of sialic acid at the termini of glycolipids and glycoproteins. Although tumor-associated carbohydrate antigens (TACAs) result in expression of altered-self, most such carbohydrates do not elicit strong humoral responses. Various strategies had been devised to elicit increased immunogenicity of such TACA aiming for potent immunotherapeutic antibodies or cancer vaccines. However some carbohydrates are immunogenic in humans and hold potential for novel glycotherapies. N-Glycolylneuraminic acid (Neu5Gc) is a foreign immunogenic sugar in humans originating from the diet (e.g., red meat) and subsequently expressed on the cell surface, especially accumulating on carcinoma. Consequently, the human immune system detects this non-self carbohydrate generating a broad anti-Neu5Gc antibody response. The co-existence of Neu5Gc/anti-Neu5Gc within humans spurs chronic inflammation mediated disease, including cancer. Concurrently, anti-Neu5Gc antibodies hold potential for novel targeted therapy. αGal is another foreign immunogenic carbohydrate antigen in humans and all humans have circulating anti-Gal antibodies. This review aims to describe the immunogenicity of Neu5Gc and its implications for human diseases, highlighting differences and similarities with αGal and its potential for novel targeted theranostics.

Synthesis of sialic acid-containing saccharides

Sialic acids are a diverse family of negatively charged monosaccharides with a shared nine-carbon carboxylated backbone, and they often serve as the terminal positions of cell surface glycoproteins and glycolipids. Sialic acids play essential roles in mediating or modulating numerous pathological, biological, and immunological recognition events. Advances in synthesis have provided chemically well-defined and structurally homogeneous sialic acid-containing carbohydrates that are crucial for studying glycobiology. This review highlights recent innovations in the chemical and chemoenzymatic synthesis of difficult α-sialosides, with a particular focus on methods developed for α-selective sialylation in the synthesis of O-linked and S-linked oligosialic acids.