Ruthenium-catalyzed Azide-alkyne Cycloaddition Research Articles

Recent Developments in the Ruthenium‐Catalyzed Azide Alkyne Cycloaddition (RuAAC) Reaction

AbstractThe ruthenium‐catalyzed azide‐alkyne cycloaddition (RuAAC) gives access to 1,5‐disubstitued triazoles in a single atom‐economical step and offers advantages compared to its copper‐catalyzed counterpart in that both terminal and internal alkynes can be employed. This review summarizes recent findings in this field during the last eight years, covering mechanistic investigations, synthetic developments, as well as applications in medicinal chemistry, polymer synthesis and physical organic chemistry.

Bioinspired Selenium-Nitrogen Exchange (SeNEx) Click Chemistry Suitable for Nanomole-Scale Medicinal Chemistry and Bioconjugation.

Click chemistry is a powerful molecular assembly strategy for rapid functional discovery. The development of click reactions with new connecting linkage is of great importance for expanding the click chemistry toolbox. We report the first selenium-nitrogen exchange (SeNEx) click reaction between benzoselenazolones and terminal alkynes (Se-N to Se-C), which is inspired by the biochemical SeNEx between Ebselen and cysteine (Cys) residue (Se-N to Se-S). The formed selenoalkyne connection is readily elaborated, thus endowing this chemistry with multidimensional molecular diversity. Besides, this reaction is modular, predictable, and high-yielding, features fast kinetics (k2≥14.43 M-1 s-1), excellent functional group compatibility, and works well at miniaturization (nanomole-scale), opening up many interesting opportunities for organo-Se synthesis and bioconjugation, as exemplified by sequential click chemistry (coupled with ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) and sulfur-fluoride exchange (SuFEx)), selenomacrocycle synthesis, nanomole-scale synthesis of Se-containing natural product library and DNA-encoded library (DEL), late-stage peptide modification and ligation, and multiple functionalization of proteins. These results indicated that SeNEx is a useful strategy for new click chemistry developments, and the established SeNEx chemistry will serve as a transformative platform in multidisciplinary fields such as synthetic chemistry, material science, chemical biology, medical chemistry, and drug discovery.

Bioinspired Selenium‐Nitrogen Exchange (SeNEx) Click Chemistry Suitable for Nanomole‐Scale Medicinal Chemistry and Bioconjugation

AbstractClick chemistry is a powerful molecular assembly strategy for rapid functional discovery. The development of click reactions with new connecting linkage is of great importance for expanding the click chemistry toolbox. We report the first selenium‐nitrogen exchange (SeNEx) click reaction between benzoselenazolones and terminal alkynes (Se−N to Se−C), which is inspired by the biochemical SeNEx between Ebselen and cysteine (Cys) residue (Se−N to Se−S). The formed selenoalkyne connection is readily elaborated, thus endowing this chemistry with multidimensional molecular diversity. Besides, this reaction is modular, predictable, and high‐yielding, features fast kinetics (k2≥14.43 M−1 s−1), excellent functional group compatibility, and works well at miniaturization (nanomole‐scale), opening up many interesting opportunities for organo‐Se synthesis and bioconjugation, as exemplified by sequential click chemistry (coupled with ruthenium‐catalyzed azide‐alkyne cycloaddition (RuAAC) and sulfur‐fluoride exchange (SuFEx)), selenomacrocycle synthesis, nanomole‐scale synthesis of Se‐containing natural product library and DNA‐encoded library (DEL), late‐stage peptide modification and ligation, and multiple functionalization of proteins. These results indicated that SeNEx is a useful strategy for new click chemistry developments, and the established SeNEx chemistry will serve as a transformative platform in multidisciplinary fields such as synthetic chemistry, material science, chemical biology, medical chemistry, and drug discovery.

Synthesis of a tricyclic hexapeptide –via two consecutive ruthenium-catalyzed macrocyclization steps– with a constrained topology to mimic vancomycin's binding properties toward D-Ala-D-Ala dipeptide

A ring-closing metathesis (RCM) – peptide coupling – ruthenium-catalyzed azide alkyne cycloaddition (RuAAC) strategy was developed to synthesize a tricyclic hexapeptide in which the side chain to side chain connectivity pattern resulted in a mimic with a topology that effectively mimics the bioactivity of vancomycin as a potent binder of the bacterial cell wall D-Ala-D-Ala dipeptide sequence and more importantly being an effective inhibitor of bacterial growth.

Recent Advances in Macrocyclic Drugs and Microwave-Assisted and/or Solid-Supported Synthesis of Macrocycles.

Macrocycles represent attractive candidates in organic synthesis and drug discovery. Since 2014, nineteen macrocyclic drugs, including three radiopharmaceuticals, have been approved by FDA for the treatment of bacterial and viral infections, cancer, obesity, immunosuppression, etc. As such, new synthetic methodologies and high throughput chemistry (e.g., microwave-assisted and/or solid-phase synthesis) to access various macrocycle entities have attracted great interest in this chemical space. This article serves as an update on our previous review related to macrocyclic drugs and new synthetic strategies toward macrocycles (Molecules, 2013, 18, 6230). In this work, I first reviewed recent FDA-approved macrocyclic drugs since 2014, followed by new advances in macrocycle synthesis using high throughput chemistry, including microwave-assisted and/or solid-supported macrocyclization strategies. Examples and highlights of macrocyclization include macrolactonization and macrolactamization, transition-metal catalyzed olefin ring-closure metathesis, intramolecular C–C and C–heteroatom cross-coupling, copper- or ruthenium-catalyzed azide–alkyne cycloaddition, intramolecular SNAr or SN2 nucleophilic substitution, condensation reaction, and multi-component reaction-mediated macrocyclization, and covering the literature since 2010.

Untargeted Identification of Alkyne-Containing Natural Products Using Ruthenium-Catalyzed Azide Alkyne Cycloaddition Reactions Coupled to LC-MS/MS.

Alkyne-containing natural products have been identified from plants, insects, algae, fungi, and bacteria. This class of natural products has been characterized as having a variety of biological activities. Polyynes are a subclass of acetylenic natural products that contain conjugated alkynes and are underrepresented in natural product databases due to the fact that they decompose during purification. Here we report a workflow that utilizes alkyne azide cycloaddition (AAC) reactions followed by LC-MS/MS analysis to identify acetylenic natural products. In this report, we demonstrate that alkyne azide cycloaddition reactions with p-bromobenzyl azide result in p-bromobenzyl-substituted triazole products that fragment to a common brominated tropylium ion. We were able to identify a synthetic alkyne spiked into the extract of Anabaena sp. PCC 7120 at a concentration of 10 μg/mL after optimization of MS/MS conditions. We then successfully identified the known natural product fischerellin A in the extract of Fischerella muscicola PCC 9339. Lastly, we identified the recently identified natural products protegenins A and C from Pseudomonas protegens Pf-5 through a combination of genome mining and RuAAC reactions. This is the first report of RuAAC reactions to detect acetylenic natural products. We also compare CuAAC and RuAAC reactions and find that CuAAC reactions produce fewer byproducts compared to RuAAC but is limited to terminal-alkyne-containing compounds. In contrast, RuAAC is capable of identification of both terminal and internal acetylenic natural products, but byproducts need to be eliminated from analysis by creation of an exclusion list. We believe that both CuAAC and RuAAC reactions coupled to LC-MS/MS represent a method for the untargeted identification of acetylenic natural products, but each method has strengths and weaknesses.

A minireview of 1,2,3-triazole hybrids with O-heterocycles as leads in medicinal chemistry.

Over the past few decades, the dynamic progress in the synthesis and screening of heterocyclic compounds against various targets has made a significant contribution in the field of medicinal chemistry. Among the wide array of heterocyclic compounds, triazole moiety has attracted the attention of researchers owing to its vast therapeutic potential and easy preparation via copper and ruthenium-catalyzed azide-alkyne cycloaddition reactions. Triazole skeletons are found as major structural components in a different class of drugs possessing diverse pharmacological profiles including anti-cancer, anti-bacterial, anti-fungal, anti-viral, anti-oxidant, anti-inflammatory, anti-diabetic, anti-tubercular, and anti-depressant among various others. Furthermore, in the past few years, a significantly large number of triazole hybrids were synthesized with various heterocyclic moieties in order to gain the added advantage of the improved pharmacological profile, overcoming the multiple drug resistance and reduced toxicity from molecular hybridization. Among these synthesized triazole hybrids, many compounds are available commercially and used for treating different infections/disorders like tazobactam and cefatrizine as potent anti-bacterial agents while isavuconazole and ravuconazole as anti-fungal activities to name a few. In this review, we will summarize the biological activities of various 1,2,3-triazole hybrids with copious oxygen-containing heterocycles as lead compounds in medicinal chemistry. This review will be very helpful for researchers working in the field of molecular modeling, drug design and development, and medicinal chemistry.

1,5-Disubstituted 1,2,3-Triazole-Containing Peptidotriazolamers: Design Principles for a Class of Versatile Peptidomimetics.

Peptidotriazolamers are hybrid foldamers combining features of peptides and triazolamers-repetitive peptidomimetic structures with triazoles replacing peptide bonds. We report on the synthesis of a new class of peptidomimetics, containing 1,5-disubstituted 1,2,3-triazoles in an alternating fashion with amide bonds and the analysis of their conformation in solid state and solution. Homo- or heterochiral peptidotriazolamers were obtained from enantiomerically pure propargylamines with stereogenic centers in the propargylic position and α-azido esters by ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) under microwave conditions in high yields. With such building blocks the peptidotriazolamers are readily available by solution phase synthesis. While the conformation of the homochiral peptidotriazolamer Boc-Ala[5Tz]Phe-Val[5Tz]Ala-Leu[5Tz]Val-OBzl resembles that of a β VIa1 turn, the heterochiral peptidotriazolamer Boc-d-Ala[5Tz]Phe-d-Val[5Tz]Ala-d-Leu[5Tz]Val-OBzl adopts a polyproline-like repetitive structure.

Synthesis of bicyclic tripeptides inspired by the ABC-ring system of vancomycin through ruthenium-based cyclization chemistries

The synthesis of a bicyclic tripeptide that mimics the ABC ring system of vancomycin is described by using a ring closing metathesis (RCM) – peptide coupling – ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) strategy.

Ruthenium-Catalyzed Azide Alkyne Cycloaddition Reaction: Scope, Mechanism, and Applications.

The ruthenium-catalyzed azide alkyne cycloaddition (RuAAC) affords 1,5-disubstituted 1,2,3-triazoles in one step and complements the more established copper-catalyzed reaction providing the 1,4-isomer. The RuAAC reaction has quickly found its way into the organic chemistry toolbox and found applications in many different areas, such as medicinal chemistry, polymer synthesis, organocatalysis, supramolecular chemistry, and the construction of electronic devices. This Review discusses the mechanism, scope, and applications of the RuAAC reaction, covering the literature from the last 10 years.

Synthesis of Substituted 1,2,3-Triazoles via Metal-Free Click Cycloaddition Reactions and Alternative Cyclization Methods

Click chemistry is one of the most efficient and widely used methodologies for selective C–N bond forming reactions, encompassing the synthesis of 1,2,3-triazoles, and it has been continued to be of great interest in synthetic and medicinal chemistry since its discovery. Diversely substituted 1,2,3-triazoles synthesized by classical Huisgen 1,3-dipolar cycloaddition of azides with alkynes have a broad spectrum of biological properties and they are widely accessed using transition-metal catalysts, such as copper- and ruthenium-catalyzed azide-alkyne cycloaddition [MAAC] reactions. In contrast, the development of organocatalyzed click chemistry has recently emerged as one of the most important alternative strategies to transition-metal-mediated reactions, because of its sustainability, environmentally benign processes and, moreover the absence of metals toxic to biological systems including living cells and biomolecules such as DNA. The focus of this review is on the recent advances in the organocatalytic click chemistry of enamine and enolate-mediated [3+2] cycloadditions and alternative metal-free cyclization reactions for the preparation of diversely functionalized 1,2,3-triazoles. The scope and limitations of these methods and also their applications for the synthesis of bioactive hybrid molecules are discussed. 1 Introduction 2 Enamine-Mediated Triazole Synthesis 3 Enolate-Mediated Triazole Synthesis 4 Alternative Metal-Free Triazole Synthesis 5 Applications to the Synthesis of Bioactive Compounds 6 Summary

1,2,3-Triazole-based analogue of benznidazole displays remarkable activity against Trypanosoma cruzi

The current treatment of Chagas disease is based on the use of two drugs, nifurtimox and benznidazole, which present limited efficacy in the chronic stage of the disease and toxic side effects. Although some progress has been made in the development of new drugs to treat this disease, the discovery of novel compounds is urgently required. In this work we report the synthesis and biological evaluation of 1,2,3-triazole-based analogues of benznidazole. A small series of 27 compounds was successfully synthesized via microwave-assisted copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) and ruthenium-catalyzed azide–alkyne cycloaddition (RuAAC) from N-benzyl-2-azidoacetamide (1) and a set of commercial terminal alkynes. Analogues 24 (IC50 40μM) and 28 (IC50 50μM) showed comparable activities to benznidazole (IC50 34μM) against trypomastigote form and analogue 15 (IC50 7μM) was found to be the most active. Regarding the cytotoxicity assessment of the series, most compounds were not cytotoxic. This work shows that the designed strategy is efficiently capable of generating novel benzindazole analogues and reveals one analogue is more active than benznidazole.

Synthesis of Fluorinated 1,4,5-Substituted 1,2,3-Triazoles by RuAAC­ Reaction

Herein, we report a convenient methodology for the synthesis of fluorinated 1,4,5-substituted 1,2,3-triazoles. The azide–alkyne cycloaddition reaction of internal alkynes catalyzed by a ruthenium complex efficiently afforded 2,2,2-trifluoroethyl- and (trifluoromethyl)thio-substituted 1,2,3-triazoles. Two types of internal alkyne, 1-aryl-2-(2,2,2-trifluoroethyl)acetylenes and 1-aryl-2-[(trifluoromethyl)thio]acetylenes, were used. This ruthenium-catalyzed azide–alkyne cycloaddition reaction was highly regioselective giving 4-aryl-5-(2,2,2-trifluoroethyl)- or 4-aryl-5-[(trifluoromethyl)thio]-1H-1,2,3-triazoles. Various functionalities were tolerated in the alkyl and aryl azides by this Huisgen 1,3-dipolar cycloaddition. All the triazoles were characterized by 1H, 13C, and 19F NMR, IR, and HRMS (or elemental analysis). Several triazoles were characterized by single-crystal X-ray structural analysis to confirm the regioselectivity of 1,2,3-triazole formation.

δ‐Peptides from RuAAC‐Derived 1,5‐Disubstituted Triazole Units

AbstractNon‐natural peptides with structures and functions similar to natural peptides have emerged lately in biomedical as well as nanotechnological contexts. They are interesting for pharmaceutical applications since they can adopt structures with new targeting potentials and because they are generally not prone to degradation by proteases. We report here a new set of peptidomimetics derived from δ‐peptides, consisting of n units of a 1,5‐disubstituted 1,2,3‐triazole amino acid (5Tzl). The monomer was prepared using ruthenium‐catalyzed azide–alkyne cycloaddition (RuAAC) chemistry using [RuCl2Cp*]x as the catalyst, allowing for simpler purification and resulting in excellent yields. This achiral monomer was used to prepare peptide oligomers that are water soluble independent of peptide chain length. Conformational analysis and structural investigations of the oligomers were performed by 2D NOESY NMR experiments, and by quantum chemical calculations using the ωB97X‐D functional. These data indicate that several conformations may co‐exist with slight energetic differences. Together with their increased hydrophilicity, this feature of homo‐5Tzl may prove essential for mimicking natural peptides composed of α‐amino acids, where the various secondary structures are achieved by side chain effects and not by the rigidity of the peptide backbone. The improved synthetic method allows for facile variation of the 5Tzl amino acid side chains, further increasing the versatility of these compounds.

Electroactive tetrathiafulvalene based pyridine-mono and -bis(1,2,3-triazoles) click ligands: synthesis, crystal structures and coordination chemistry

Electroactive ligands picoline-1,2,3-triazole-5-DM-TTF and lutidine-bis(1,2,3-triazole-5-DM-TTF) are synthesized by click chemistry following the ruthenium catalyzed azide–alkyne cycloaddition strategy. In the solid state structure of the bis(triazole) derivative, a tweezer conformation is observed, with the two TTF units arranged in a parallel manner. Cyclic voltammetry measurements show a splitting of the first oxidation wave, demonstrating sequential oxidation of the TTF donors in the intramolecular in through space mixed valence radical cation and then dication, suggesting that the tweezer conformation is maintained also in solution. A cobalt(II) complex based on the chelating picoline-triazole ligand is described. The lutidine-bis-triazole ligand shows no chelating behavior in the two structurally reported cadmium(II) complexes. In one of the complexes, the Cd(II) center is coordinated by four triazole rings from four different ligands, while the other four triazole units establish hydrogen bonds with one axially coordinated water molecule. The second complex is a one-dimensional coordination polymer, containing the CdCl2 fragment, in which the repeating motif is a pentanuclear “Cd5Cl10” unit decorated with two TTF ligands coordinated in a ditopical mode through the triazole units. The TTF donors establish intra- and intermolecular S⋯S contacts.

Tetrathiafulvalene mono- and bis-1,2,3-triazole precursors by click chemistry: structural diversity and reactivity

The donor ortho-dimethyl-TTF-(N-n-Bu-1,2,3-triazole) 1,5-isomer has been synthesized by click chemistry following a ruthenium-catalyzed azide-alkyne cycloaddition procedure. The single crystal X-ray analysis showed a planar conformation between the TTF and triazole units and a set of intermolecular interactions at the supramolecular level in the solid state. The same procedure allowed the preparation of the corresponding ortho-dimethyl-TTF-bis(triazole) which was also structurally characterized. Because of the steric hindrance, the triazole units are no longer planar with the TTF backbone. The reactivity of the triazole ring has been investigated in protonation and alkylation reactions, monitored by UV-visible spectroscopy, which clearly showed the red shift of the intramolecular charge transfer band. A TTF-methyl-triazolium salt has been isolated and analyzed by single crystal X-ray analysis. All of the TTF-triazoles and triazolium salts are valuable precursors for radical cation salts due to their oxidation potentials and variety of possible intermolecular interactions.

Reaction mechanism of ruthenium-catalyzed azide–alkyne cycloaddition reaction: A DFT study

The ruthenium catalyzed reaction of alkynes with azides (RuAAC) have enabled a facile route to obtain substituted triazoles, in contrast to thermal 1,3-dipolar cycloaddition which requires high activation barrier and is slow with limited regioselectivity. In this study, ruthenium catalyzed azide–alkyne cycloaddition reaction mechanism has been modeled by quantum mechanical methods. The reactions of benzyl azide and substituted alkynes have been modeled by following model mechanism with quantum mechanical calculations at the B3LYP/6-31G* level of theory with LANL2DZ on Ru. The reaction has been investigated for terminal and internal alkynes, with Cp (cyclopentadiene) and Cp* (pentamethylcyclopentadiene) ligand and compared with the experimental results. The calculations in this study have reproduced the experimental regioselectivity and allowed us to account on the electronic and steric effects. Both thermodynamic and kinetic parameters appeared to be important in these reactions. The thermodynamic stability of the Ru–azide–alkyne complexes and the relative ease of the complex to undergo reaction determined the product distribution.

Synthesis of new C5-(1-substituted-1,2,3-triazol-4 or 5-yl)-2′-deoxyuridines and their antiviral evaluation

The synthesis and antiviral evaluation of a series of C5-(1,4- and 1,5-disubstituted-1,2,3-triazolo)-nucleoside derivatives is described. The key steps of this synthesis are regioselective Huisgen’s 1,3-dipolar cycloaddition, using either copper-catalyzed azide-alkyne cycloaddition (CuAAC) or ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) under microwave activation. Some compounds among the 5a– l series possess activity against herpes simplex viruses 1 and 2, varicella-zoster virus, human cytomegalovirus and vaccinia virus. Their cytostatic activities were determined against murine leukemia cells, human T-lymphocyte cells and cervix carcinoma cells. Compounds were also evaluated on a wide panel of RNA viruses, including Vesicular stomatitis virus, influenza viruses type A (H1N1 and H3N2) and B in MDCK cell cultures, parainfluenza-3 virus, reovirus-1, Sindbis virus and Punta Toro virus in Vero cell cultures and Vesicular stomatitis, Coxsackie B4 and respiratory syncytial virus, with no specific antiviral effect.

Electronic effects of ruthenium-catalyzed [3+2]-cycloaddition of alkynes and azides

A combined experimental and theoretical study of ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) reactions is presented and various electronic analyses were conducted to provide a basis in understanding the observed regioselectivity of the 1,2,3-triazole products. Computational studies using density functional theory (DFT) and atoms in molecules quantum theory (AIM) further yield fresh details on the electronic factors that determine the regioselectivity in the RuAAC. It is found that the formation of 1,2,3-triazole products is irreversible and from the Hammett study, the pathway involving a vinyl cationic intermediate is ruled out. The electronic effect favors the formation of 5-electron-donating-group substituted-1,2,3-trizoles.

Copper(II)-Catalyzed Conversion of Aryl/Heteroaryl Boronic Acids, Boronates, and Trifluoroborates into the Corresponding Azides: Substrate Scope and Limitations

We report the copper(II)-catalyzed conversion of organoboron compounds into the corresponding azide derivatives. A systematic series of phenylboronic acid derivatives is evaluated to examine the importance of steric and electronic effects of the substituents on reaction yield as well as functional group compatibility. Heterocyclic substrates are also shown to participate in this mild reaction while compounds incorporating B-C(sp(3)) bonds are unreactive under the reaction conditions. The copper(II)-catalyzed boronic acid-azide coupling reaction is further extended to both boronate esters and potassium organotrifluoroborate salts. The method described herein complements existing procedures for the preparation of aryl azides from the respective amino, triazene, and halide derivatives and we expect that it will greatly facilitate copper- and ruthenium-catalyzed azide-alkyne cycloaddition reactions for the preparation of diversely functionalized 1-aryl- or 1-heteroaryl-1,2,3-triazoles derivatives.