Click Reaction Inspired Enzyme Inhibitors in Diabetes Care: An Update in the Field of Chronic Metabolic Disorder

Diabetes is a chronic metabolic disorder that impacts all age groups and affects a large population worldwide. Humans receive glucose from almost every food source, and after absorption from the gut, it reaches the liver, which functions as the distribution center for it. The insulin-responsive glucose transporter type 4 (GLUT-4) is a major carrier of glucose to the various cells (majorly expressed in myocytes, adipocytes, and cardiomyocytes) in a well-fed state. In fasting periods, the glucose supply is maintained by glycogenolysis and gluconeogenesis. In diabetes, the distribution of glucose is hampered due to several reasons. Furthermore, to treat this disorder, several drugs have been synthesized, and click chemistry plays an important role. A more recent concept for producing pharmaceuticals with a click chemistry approach makes any reaction more practical and stereospecific, along with a higher yield of products and a smaller number of by-products. This approach comprises a compiled study of the activity of numerous compelling antidiabetic drugs containing 1,2,3-triazole derivatives supported by click chemistry. In this review, we discuss the synthetic antidiabetic drugs made via click chemistry and their commendable role in improving diabetes care.

Copper-Mediated Intramolecular Interrupted CuAAC Selanylation

We, herein, describe a copper-mediated domino CuAAC intramolecular selanylation for the synthesis of unprecedented fused benzo[4,5][1,3]selenazolo[3,2-c][1,2,3]triazoles from 1,2-bis(2-azidoaryl)diselenides and terminal alkynes under microwave irradiation. This is the seminal method for the synthesis of these fused heterocycles, and it proceeds under mild conditions, tolerates several functional groups, and can be carried out using environmentally benign solvents such as dimethyl carbonate. This transformation has been successfully extended to TMS-protected alkynes and to bioactive alkynes. A plausible reaction mechanism is proposed based on several control experiments and previous reports.

Theoretical Study on Cooperation Catalysis of Chiral Guanidine/ Copper(I) in Asymmetric Azide-Alkyne Cycloaddition/[2 + 2] Cascade Reaction

Density functional theory (DFT) calculations with BP86-D3(BJ) functionals were employed to reveal the mechanism and stereoselectivity of chiral guanidine/copper(I) salt-catalyzed stereoselective three-component reaction among N-sulfonyl azide, terminal alkyne, and isatin-imine for spiroazetidinimines that was first reported by Feng and Liu (Angew. Chem. Int. Ed. 2018, 57, 16852-16856). For the noncatalytic cascade reaction, the denitrogenation to generate ketenimine species was the rate-determining step, with an activation barrier of 25.8-34.8 kcal mol^-1. Chiral guanidine-amide promoted the deprotonation of phenylacetylene, generating guanidine-Cu(I) acetylide complexes as active species. In azide-alkyne cycloaddition, copper acetylene coordinated to the O atom of the amide moiety in guanidium, and TsN₃ was activated by hydrogen bonding, affording the Cu(I)-ketenimine species with an energy barrier of 3.5∼9.4 kcal mol^-1. The optically active spiroazetidinimine oxindole was constructed via a stepwise four-membered ring formation, followed by deprotonation of guanidium moieties for C-H bonding in a stereoselective way. The steric effect of the bulky CHPh₂ group and chiral backbone in the guanidine, combined with the coordination between the Boc group in isatin-imine with a copper center, played important roles in controlling the stereoselectivity of the reaction. The major spiroazetidinimine oxindole product with an SS configuration was formed in a kinetically more favored way, which was consistent with the experimental observation.

Understanding the Fate of the Banert Cascade of Propargylic Azides: Sigmatropic versus Prototropic Pathway

The Banert cascade is an efficient synthetic strategy for obtaining 4,5-disubstituted 1,2,3-triazoles. The reaction can proceed via a sigmatropic or prototropic mechanism depending on the substrate and the conditions. In this work, the mechanisms of both pathways from propargylic azides with different electronic features were investigated using density functional theory, quantum theory of atoms in molecules, and natural bond orbital approaches. The calculated energy barriers were consistent with the experimental data. Three patterns of electron density distribution on the transition structures were observed, which reflected the behaviors of the reactants in the Banert cascade. The stronger conjugative effects were associated with lower/higher free activation energies of sigmatropic/prototropic reactions, respectively. A clear relationship between the accumulation of the charge at the C³ atom of propargylic azides with the energy barriers for prototropic reactions was found. Thus, the obtained results would allow the prediction of the reaction's course by evaluating reactants.

Mechanochemical Approach towards Multi-Functionalized 1,2,3-Triazoles and Anti-Seizure Drug Rufinamide Analogs Using Copper Beads

Highly regiospecific, copper-salt-free and neat conditions have been demonstrated for the 1,3-dipolar azide-alkyne cycloaddition (AAC) reactions under mechanochemical conditions. A group of structurally challenging alkynes and heterocyclic derivatives was efficiently implemented to achieve highly functionalized 1,4-disubstituted-1,2,3-triazoles in good to excellent yield by using the Cu beads without generation of unwanted byproducts. Furthermore, the high-speed ball milling (HSBM) strategy has also been extended to the synthesis of the commercially available pharmaceutical agent, Rufinamide, an antiepileptic drug (AED) and its analogues. The same strategy was also applied for the synthesis of the Cl-derivative of Rufinamide. Analysis of the single crystal XRD data of the triazole was also performed for the final structural confirmation. The Cu beads are easily recoverable from the reaction mixture and used for the further reactions without any special treatment.

The straightforward synthesis of N-coordinated ruthenium 4-aryl-1,2,3-triazolato complexes by [3 + 2] cycloaddition reactions of a ruthenium azido complex with terminal phenylacetylenes and non-covalent aromatic interactions in structures

The straightforward preparation of N-coordinated ruthenium triazolato complexes by [3 + 2] cycloaddition reactions of a ruthenium azido complex [Ru]-N3 (1, [Ru] = (η5-C5H5)(dppe)Ru, dppe = Ph2PCH2CH2PPh2) with a series of terminal phenylacetylenes is reported. The reaction products, N(2)-bound ruthenium 4-aryl-1,2,3-triazolato complexes such as [Ru]N3C2H(4-C6H4CN) (2), [Ru]N3C2H(4-C6H4CHO) (3), [Ru]N3C2H(4-C6H4F) (4), [Ru]N3C2H(Ph) (5) and [Ru]N3C2H(4-C6H4CH3) (6) were produced from 4-ethynylbenzonitrile, 4-ethynylbenzaldehyde, 1-ethynyl-4-fluorobenzene, phenylacetylene and 4-ethynyltoluene, respectively, at 80 °C or above under an atmosphere of air. To the best of our knowledge, this is the first example of the preparation of N-coordinated ruthenium aryl-substituted 1,2,3-triazolato complexes by the [3 + 2] cycloaddition of a metal-coordinated azido ligand and a terminal aryl acetylene, less electron-deficient terminal aryl alkynes. All of the compounds have been fully characterized and the structures of complexes 2, 3, 5 and 6 were confirmed by single-crystal X-ray diffraction analysis. Each compound participates in non-covalent aromatic interactions in the solid-state structure which can be favorable in the binding of DNA/biomolecular targets and has shown great potential in the development of biologically active anticancer drugs.

Copper-Containing Catalysts for Azide–Alkyne Cycloaddition in Supercritical CO2

Background: Chemical industry has increased the investment into and innovation capacity to supply chemicals from safe and sustainable sources, which will be essential to offering new solutions and supporting the green transition of the global economy and society. In this sense, the use of green solvents and reusable heterogeneous catalysts has emerged as a promising sustainable process strategy for engineering, chemistry and the environment. In this work, different homogeneous (copper bromide, CuBr and copper(II) acetate, Cu (CH3COO)2·H2O) and heterogeneous (Cu Wire, Cu Plate, Cu/β-SiC, pre-treated Cu Wire and pre-treated Cu Plate) copper catalysts were tested for the copper(I)-catalyzed alkyne–azide cycloaddition (CuAAC) reaction. In addition, the influence of different reaction media was analyzed, comparing the use of an organic solvent such as toluene and a green solvent such as supercritical CO2 (scCO2). Methods: Characterization of the catalysts includes by X-ray diffraction (XRD), Scan Electron Microscopy (SEM), Atomic absorption spectrophotometry (AA) and Temperature Programmed Reduction (TPR). Parameters such as catalyst loading, reaction time, reusability and leaching of the catalysts were studied to obtain more information on the CuAAC reaction in scCO2. Results: The pre-treated copper plate achieved a 57% increase in reaction yield compared to the non pre-treated copper plate. However, the recovery and reuse of the pre-treated copper plate showed a severe deterioration and a considerable change in its surface. Cu Wire (without pre-treatment) achieved yields of up to 94.2% after reusing it for five cycles. Conclusions: These results suggest the possibility to exploit the combination of heterogeneous catalysts and scCO2 and justify further research to highlight green solvents and simultaneously address the challenges of reaction, purification and recycling.

Reductive Alkylation of Azides and Nitroarenes with Alcohols: A Selective Route to Mono- and Dialkylated Amines

Herein, we demonstrated an efficient protocol for reductive alkylation of azides/nitro compounds via a borrowing hydrogen (BH) method. By following this protocol, selective mono- and dialkylated amines were obtained under mild and solvent-free conditions. A series of control experiments and deuterium-labeling experiments were performed to understand this catalytic process. Mechanistic studies suggested that the Ir(III)-H was the active intermediate in this reaction. KIE study revealed that the breaking of the C-H bond of alcohol might be the rate-limiting step. Notably, this solvent-free strategy disclosed a high TON of around 5600. Based on kinetic studies and control experiments, a metal-ligand cooperative mechanism was proposed.

Electron Distribution in 1,2,3-Benzotriazole and 1,2,3-Triazole Anion Radical Isomers: An EPR and DFT Study

The anion radicals of N1- and N2-alkylbenzotriazoles and alkyltriazoles (alkyl = methyl or isopropyl) have been generated by low-temperature potassium metal reduction in tetrahydrofuran. Electron paramagnetic resonance (EPR) analysis and density functional theory calculations reveal that the electron spin distribution within the triazole ring of these systems is markedly different. The magnitude of the electron-nitrogen couplings along with the calculated spin densities reveals that the N2-alkylbenzotriazole and N2-alkyltriazole anion radicals have significantly greater electron spin residing within the N₃ portion of the triazole ring compared with that of the respective N1 isomers. These differences impact the overall geometry of the triazole ring where both N2-isomers lose planarity upon reduction. Experimental and computational results reveal that the N2-methyltriazole anion radical has the largest concentration of electron spin residing in the N₃ moiety compared to that of the other three anion radicals studied. Significant anisotropic line broadening is observed in the EPR spectrum of the N2-methyltriazole anion radical, which is a consequence of the large nitrogen hyperfine couplings and sufficiently slow rotational motion of this species in solution.

Fabrication of copper(II)-coated magnetic core-shell nanoparticles Fe3O4@SiO2-2-aminobenzohydrazide and investigation of its catalytic application in the synthesis of 1,2,3-triazole compounds

In the present work, an attempt has been made to synthesize the 1,2,3-triazole derivatives resulting from the click reaction, in a mild and green environment using the new copper(II)-coated magnetic core-shell nanoparticles Fe3O4@SiO2 modified by isatoic anhydride. The structure of the catalyst has been determined by XRD, FE-SEM, TGA, VSM, EDS, and FT-IR analyzes. The high efficiency and the ability to be recovered and reused for at least up to 6 consecutive runs are some superior properties of the catalyst.

Three-component coupling reaction for the synthesis of fully substituted triazoles: reactivity control of Cu-acetylide toward alkyl azides and diazo compounds

Herein, we report a Cu-catalyzed three-component coupling reaction of alkynes, azides, and diazo compounds for the synthesis of fully substituted triazoles.

Assembling triazolated calix[4]semitubes by means of copper(i)-catalyzed azide–alkyne cycloaddition

Calix[4]arene-based bis(alkynes) and bis(azides) react with each other under thoroughly tuned CuAAC conditions and produce diverse biscalixarene semitube assemblies linked by two triazole units, which are capable of binding transition metal cations.

One-pot synthesis of 3-substituted-4H-[1,2,3] triazolo[5,1-c][1,4]oxazin-6(7H)-ones from propargyl alcohols, chloroacetyl chloride, and sodium azide

An efficient, one-pot synthesis of 3-substituted-4 H-[1,2,3]triazolo[5,1- c][1,4]oxazin-6(7 H)-ones is developed via sequential esterification, substitution, and 1,3-dipolar cycloaddition processes of various propargyl alcohols, chloroacetyl chloride, and sodium azide. This method provides a variety of novel 1,2,3-triazole-fused oxazinones and has several advantages including simple operation, high efficiency, and good-to-excellent product yields (80%–95%) without the need to isolate the ester and azide intermediates.

Access to molecular complexity. Multicomponent reactions involving five or more components

The evaluation of the significance of a chemical transformation addresses many factors, including such important characteristics as the number of chemical bonds formed in one step, the reaction time, labour intensity, the cost of reactants and catalysts and so on. The amount of waste produced in the reaction has also gained increasing importance in recent years. Multicomponent reactions (MCRs) occupy a special place as a synthetic tool in modern organic chemistry. These reactions allow the synthesis of target products with complex structures, minimizing labour costs. This review summarizes the literature on multicomponent reactions involving five or more components. The data in the review are classified according to the number of reactants participating in the reaction and the types of reactions. It is worth noting that in some cases, these transformations can be a part of a domino process, making this classification difficult, if not impossible. The structural diversity of the reaction products greatly increases with increasing number of components involved in the MCR, which becomes virtually unlimited when using combinations of MCRs. This review highlights the main trends of past decades in the field of MCRs. The last two decades have witnessed an explosive growth in the number of publications in this area of chemistry.

The bibliography includes 309 references.

2H-Bis-1,2,3-triazolo-isoquinoline: Design, Synthesis, and Photophysical Study

An efficient three-step synthesis of a new heterocyclic system is described wherein the 2H-bis([1,2,3]triazolo)[5,1-a:4',5'-c]isoquinoline ring system is elaborated using a simple synthetic strategy. The approach permits the preparation of target compounds in high yields using readily available arylhydrazines and o-alkynylbenzaldehydes as starting materials. The photophysical properties of the prepared heterocycles were studied to demonstrate that the prepared compounds are attractive blue-emitting fluorophores, exhibiting quantum yields up to 98% and Stokes shifts up to 67 nm. A strong effect of the steric hindrance on the absorption and emission spectra was revealed.

Synthesis and Biological Evaluation of Hapten-Clicked Analogues of The Antigenic Peptide Melan-A/MART-126(27L)-35

A click-chemistry-based approach was implemented to prepare peptidomimetics designed in silico and made from aromatic azides and a propargylated GIGI-mimicking platform derived from the altered Melan-A/MART-126(27L)-35 antigenic peptide ELAGIGILTV. The CuI -catalyzed Huisgen cycloaddition was carried out on solid support to generate rapidly a first series of peptidomimetics, which were evaluated for their capacity to dock at the interface between the major histocompatibility complex class-I (MHC-I) human leucocyte antigen (HLA)-A2 and T-cell receptors (TCRs). Despite being a weak HLA-A2 ligand, one of these 11 first synthetic compounds bearing a p-nitrobenzyl-triazole side chain was recognized by the receptor proteins of Melan-A/MART-1-specific T-cells. After modification of the N and C termini of this agonist, which was intended to enhance HLA-A2 binding, one of the resulting seven additional compounds triggered significant T-cell responses. Thus, these results highlight the capacity of naturally circulating human TCRs that are specific for the native Melan-A/MART-126-35 peptide to cross-react with peptidomimetics bearing organic motifs structurally different from the native central amino acids.

Copper(I)-Catalyzed Ketenimine Formation/Aza-Claisen Rearrangement Cascade for Stereoselective Synthesis of α-Allylic Amidines

A copper-catalyzed three-component reaction of terminal alkynes, TsN₃, and tertiary allylic amines is developed toward the one-pot synthesis of α-allylic amidines. The product was synthesized on gram scale under 1 mol % of catalyst loading. Transformations of products into alkenyl amine and other nitrogen-containing compounds are demonstrated without any loss of stereochemical information.

A route to virtually unlimited functionalization of water-soluble p-sulfonatocalix[4]arenes

The functionality of p-sulfonatocalix[4]arenes can be easily extended using the propargylation/CuAAC reaction sequence, which allows the introduction of up to four substituted triazole units to the narrow rims of the macrocycles while maintaining their cone shapes and water solubility and, thus, biomedical applicability.

Synthesis and Spectral Study of a New Family of 2,5-Diaryltriazoles Having Restricted Rotation of the 5-Aryl Substituent

Efficient synthesis of 2,5-diaryl substituted 4-azido-1,2,3-triazoles by the reaction of sodium azide with dichlorosubstituted diazadienes was demonstrated. The optical properties of the prepared azidotriazoles were studied to reveal a luminescence maximum in the 360-420 nm region. To improve the luminescence quantum yields a family of 4-azido-1,2,3-triazoles bearing ortho-propargyloxy substituents in the 5 position was prepared. Subsequent intramolecular thermal cyclization permits to construct additional triazole fragment and obtain unique benzoxazocine derivatives condensed with two triazole rings. This new family of condensed heterocycles has a flattened heterocyclic system structure to provide more conjugation of the 5-aryl fragment with the triazole core. As a result, a new type of UV/"blue light-emitting" materials with better photophysical properties was obtained.

A Novel Approach to Functionalization of Aryl Azides through the Generation and Reaction of Organolithium Species Bearing Masked Azides in Flow Microreactors

A novel straightforward method for aryl azides having functional groups based on generation and reactions of aryllithiums bearing a triazene group from polybromoarenes using flow microreactor systems was achieved. The present approach will serve as a powerful method in organolithium chemistry and open a new possibility in the synthesis of polyfunctional organic azides.

K2S2O8 mediated C-3 arylation of quinoxalin-2(1H)-ones under metal-, photocatalyst- and light-free conditions

Two facile and effective C-3 arylation protocols of quinoxalin-2(1H)-ones with arylhydrazines and aryl boronic acids respectively via free radical cross-coupling reactions under metal-, photocatalyst- and light-free conditions have been unveiled. K2S2O8 has been used as an efficient oxidant to generate aryl radicals from arylhydrazines and aryl boronic acids under two different reaction conditions. The generated aryl radicals undergo a free radical coupling reaction at the C-3 position of quinoxalin-2(1H)-ones producing 3-arylquinoxalin-2(1H)-ones in good to excellent yields. The involvement of radicals in the course of the reaction has been demonstrated by radical trapping experiments with 2,2,6,6-tetramethylpiperidine-1-oxyl.

Pharmacological and Cellular Significance of Triazole-Surrogated Compounds

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Heterocyclic compounds have been at the hierarchy position in academia, and industrial arena, particularly the compounds containing triazole-core are found to be potent with a broad range of biological activities. The resistance of triazole ring towards chemical (acid and base) hydrolysis, oxidative and reductive reaction conditions, metabolic degradation and its higher aromatic stabilization energy makes it a better heterocyclic core as therapeutic agents. These triazole-linked compounds are used for clinical purposes for antifungal, anti-mycobacterium, anticancer, anti-migraine and antidepressant drugs. Triazole scaffolds are also found to act as a spacer for the sake of covalent attachment of the high molecular weight bio-macromolecules with an experimental building blocks to explore structure-function relationships. Herein, several methods and strategies for the synthesis of compounds with 1,2,3-triazole moiety exploring Hüisgen, Meldal and Sharpless 1,3-dipolar cycloaddition reaction between azide and alkyne derivatives have been deliberated for a series of representative compounds. Moreover, this review article highlights in-depth applications of the [3+2]-cycloaddition reaction for the advances of triazole-containing antibacterial as well as metabolic labelling agents for the in vitro and in vivo studies on cellular level.

Cu(ii), Ir(i) and CuO nanocatalyzed mild synthesis of luminescent symmetrical and unsymmetrical bis(triazolylmethyl)quinoxalines: biocompatibility, cytotoxicity, live cell imaging and biomolecular interaction

A microwave induced Cu(ii) and Ir(i) catalysed click reaction for the synthesis of anticancer symmetrical and unsymmetrical triazoles.

Selective azide–alkyne cycloaddition reactions of azidoalkylated calixarenes

Reaction of calixarene oligo(azides) with alkynes taken in deficiency lead selectively to oligo(triazoles) under Cu(i) catalysis. Same azides react with alkynes intramolecularly under copper-free conditions forming 1,5-substituted triazole bridges.

Regioselective N1- and N2-heterocycloalkylation of N1-sulfonyl-1,2,3-triazoles

A tuneable and visible-light-driven three-component reaction between N¹-sulfonyl-1,2,3-triazoles, saturated heterocycles, and N-bromosuccinimide for regioselective synthesis of N¹- or N²-heterocycloalkylated 1,2,3-triazoles.

Greener synthesis of 1,2,3-triazoles using a copper(i)-exchanged magnetically recoverable β-zeolite as catalyst

A novel magnetically recoverable Cu(i)-exchanged β-zeolite catalyst was prepared, characterized and applied for the synthesis of 1,2,3-triazoles via the one-pot three-component reaction.

Biosynthetic Pathways to Nonproteinogenic α-Amino Acids

Natural nonproteinogenic amino acids vastly outnumber the well-known 22 proteinogenic amino acids. Such amino acids are generated in specialized metabolic pathways. In these pathways, diverse biosynthetic transformations, ranging from isomerizations to the stereospecific functionalization of C-H bonds, are employed to generate structural diversity. The resulting nonproteinogenic amino acids can be integrated into more complex natural products. Here we review recently discovered biosynthetic routes to freestanding nonproteinogenic α-amino acids, with an emphasis on work reported between 2013 and mid-2019.

Influence of the substituent D/A at the 1,2,3-triazole ring on novel terpyridine derivatives: synthesis and properties

In this study, we newly designed and developed a synthesis route based on the 1,3-dipolar cycloaddition of the derivatives of 4'-(1,2,3-triazol-4-yl)phenyl-2,2':6',2''-terpyridine with various (hetero)aryl substituents, differing in electronic character, on a triazol ring. The obtained compounds were comprehensively characterized by UV-Vis spectroscopy and electrochemical and thermal studies. Moreover, preliminary biological tests were conducted. The investigation allowed the selection of materials with the most promising properties with particular emphasis on the nature of the substituents. In addition, theoretical studies (DFT and TD-DFT) were performed to verify the comprehensive understanding of experimental results.