Mechanism of thio acid/azide amidation

Synthetic Sialosides Terminated with 8-N-Substituted Sialic Acid as Selective Substrates for Sialidases from Bacteria and Influenza Viruses

Sialosides containing C8-modified sialic acids are challenging synthetic targets but potentially useful probes for diagnostic substrate profiling of sialidases and elucidating the binding specificity of sialic acid-interacting proteins. Here, we demonstrate efficient chemoenzymatic methods for synthesizing para-nitrophenol-tagged α2-3- and α2-6-linked sialyl galactosides containing C8-acetamido, C8-azido, or C8-amino derivatized N-acetylneuraminic acid (Neu5Ac). High-throughput substrate specificity studies showed that the C8-modification of sialic acid significantly changes its recognition by sialidases from humans, various bacteria, and different influenza A and B viruses. Sialosides carrying Neu5Ac with a C8-azido modification were generally well tolerated by all the sialidases we tested, whereas sialosides containing C8-acetamido-modified Neu5Ac were only cleaved by selective bacterial sialidases. In contrast, sialosides with C8-amino-modified Neu5Ac were cleaved by a combination of selective bacterial and influenza A virus sialidases. These results indicate that sialosides terminated with a C8-amino or C8-acetamido-modified sialic acid can be used with other sialosides for diagnostic profiling of disease-causing sialidase-producing pathogens.

N-Acylsulfonamide: a valuable moiety to design new sulfa drug analogues

Sulfonamides are the oldest class of antibiotics, discovered more than 80 years ago. They are still used today despite the appearance of drug resistance phenomena that limit their prescription. Since the discovery and use of the first sulfa drugs, many analogues have been synthesized in order to obtain new active molecules able to circumvent bacterial resistance. Structurally similar to sulfonamide, the N-acylsulfonamide group arouses interest in the field of medicinal chemistry due to specific physico-chemical properties. We report here the synthesis and antibacterial/antibiofilm activities of 18 sulfa drug analogues with an N-acylsulfonamide moiety. These derivatives were obtained efficiently by sulfo-click reactions between readily available thioacid and sulfonyl azide synthons.

Redox-Neutral Dehydrogenative Cross-Coupling of Alcohols and Amines Enabled by Nickel Catalysis

Presented herein is a facile and straightforward synthetic method for the construction of amides via Ni/NHC-catalyzed amidation of alcohols with amines. The strategy exhibits various advantages over existing methods, including...

Pd-Catalyzed Oxidative Aminocarbonylation of Arylboronic Acids with Unreactive Tertiary Amines via C-N Bond Activation

An efficient synthesis of tertiary amides from aryl boronic acids and inert tertiary amines through the oxidative carbonylation via C(sp³)-N bond activation is presented. This protocol significantly restricts the homocoupling biarylketone product. It involves the use of a homogeneous PdCl₂/CuI catalyst and a heterogeneous Pd/C based catalyst, which promotes C(sp³)-N bond activation of tertiary amines with aryl boronic acids. This process represents a ligand-free, base-free, and recyclable catalyst along with an ideal oxidant like molecular oxygen.

The effect of deoxyfluorination and O-acylation on the cytotoxicity of N-acetyl-D-gluco- and D-galactosamine hemiacetals

Fully acetylated deoxyfluorinated hexosamine analogues and non-fluorinated 3,4,6-tri-O-acylated N-acetyl-hexosamine hemiacetals have previously been shown to display moderate anti-proliferative activity. We prepared a set of deoxyfluorinated GlcNAc and GalNAc hemiacetals that comprised both features: O-acylation at the non-anomeric positions with an acetyl, propionyl and butanoyl group, and deoxyfluorination at selected positions. Determination of the in vitro cytotoxicity towards the MDA-MB-231 breast cancer and HEK-293 cell lines showed that deoxyfluorination enhanced cytotoxicity in most analogues. Increasing the ester alkyl chain length had a variable effect on the cytotoxicity of fluoro analogues, which contrasted with non-fluorinated hemiacetals where butanoyl derivatives had always higher cytotoxicity than acetates. Reaction with 2-phenylethanethiol indicated that the recently described S-glyco-modification is an unlikely cause of cytotoxicity.

Synthesis of multiply fluorinated N-acetyl-D-glucosamine and D-galactosamine analogs via the corresponding deoxyfluorinated glucosazide and galactosazide phenyl thioglycosides

Multiple fluorination of glycostructures has emerged as an attractive way of modulating their protein affinity, metabolic stability, and lipophilicity. Here we described the synthesis of a series of mono-, di- and trifluorinated N-acetyl-ᴅ-glucosamine and ᴅ-galactosamine analogs. The key intermediates are the corresponding multiply fluorinated glucosazide and galactosazide thioglycosides prepared from deoxyfluorinated 1,6-anhydro-2-azido-β-ᴅ-hexopyranose precursors by ring-opening reaction with phenyl trimethylsilyl sulfide. Nucleophilic deoxyfluorination at C4 and C6 by reaction with DAST, thioglycoside hydrolysis and azide/acetamide transformation completed the synthesis.

Synthetic Studies on the Incorporation of N-Acetylallosamine in Hyaluronic Acid-Inspired Thiodisaccharides

Two approaches for the synthesis of the thiodisaccharide β-S-GlcA(1→3)β-S-AllNAc are described here. The target disaccharide was a C-3 epimer and thio-analogue of the hyaluronic acid repetitive unit, tuned with a thiopropargyl anomeric group for further click conjugation. Thus, we analysed and tested two convenient sequences, combining the two key steps required to introduce the thioglycosidic bonds and consequently reach the target molecule: the SN2 substitution of a good leaving group (triflate) present at C-3 of a GlcNAc derivative and the introduction of the anomeric thiopropargyl substituent. The use of a 2-azido precursor showed to be a convenient substrate for the SN2 step. Nevertheless, further protecting group manipulation and the introduction of the thiopropargyl anomeric residue were then required. This approach showed to provide access to a variety of thiodisaccharide derivatives as interesting building blocks for the construction of neoglycoconjugates.

The Sulfo-Click Reaction and Dual Labeling of Nucleosides

This article contains detailed synthetic procedures for the implementation of the sulfo-click reaction to nucleoside derivatives. First, 3'-O-TBDMS-protected nucleosides are converted to their corresponding 4'-thioacid derivatives in three steps. Then, various conjugates are synthetized via a biocompatible and chemoselective coupling procedure using sulfonyl azide partners. Finally, to illustrate the potential of the sulfo-click reaction, a nucleoside bearing two orthogonal azido groups is synthesized and engaged in one-pot dual labeling through a sulfo-click/copper-catalyzed azide-alkyne cycloaddition (CuAAC) cascade. The high efficiency of the sulfo-click reaction as applied to nucleosides opens up new possibilities in the context of bioconjugation. © 2020 Wiley Periodicals LLC. Basic Protocol 1: General protocol for the synthesis of 4'-thioacid-nucleoside derivatives Basic Protocol 2: Implementation of the sulfo-click reaction Basic Protocol 3: Synthesis of 3'-azido-4'-(carboxamido)ethane-sulfonyl azide-3'-deoxythymidine Basic Protocol 4: Detailed synthetic procedure for one-pot double-click conjugations.

Electrophilic Azides for Materials Synthesis and Chemical Biology

Organic azides are involved in a variety of useful transformations, including nitrene chemistry, reactions with nucleophiles and electrophiles, and cycloadditions. The 1,3-dipolar cycloadditions of azides constitute a major class of highly reliable and versatile reactions, as shown by the development and rapid adoption of click chemistry and bioorthogonal chemistry. Metal-catalyzed azide-alkyne cycloaddition (Cu/RuAAC), the prototypical click reaction, has found wide utility in pharmaceutical, biomedical, and materials sciences. The strain-promoted, or distortion-accelerated, azide-alkyne cycloaddition eliminates the need for a metal catalyst.In the azide-mediated 1,3-dipolar cycloaddition reactions, azides are ambiphilic, i.e., HOMO-LUMO-controlled dipoles where both the HOMO and LUMO interact strongly with the dipolarophile. Azide-alkyne cycloaddition proceeds primarily through the HOMO_azide-LUMO_{dipolarophile} interaction, and electron-deficient dipolarophiles react more readily. The inverse-electron-demand reaction, involving the LUMO_azide-HOMO_{dipolarophile} interaction, is less common because of the low stability of electron-deficient azides such as acyl, sulfonyl, and phosphoryl azides. Nevertheless, there have been reports since the 1960s showing enhanced reaction kinetics between electron-poor azides and electron-rich dipolarophiles. Our laboratory has developed the use of perfluoroaryl azides (PFAAs), a class of stable electron-deficient azides, as nitrene precursors and for reactions with nucleophiles and electron-rich dipolarophiles. Perfluorination on the aryl ring also facilitates the synthesis of PFAAs and quantitative analysis of the products by ¹⁹F NMR spectroscopy.In this Account, we summarize key reactions involving electrophilic azides and applications of these reactions in materials synthesis and chemical biology. These electron-deficient azides exhibit unique reactivity toward nucleophiles and electron-rich or strained dipolarophiles, in some cases leading to new transformations that do not require any catalysts or products that are impossible to obtain from the nonelectrophilic azides. We highlight work from our laboratories on reactions of PFAAs with enamines, enolates, thioacids, and phosphines. In the reactions of PFAAs with enamines or enolates, the triazole or triazoline cycloaddition products undergo further rearrangement to give amidines or amides as the final products at rates of up to 10⁵ times faster than their non-fluorinated anlogues. Computational investigations by the distortion/interaction activation strain model reveal that perfluorination lowers the LUMO of the aryl azide as well as the overall activation energy of the reaction by decreasing the distortion energies of the reactants to reach the transition states. The PFAA-enamine reaction can be carried out in a one-pot fashion using readily available starting materials of aldehyde and amine, making the reaction especially attractive, for example, in the functionalization of nanomaterials and derivatization of antibiotics for the preparation of theranostic nanodrugs. Similar fast kinetics was also observed for the PPAA-mediated Staudinger reaction, which proceeds at 10⁴ times higher rate than the classic Staudinger ligation, giving stable phosphoimines in high yields. The reaction is biorthogonal, allowing cell-surface labeling with minimal background noise.

Synthesis of (1→3) Thiodisaccharides of GlcNAc and the Serendipitous Formation of 2,3-Dideoxy-(1→2)-thiodisaccharides through a Vinyl Azide Intermediate

The syntheses of β-S-GlcA(1→3)GlcNAc and β-S-Gal(1→3)GlcNAc thiodisaccharides, which can be considered mimetics of the repeating units of hyaluronan and keratan respectively, were achieved by S_N2 displacement of a triflate group allocated at the 3-position of a convenient 2-azido-4,6-O-benzylidene-2-deoxy-β-d-allopyranose precursor by the corresponding nucleophilic suitable protected thioaldoses derived from glucuronic acid (GlcA) and galactose (Gal). The study of the reaction led to the finding that the vinyl azide formed by competitive E2 reaction of the mentioned triflate was an interesting precursor of a new kind of 2,3-dideoxy-2-azido-(1→2) thiodisaccharides through an addition reaction. Determination of the stereochemistry of the new stereocenter at C-2 was achieved by NOESY experiments. Final protecting group manipulation of the (1→3) thiodisaccharides led to a family of derivatives that could be used as building blocks for the synthesis of complex glycomimetics.

Synthesis of thioureido peptidomimetics employing alkyl azides and dithiocarbamates

An unprecedented approach for the assembly of thioureido peptidomimetics is developed employing alkyl azides and dithiocarbamates. Dithiocarbamates react with alkyl azides with the liberation of N2 and elemental sulfur thereby leading to thiourea in a traceless manner. Thioureido peptidomimetics are thus furnished in good yields with no epimerization. This process is mild, free from the use of a base, scalable and step economic. The practicability of this methodology has been highlighted by the synthesis of N,N'-orthogonally protected thioureido peptidomimetics.

Efficient and phosphine-free bidentate N-heterocyclic carbene/ruthenium catalytic systems for the dehydrogenative amidation of alcohols and amines

An efficient and phosphine-free bidentate NHC/Ru catalytic system was discovered for the dehydrogenative amide synthesis from alcohols and amines.

Sulfo-click chemistry with 18F-labeled thio acids

The first application of sulfo-click chemistry with ¹⁸F-labeled thio acids is described.

Assembly of glycoamino acid building blocks: a new strategy for the straightforward synthesis of heparan sulfate mimics

A new strategy that enables a modular straightforward synthesis of heparan sulfate oligosaccharide mimics by the assembly of simple glycoamino acid building blocks is described. The coupling between units is readily carried out by an amidation reaction. Several glycoamino acid oligomers were prepared and their interaction with the FGF2 protein was analyzed.

Ruthenium-Based Catalytic Systems Incorporating a Labile Cyclooctadiene Ligand with N-Heterocyclic Carbene Precursors for the Atom-Economic Alcohol Amidation Using Amines

Transition-metal-catalyzed amide-bond formation from alcohols and amines is an atom-economic and eco-friendly route. Herein, we identified a highly active in situ N-heterocyclic carbene (NHC)/ruthenium (Ru) catalytic system for this amide synthesis. Various substrates, including sterically hindered ones, could be directly transformed into the corresponding amides with the catalyst loading as low as 0.25 mol.%. In this system, we replaced the p-cymene ligand of the Ru source with a relatively labile cyclooctadiene (cod) ligand so as to more efficiently obtain the corresponding poly-carbene Ru species. Expectedly, the weaker cod ligand could be more easily substituted with multiple mono-NHC ligands. Further high-resolution mass spectrometry (HRMS) analyses revealed that two tetra-carbene complexes were probably generated from the in situ catalytic system.

An Optimized Preparation of 1,1-Dimethylallyl Esters and Their Application to Solid-Phase Peptide Synthesis

A one-step preparation of 1,1-dimethylallyl (DMA) esters was optimized for the C-terminal protection of a range of Fmoc-protected amino acids. This preparation is not sensitive to the scale of reaction and affords the corresponding DMA esters in 70-99% yield with high regioselectivity. Additionally, these DMA-protected amino acids were used with the backbone amide linker (BAL) of Albericio and Barany and found to resist diketopiperazine formation during the synthesis of a series of tripeptide esters. C-terminal DMA protection is compatible with the BAL linkage and allows for standard Fmoc-based methods to be used throughout the synthesis.

Total Synthesis of Zwitterionic Tetrasaccharide Repeating Unit from Bacteroides fragilis ATCC 25285/NCTC 9343 Capsular Polysaccharide PS A1 with Alternating Charges on Adjacent Monosaccharides

The tetrasaccharide repeating unit of zwitterionic polysaccharide A1 (PS A1) from Bacteroides fragilis ATCC 25285/NCTC 9343 has been synthesized using a linear glycosylation approach. One key step includes an α(1,4)-stereoselective [2 + 1] glycosylation of a 2,4,6-trideoxy-2-acetamido-4-amino-d-Gal p (AAT) donor with a poorly reactive axial C4-OH disaccharide acceptor. Mild acid-mediated deacetylation and a challenging [3 + 1] glycosylation are also highlighted. The strategy is inclusive of a single-pot, three-step deprotection affording PS A1 with alternating charges on adjacent monosaccharide units.

Computational study of the mechanism of amide bond formation via CS2-releasing 1,3-acyl transfer

A systematic computational study on CS₂-releasing 1,3-acyl transfer was performed for the first time and provided deeper mechanistic insights.

Thioacids – synthons for amide bond formation and ligation reactions: assembly of peptides and peptidomimetics

The synthesis of α-amino thioacids and peptide thioacids and their applications in chemoselective amide bond formation, ligation of peptides/proteins/glycopeptides and synthesis of peptidomimetics are reviewed.

Thio acid-mediated conversion of azides to amides - exploratory studies en route to oroidin alkaloids

The utility of the thio acid-azide coupling reaction to afford amides is explored in imidazole-containing substrates for application in the total synthesis of examples of oroidin alkaloids. Good yields of the expected amides are obtained in both monomeric and dimeric substrates. Bis azides react preferentially at the 2-azido position but hydrosulfenylation and reduction interfere. 2-Thiophenyl and 2-oxo groups were evaluated as 2-amino surrogates, the thioether delivered the expected amide, whereas 2-imidazolone gave a mixture of the expected amide and the hydrosulfenylation product.

N-Alkylation and Aminohydroxylation of 2-Azidobenzenesulfonamide Gives a Pyrrolobenzothiadiazepine Precursor Whereas Attempted N-Alkylation of 2-Azidobenzamide Gives Benzotriazinones and Quinazolinones

N-Alkylation of 2-azidobenzenesulfonamide with 5-bromopent-1-ene gave an N-pentenyl sulfonamide, which underwent intramolecular aminohydroxylation to give an N-(2-azidoaryl)sulfonyl prolinol, a precursor for the synthesis of a pyrrolobenzothiadiazepine. The attempted N-alkylation of 2-azidobenzamide gave a separable mixture (∼1:1) of a benzotriazinone and a quinazolinone in a 72% combined yield. Other primary alkyl halides (3 examples) gave similar mixtures of benzotriazinones and quinazolinones. Benzylic, allylic, and secondary and tertiary alkyl halides (5 examples) gave only benzotriazinones in moderate yields. The results of mechanistic studies show the likely involvement of nitrene intermediates in the quinazolinone pathway and a second pathway involving a dimethylsulfoxide or dimethylsulfide-mediated conversion of 2-azidobenzamide into benzotriazinones.