Acceptors In Reactions Research Articles

Synthesis of 2-Amino-2-deoxy Sugars via Boron-Catalyzed Coupling of Glycosyl Fluorides and Silyl Ether Acceptors.

Although aminosugars are important components in a variety of bioactive molecules, their stereoselective formation is made challenging by the Lewis basic nature of amino substituents. Additionally, the use of N-acyl protecting groups is often problematic due to the competing formation of oxazolines during the glycosylation of 2-aminosugar derivatives. Herein, we report a boron-catalyzed strategy utilizing silyl ether glycosyl acceptors and 2-aminosugar donors that employs the 2,2,2-trichloroethoxycarbonyl (Troc) protecting group for the C2 amino functionality in glycosyl fluorides. This modification allows for operationally simple room-temperature glycosylations and features a rapid reaction profile that addresses some of the limitations in the synthesis of 2-amino-2-deoxy sugar-containing glycosides. Tailoring the order of reactivity of the silyl acceptors enables one-pot iterative glycosylations, thus streamlining the synthesis of complex oligosaccharides while allowing fewer intermediates and purification steps.

Chemical Synthesis and Antigenicity Evaluation of an Aminoglycoside Trisaccharide Repeating Unit of Pseudomonas aeruginosa Serotype O5 O-Antigen Containing a Rare Dimeric-ManpN3NA.

Pseudomonas aeruginosa bacteria are becoming increasingly resistant against multiple antibiotics. Therefore, the development of vaccines to prevent infections with these bacteria is an urgent medical need. While the immunological activity of lipopolysaccharide O-antigens in P. aeruginosa is well-known, the specific protective epitopes remain unidentified. Herein, we present the first chemical synthesis of highly functionalized aminoglycoside trisaccharide 1 and its acetamido derivative 2 found in the P. aeruginosa serotype O5 O-antigen. The synthesis of the trisaccharide targets is based on balancing the reactivity of disaccharide acceptors and monosaccharide donors. Glycosylations were analyzed by quantifying the reactivity of the hydroxyl group of the disaccharide acceptor using the orbital-weighted Fukui function and dual descriptor. The stereoselective formation of 1,2-cis-α-fucosylamine linkages was achieved through a combination of remote acyl participation and reagent modulation. The simultaneous SN2 substitution of azide groups at C2' and C2″ enabled the efficient synthesis of 1,2-cis-β-linkages for both 2,3-diamino-D-mannuronic acids. Through a strategic orthogonal modification, the five amino groups on target trisaccharide 1 were equipped with a rare acetamidino (Am) and four acetyl (Ac) groups. Glycan microarray analyses of sera from patients infected with P. aeruginosa indicated that trisaccharides 1 and 2 are key antigenic epitopes of the serotype O5 O-antigen. The acetamidino group is not an essential determinant of antibody binding. The β-D-ManpNAc3NAcA residue is a key motif for the antigenicity of serotype O5 O-antigen. These findings serve as a foundation for the development of glycoconjugate vaccines targeting P. aeruginosa serotype O5.

Synthesis of the H-type 1 and Lewis B antigens as 6-aminohexyl glycosides.

The LebLea heptasaccharide is a TACA that was isolated from the human colonic adenocarcinoma cell line Colo205 and is a good target for the development of anti-cancer immunotherapeutics. However, it displays on its reducing end the Leb tetrasaccharide: α-L-Fucp-(1→2)-β-D-Galp-(1→3)-[α-L-Fucp-(1→4)]-D-GlcNAcp and the H-type 1 (H-1 antigen) trisaccharide: α-L-Fucp-(1→2)-β-D-Galp-(1→3)-D-GlcNAcp that are also found on non cancerous tissues. To discover analogues or fragments of LebLea that could be used as immunotherapeutics while not triggering immune responses against Leb and the H-1 antigen, we have synthesised the Leb tetrasaccharide hexyl glycoside and the Leb and H-1 antigens aminohexyl glycosides to be used in ELISA experiments. We describe an improved preparation of the 6-O-benzyl-2,3,4-tri-O-acetyl-α-D-galactopyranosyl bromide in neutral conditions and demonstrate the importance of appropriately “matching” the reactivity of acceptors with that of glycosyl donors. Mono- and di-fucosylation of a disaccharide diol acceptor with per-benzylated thioethyl fucoside activated in situ with bromine and under halide ion catalysis is described and our results are compared to literature reports. We observed that our fucosylation reactions required higher equivalents of fucosyl donor and extended reaction times than previously reported. We propose that the protecting groups on the galactosyl unit led to a reduced reactivity of the acceptor. The protected intermediates were converted to 6-azido hexyl glycosides and submitted to dissolving metal conditions to give 6-aminohexyl glycosides. We also prepared the n-hexyl trisaccharide glycoside Leb that will be used as a soluble antigen in competitive ELISA experiments.

The impacts of benzoyl and benzyl groups on the non-covalent interactions and electronic properties of glycosyl acceptors

Modulating the reactivity of glycosyl acceptors has become a key in promoting the chemical synthesis of complex glycans. Herein, computational chemistry was employed to explore the impacts of protecting groups on the non-covalent interactions and electronic properties of glycosyl acceptors. Wavefunction analyses showed that substituting benzoyl groups with benzyl groups and introducing a benzyl group to the C2 amine of a D-GlcpNAc residue can eliminate intra-/intermolecular hydrogen bonds, thereby altering the charge distribution significantly. This protecting group-induced charge distribution increases the nucleophilicity of hydroxyl group. This study may contribute to understanding the assistance of computational chemistry in glycan synthesis.

Site-Selective Modification of Secondary Amine Moieties on Native Peptides, Proteins, and Natural Products with Ynones.

Site-selective modification of biologically relevant secondary amines in peptides, proteins, and natural products has been challenging due to the similar reactivity between primary and secondary amines. Even for the secondary amines, their reactivities are significantly influenced by their structures and environment. Herein, we report a ynone Michael bioconjugation method for selective modification of secondary amines in unprotected peptides and proteins and complex natural products. We show that fine tuning the electronic effect of the ynones enables controlling the Michael acceptor reactivity for the selective reaction with the structurally different secondary amines in densely functionalized complex structures and complicated biological environment.

Multisite Labeling of Proteins Using the Ligand-Directed Reactivity of Triggerable Michael Acceptors.

Targeted modification of endogenous proteins without genetic manipulation of protein expression machinery has a range of applications from chemical biology to drug discovery. Despite being demonstrated to be effective in various applications, target-specific protein labeling using ligand-directed strategies is limited by stringent amino acid selectivity. Here, we present highly reactive ligand-directed triggerable Michael acceptors (LD-TMAcs) that feature rapid protein labeling. Unlike previous approaches, the unique reactivity of LD-TMAcs enables multiple modifications on a single target protein, effectively mapping the ligand binding site. This capability is attributed to the tunable reactivity of TMAcs that enable the labeling of several amino acid functionalities via a binding-induced increase in local concentration while remaining fully dormant in the absence of protein binding. We demonstrate the target selectivity of these molecules in cell lysates using carbonic anhydrase as the model protein. Furthermore, we demonstrate the utility of this method by selectively labeling membrane-bound carbonic anhydrase XII in live cells. We envision that the unique features of LD-TMAcs will find use in target identification, investigation of binding/allosteric sites, and studying membrane proteins.

Mapping the effect of configuration and protecting group pattern on glycosyl acceptor reactivity.

The reactivity of the acceptor alcohol can have a tremendous influence on the outcome of a glycosylation reaction, both in terms of yield and stereoselectivity. Through a systematic survey of 67 acceptor alcohols in glycosylation reactions with two glucosyl donors we here reveal how the reactivity of a carbohydrate acceptor depends on its configuration and substitution pattern. The study shows how the functional groups flanking the acceptor alcohol influence the reactivity of the alcohol and show that both the nature and relative orientation play an essential role. The empiric acceptor reactivity guidelines revealed here will aid in the rational optimization of glycosylation reactions and be an important tool in the assembly of oligosaccharides.

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.

Synthesis of 7-Arylthiomethyl Dibenzo[b,d]azepines through Imidoylative Heck Cyclization and CPA-Catalyzed Thio-Michael Addition/Enantioselective Protonation.

A chiral phosphoric acid-catalyzed thio-Michael addition/enantioselective protonation has been developed for the first time. The reaction applies 7-methylene-6-aryl-7H-dibenzo[b,d]azepines, products of Pd-catalyzed imidoylative Heck cyclization, as Michael acceptors in reactions with a wide range of aryl thiols. Diversified 7-[(arylthio)methyl]-7H-dibenzo[b,d]azepines bearing a benzylic stereocenter and a thermodynamically regulated biaryl axis were produced with good to excellent enantioselectivity and 14-25:1 diastereoisomeric ratios.

Reactivity-informed design, synthesis, and Michael addition kinetics of C-ring andrographolide analogs

Andrographolide, a labdane diterpenoid lactone natural product extracted from Andrographis paniculata, has demonstrated potent biological activity and therapeutic potential against cancer, Alzheimer's disease, diabetes, and multiple sclerosis. Andrographolide is reported to significantly inhibit the NF-κB signaling pathway, which is active in immune system function and regulation of inflammatory cells. However, andrographolide use is not optimized for human systems. In order to improve upon these aspects, we designed and semi- synthesized a library of andrographolide analogues with modified electronics of the 12,13 unsaturated lactone to increase cytotoxicity via the addition of electron-withdrawing groups, thereby influencing reactivity of the compound. Reactivity of the top piece butenolide warhead was quantified via a time- resolved colorimetric ex vivo Michael addition assay using Elleman’s Method, wherein reduced-glutathione was the electron donor. The results demonstrate that the installation of an acetate ester at C14 did not result in greater Michael acceptor reactivity, whereas changing the electrophile to C14 with a C11-C12 alkene resulted in a significantly higher rate of reaction with glutathione. Further, computational studies were employed to model the Michael addition energetic pathways of andrographolide and each analog. We found the energetic pathways for all analogs were relatively consistent, suggesting that changes in sterics may play a more significant role in defining Michael addition kinetics. Through understanding the kinetic mechanisms of the Michael acceptor pharmacophore in andrographolide, we hope to further inform directed semisyntheses in order to optimize this incredibly potent natural product for clinical use.

Synthesis of Complex Diterpenes: Strategies Guided by Oxidation Pattern Analysis.

With complex molecular architectures, intriguing oxidation patterns, and wide-ranging biological activities, diterpene natural products have greatly impacted research in organic chemistry and drug discovery. Our laboratory has completed total syntheses of several highly oxidized diterpenes, including the ent-kauranoids maoecrystal Z, trichorabdal A, and longikaurin E; the antibiotic pleuromutilin; and the insecticides ryanodol, ryanodine, and perseanol. In this Account, we show how analysis of oxidation patterns and inherent functional group relationships can inform key C-C bond disconnections that greatly simplify the complexity of polycyclic structures and streamline their total syntheses. In articulating these concepts, we draw heavily from the approaches to synthetic strategy that were codified by Evans, Corey, Seebach, and others, based on the formalism that heteroatoms impose an alternating acceptor and donor reactivity pattern upon a carbon skeleton. We find these ideas particularly useful when considering oxidized diterpenes as synthetic targets.In the first part of the Account, we describe the use of reductive cyclizations as strategic tactics for building polycyclic systems with γ-hydroxyketone motifs. We have leveraged Sm-ketyl radical cyclizations as "reactivity umpolungs" to generate γ-hydroxyketones in our total syntheses of the Isodon ent-kauranoid diterpenes (-)-maoecrystal Z, (-)-longikaurin E, and (-)-trichorabdal A. Following this work, we identified the same γ-hydroxyketone pattern in the diterpene antibiotic (+)-pleuromutilin, which again inspired the use of a SmI2-mediated reductive cyclization, this time to construct a bridging eight-membered ring. This collection of four total syntheses highlights how reductive cyclizations are particularly effective umpolung tactics when used to simultaneously form rings and introduce 1,4-dioxygenation patterns.In the second part of the Account, we detail the syntheses of the complex and highly oxidized ryanodane and isoryanodane diterpenes and present the oxidation pattern analysis that guided our synthetic designs. We first discuss our 15-step total synthesis of (+)-ryanodol, which incorporated five of the eight oxygen atoms in just two transformations: a dihydroxylation of (S)-pulegone and a SeO2-mediated trioxidation of the A-ring cyclopentenone. This latter transformation gave rise to an independent investigation of SeO2-mediated peroxidations of simple bicyclic cyclopent-2-en-1-ones. The syntheses of (+)-ryanodine and (+)-20-deoxyspiganthine are also presented, which required modified end-game strategies to selectively incorporate the key pyrrole-2-carboxylate ester. Finally, we describe our fragment coupling approach to prepare the isoryanodane diterpene (+)-perseanol. Using a similar oxidation pattern analysis to that developed in the synthesis of ryanodol, we again identified a two-stage strategy to install the five hydroxyl groups. This strategy was enabled by a Pd-mediated carbopalladation/carbonylation cascade and leveraged unexpected, emergent reactivity to sequence a series of late-stage oxidations.While each of the diterpene natural products discussed in this Account present unique synthetic questions, we hope that through their collective discussion, we provide a conceptual framework that condenses and summarizes the chemical knowledge we have learned and inspires future discourse and innovations in strategy design and methodology development.

Synthesis of the hyper-branched core tetrasaccharide motif of chloroviruses.

Chemical synthesis of complex oligosaccharides, especially those possessing hyper-branched structures with one or multiple 1,2-cis glycosidic bonds, is a challenging task. Complementary reactivity of glycosyl donors and acceptors and proper tuning of the solvent/temperature/activator coupled with compromised glycosylation yields for sterically congested glycosyl acceptors are among several factors that make such syntheses daunting. Herein, we report the synthesis of a semi-conserved hyper-branched core tetrasaccharide motif from chloroviruses which are associated with reduced cognitive function in humans as well as in mouse models. The target tetrasaccharide contains four different sugar residues in which l-fucose is connected to d-xylose and l-rhamnose via a 1,2-trans glycosidic bond, whereas with the d-galactose residue is connected through a 1,2-cis glycosidic bond. A thorough and comprehensive study of various accountable factors enabled us to install a 1,2-cis galactopyranosidic linkage in a stereoselective fashion under [Au]/[Ag]-catalyzed glycosidation conditions en route to the target tetrasaccharide motif in 14 steps.

Determining Michael Acceptor Reactivity from Kinetic, Mechanistic, and Computational Analysis for the Base-catalyzed Thiol-Michael Reaction.

A combined experimental and computational study of the reactivities of seven commonly used Michael acceptors paired with two thiols within the framework of photobase-catalyzed thiol-Michael reactions is reported. The rate coefficients of the propagation (kP), reverse propagation (k-P), chain-transfer (kCT), and overall reaction (koverall) were experimentally determined and compared with the well-accepted electrophilicity parameters of Mayr and Parr, and DFT-calculated energetics. Both Mayr's and Parr's electrophilicity parameters predict the reactivities of these structurally varying vinyl functional groups well, covering a range of overall reaction rate coefficients from 0.5 to 6.2 s-1. To gain insight into the individual steps, the relative energies have been calculated using DFT for each of the stationary points along this step-growth reaction between ethanethiol and the seven alkenes. The free energies of the individual steps reveal the underlying factors that control the reaction barriers for propagation and chain transfer. Both the propagation and chain transfer steps are under kinetic control. These results serve as a useful guide for Michael acceptor selection to design and predict thiol-Michael-based materials with appropriate kinetic and material properties.

Azidophenylselenylation of glycals towards 2-azido-2-deoxy-selenoglycosides and their application in oligosaccharide synthesis

Abstract 2-Amino-2-deoxy-pyranosyl units are important structural components of cell-wall polymers in prokaryotes, fungi and mammals. With respect to the need for development of novel and efficient vaccines and tools for serodiagnosis of infectious diseases, of particular interest are the oligosaccharide cell-wall antigens of pathogenic bacteria and fungi, which comprise 2-amino-2-deoxy-D-glucopyranose and 2-amino-2-deoxy-D-galactopyranose units as α- or β-anomers. Synthesis of N-acylated α-GlcN and α-GalN containing oligosaccharides is a special challenge due to the presence of a participating group at C2 which favors the formation of β- rather than α-glycoside bond. Herein we overview the efficient two-step approach for preparation of 1,2-cis-glycosides of 2-amino-2-deoxy-D-glucopyranose and 2-amino-2-deoxy-D-galactopyranose, which was recently developed in our laboratory. In the first step, an efficient and straightforward azidophenylselenylation procedure of glycals gives phenyl 2-azido-2-deoxy-1-selenoglycosides as versatile glycosyl donors. In the second step, these donors can be efficiently transformed into α- or β-glycosides depending on the choice of the solvent. In acetonitrile, total β-stereocontrol was achieved, and the use of diethyl ether as a solvent favouring α-stereoselectivity of glycosylations with phenyl 2-azido-2-deoxy-1-selenoglycosides. Besides, it was shown, that low reactivity and nucleophilicity of glycosyl acceptors which are glycosylated with phenyl 2-azido-2-deoxy-1-selenogalactosides facilitated the formation of α-GalN derivatives. To date, homogenous azidophenylselenylation of glycals and glycosylation with phenyl 2-azido-2-deoxy-1-seleno-α-D-glycopyranosides can be regarded as most useful tool for introduction of 2-amino-2-deoxy-D-glycopyranoside residues into complex synthetic oligosaccharides.

Relationship among structure, cytotoxicity, and Michael acceptor reactivity of quinocidin

Quinocidin (QCD) is a cytotoxic antibiotic with an unusual 3,4-dihydroquinolizinium skeleton. We previously found that QCD captures thiols in neutral aqueous media via a Michael addition-type reaction. However, it remains unclear whether the Michael acceptor reactivity of QCD is responsible for its cytotoxicity. In this study, we synthesized thirteen analogs of QCD to examine the relationship among its structure, cytotoxicity, and reactivity toward thiols. Thiol-trapping experiments and cytotoxicity tests collectively suggested that the Michael acceptor function of QCD is independent of its cytotoxic activity, and that the pyridinium moiety with the hydrophobic side chain is a key structural factor for cytotoxicity. These findings further led us to demonstrate that incorporation of an amide group into the side chain of QCD significantly reduced its toxicity but hardly affected the Michael acceptor function. The present study lays the foundation for QCD-based drug design and highlights the potential of QCD as a unique electrophile for use in the development of covalent inhibitors and protein-labeling probes.

α-Selective glycosylations using glycosyl N-(ortho-methoxyphenyl)trifluoroacetimidates.

Six N-(o-methoxyphenyl)trifluoroacetimidate glycosyl donors have been synthesized and their role as glycosyl donors has been investigated. The donors were synthesized with complete β-selectivity, except in one case, and were found to be stable. When Bi(OTf)3, Fe(OTf)2, and Zn(OTf)2 were employed as catalysts, the glycosylations were found to be highly α-selective in Et2O. The selectivity and reaction rate changed with a change in the acceptor reactivity.

The first total synthesis of rebaudioside R.

With easily available monosaccharides and steviol as starting materials, the first total synthesis of rebaudioside R with a xylosyl core in the C13-OH linked sugar chain was accomplished via two distinct approaches. The first approach features the stepwise installation of branch-sugar residues via an order of C2-OH first and then C3-OH of the xylosyl core, laying a firm foundation for the synthesis of analogues with different branch sugars, while the second route features the introduction of the C13 trisaccharide sugar chain via a convergent strategy, securing the overall synthetic efficiency. Through the synthetic study, the effect of protecting groups (PGs) at the vicinal hydroxy group on the reactivity of OH acceptors was illustrated.

Use of remote acyl groups for stereoselective 1,2-cis-glycosylation with fluorinated glucosazide thiodonors.

Fluorinated glycans are valuable probes for studying carbohydrate-protein interactions at the atomic level. Glucosamine is a ubiquitous component of glycans, and the stereoselective synthesis of α-linked fluorinated glucosamine is a challenge associated with the chemical synthesis of fluorinated glycans. We found that introducing a 6-O-acyl protecting group onto 3-fluoro and 4-fluoro glucosazide thiodonors endowed them with moderate α-selectivity in the glycosylation of carbohydrate acceptors, which was further improved by adjusting the acceptor reactivity via O-benzoylation. Excellent stereoselectivity was achieved for 3,6-di-O-acyl-4-fluoro analogues. The glycosylation of threonine-derived acceptors enabled the stereoselective synthesis of the protected fluorinated analogue of α-GlcNAc-O-Thr, a moiety abundant in cell-surface O-glycans of the protozoan parasite Trypanosoma cruzi. DFT calculations supported the involvement of transient cationic species which resulted from the stabilization of the oxocarbenium ion through O-6 acyl group participation.

Nucleophilicity of Glutathione: A Link to Michael Acceptor Reactivities.

Deprotonated glutathione is among the most potent biological nucleophiles and plays an important physiological role in cellular detoxification by forming covalent conjugates with Michael acceptors. The electrophilicity E of various Michael acceptors was characterized recently according to the Patz–Mayr relation lg k 2=s N(N+E). We now determined the nucleophilic reactivity (N, s N) of glutathione (GSH) in aqueous solution at 20 °C to connect published GSH reactivities (k GSH) with Mayr's electrophilicity scale (E). In this way, electrophilicities E of more than 70 Michael acceptors could be estimated, which can now be used to systematically predict novel reactions with the multitude of nucleophiles whose nucleophilicity parameters N/s N are known.

Activation of Cellobiose Dehydrogenase Bioelectrocatalysis by Carbon Nanoparticles

AbstractProtein‐nanoparticle (NP) interactions offer a powerful tool to modulate enzymatic activity and activate biocatalysis. However, strong scattering effects may complicate conventional spectrophotometric studies of NP‐assisted biocatalysis. Here, bioelectrocatalytic oxidation of cellobiose by the Humicola insolens cellobiose dehydrogenase (CDH) was electrochemically interrogated with two redox mediators, ferricyanide and cytochrome c, and two types of carbon NPs (CNPs), unmodified (CNP) and negatively charged 4‐sulfophenyl‐modified (CNP‐) ones. We show that both CNPs activate the bioelectrocatalytic oxidation of cellobiose and improve the enzymatic specificity reflected in the kf/KM relationship of 6680 (no CNPs), 15500 (CNP‐) and 10444 M−1 s−1 (CNP) shown for ferricyanide‐mediated and 1262 (no CNPs), 3630 (CNP‐), and 6375 M−1 s−1 (CNP) for cytochrome c‐mediated reactions. The bioelectrocatalysis activation by CNPs apparently results from the interfacial/conformational regulation of the CDH activity and the CDH‐electron acceptor reactivity, which may also be the case of other nanomaterials such as frequently used in bioelectrochemistry carbon nanotubes.