Copper-catalyzed Azide-alkyne Cycloaddition Research Articles

Catalytic Enantioselective Construction of Chiroptical Boron-Stereogenic Compounds.

The construction of main group heteroatom-stereogenic compounds is of great importance due to their intriguing chemical, physical, biological, and stereoelectronic properties. Despite that organoboron compounds are widely used in organic chemistry, the creation of a tetrahedral boron-stereogenic center in one enantiomeric form remains highly challenging. Given the labile nature of ligands attached to the tetracoordinate boron atom, only a handful of enantioenriched boron-stereogenic compounds have been reported via resolution or a chiral substrate-induced diastereoselective approach. To date catalytic asymmetric synthesis of boron-stereogenic compounds has remained unknown. Here, we demonstrate the first catalytic enantioselective construction of boron-stereogenic compounds via an asymmetric copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. This enantioselective CuAAC reaction not only gives access to a wide range of novel highly functionalized boron-stereogenic heterocycles in high yields with good to excellent enantioselectivities but also produces optically active terminal alkyne and triazole moieties with various potential application prospects. Further transformation of the chiral tetracoordinate boron compounds delivers several complex heterocyclic entities bearing boron-stereogenic centers without the loss of enantiopurity. Moreover, the X-ray structure, the barrier to racemization, and the HOMO/LUMO gap of selected tetracoordinate boron compounds are investigated. Notably, these novel N,N π-conjugated boron-stereogenic compounds exhibit bright fluorescence. The optical properties, including circular dichroism, quantum yield, and circular polarized luminescence spectroscopies, are examined. These features expand the chemical space of the chiroptical boron-based dye platform, which could have great potential applications in chiral optoelectronic materials.

Highly Efficient Modular Construction of Functional Drug Delivery Platform Based on Amphiphilic Biodegradable Polymers via Click Chemistry.

Amphiphilic copolymers with pendant functional groups in polyester segments are widely used in nanomedicine. These enriched functionalities are designed to form covalent conjugates with payloads or provide additional stabilization effects for encapsulated drugs. A general method is successfully developed for the efficient preparation of functional biodegradable PEG-polyester copolymers via click chemistry. Firstly, in the presence of mPEG as initiator, Sn(Oct)2-catalyzed ring-opening polymerization of the α-alkynyl functionalized lactone with D,L-lactide or ε-caprolactone afforded linear mPEG-polyesters bearing multiple pendant alkynyl groups. Kinetic studies indicated the formation of random copolymers. Through copper-catalyzed azide-alkyne cycloaddition reaction, various small azido molecules with different functionalities to polyester segments are efficiently grafted. The molecular weights, polydispersities and grafting efficiencies of azido molecules of these copolymers were investigated by NMR and GPC. Secondly, it is demonstrated that the resulting amphiphilic functional copolymers with low CMC values could self-assemble to form nanoparticles in aqueous media. In addition, the in vitro degradation study and cytotoxicity assays indicated the excellent biodegradability and low cytotoxicity of these copolymers. This work provides a general approach toward the preparation of functional PEG-polyester copolymers in a quite efficient way, which may further facilitate the application of functional PEG-polyesters as drug delivery materials.

Click and Cut: a click chemistry approach to developing oxidative DNA damaging agents.

Metallodrugs provide important first-line treatment against various forms of human cancer. To overcome chemotherapeutic resistance and widen treatment possibilities, new agents with improved or alternative modes of action are highly sought after. Here, we present a click chemistry strategy for developing DNA damaging metallodrugs. The approach involves the development of a series of polyamine ligands where three primary, secondary or tertiary alkyne-amines were selected and ‘clicked’ using the copper-catalysed azide-alkyne cycloaddition reaction to a 1,3,5-azide mesitylene core to produce a family of compounds we call the ‘Tri-Click’ (TC) series. From the isolated library, one dominant ligand (TC1) emerged as a high-affinity copper(II) binding agent with potent DNA recognition and damaging properties. Using a range of in vitro biophysical and molecular techniques—including free radical scavengers, spin trapping antioxidants and base excision repair (BER) enzymes—the oxidative DNA damaging mechanism of copper-bound TC1 was elucidated. This activity was then compared to intracellular results obtained from peripheral blood mononuclear cells exposed to Cu(II)–TC1 where use of BER enzymes and fluorescently modified dNTPs enabled the characterisation and quantification of genomic DNA lesions produced by the complex. The approach can serve as a new avenue for the design of DNA damaging agents with unique activity profiles.

Chemoenzymatic and Protecting-Group-Free Synthesis of 1,4-Substituted 1,2,3-Triazole-α-d-glucosides with Potent Inhibitory Activity toward Lysosomal α-Glucosidase.

α-Glucosyl triazoles have rarely been tested as α-glucosidase inhibitors, partly due to inefficient synthesis of their precursor α-d-glucosylazide (αGA1). Glycosynthase enzymes, made by nucleophile mutations of retaining β-glucosidases, produce αGA1 in chemical rescue experiments. Thermoanaerobacterium xylanolyticus glucosyl hydrolase 116 β-glucosidase (TxGH116) E441G nucleophile mutant catalyzed synthesis of αGA1 from sodium azide and pNP-β-d-glucoside (pNPGlc) or cellobiose in aqueous medium at 45 °C. The pNPGlc and azide reaction product was purified by Sephadex LH-20 column chromatography to yield 280 mg of pure αGA1 (68% yield). αGA1 was successfully conjugated with alkynes attached to different functional groups, including aryl, ether, amine, amide, ester, alcohol, and flavone via copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry reactions. These reactions afforded the 1,4-substituted 1,2,3-triazole-α-d-glucoside derivatives AGT2-14 without protection and deprotection. Several of these glucosyl triazoles exhibited strong inhibition of human lysosomal α-glucosidase, with IC50 values for AGT4 and AGT14 more than 60-fold lower than that of the commercial α-glucosidase inhibitor acarbose.

Molecules, the Ultimate Nanomotor: Linking Chemical Reaction Intermediates to their Molecular Diffusivity.

The intellectual community focused on nanomotors has recently become interested in extending these concepts to individual molecules. Here, we study a chemical reaction according to whose mechanism some intermediate species should speed up while others slow down in predictable ways, if the nanomotor hypothesis of boosted diffusion holds. Accordingly, we scrutinize the absolute diffusion coefficient (D) during intermediate steps of the catalytic cycle for the CuAAC reaction (copper-catalyzed azide-alkyne cycloaddition click reaction), using proton pulsed field-gradient nuclear magnetic resonance to discriminate between the diffusion of various reaction intermediates. We observe time-dependent diffusion that is enhanced for some intermediate molecular species and depressed for those whose size increases owing to complex formation. These findings point to the failure of the conventional Stokes-Einstein equation to fully explain diffusivity during chemical reaction. Without attempting a firm explanation, this paper highlights aspects of the physics of chemical reactions that are imperfectly understood and presents systematic data that can be used to assess hypotheses.

An Advanced 'clickECM' That Can be Modified by the Inverse-Electron-Demand Diels-Alder Reaction.

The extracellular matrix (ECM) represents the natural environment of cells in tissue and therefore is a promising biomaterial in a variety of applications. Depending on the purpose, it is necessary to equip the ECM with specific addressable functional groups for further modification with bioactive molecules, for controllable cross‐linking and/or covalent binding to surfaces. Metabolic glycoengineering (MGE) enables the specific modification of the ECM with such functional groups without affecting the native structure of the ECM. In a previous approach (S. M. Ruff, S. Keller, D. E. Wieland, V. Wittmann, G. E. M. Tovar, M. Bach, P. J. Kluger, Acta Biomater. 2017, 52, 159–170), we demonstrated the modification of an ECM with azido groups, which can be addressed by bioorthogonal copper‐catalyzed azide‐alkyne cycloaddition (CuAAC). Here, we demonstrate the modification of an ECM with dienophiles (terminal alkenes, cyclopropene), which can be addressed by an inverse‐electron‐demand Diels‐Alder (IEDDA) reaction. This reaction is cell friendly as there are no cytotoxic catalysts needed. We show the equipment of the ECM with a bioactive molecule (enzyme) and prove that the functional groups do not influence cellular behavior. Thus, this new material has great potential for use as a biomaterial, which can be individually modified in a wide range of applications.

Organic solvent reverse osmosis using CuAAC-crosslinked molecularly-mixed composite membranes

Molecularly-mixed composite membranes (MMCMs) incorporating amorphous scrambled porous organic cages (ASPOCs) are a rapidly emerging membrane class that are characterized by the formation of a solid solution due to a high degree of homogeneity that is not observed in other classes of polymer-particle composites. Molecular-level mixing overcomes many of the performance/processing issues typically encountered with two phase composite materials. However, chemical stabilization of the polymer matrix can deactivate the ASPOC cages. Here, we illustrate an alternative method of crosslinking Matrimid® in MMCMs using the copper catalyzed azide alkyne cycloaddition (CuAAC) click reaction. We fabricated thin film composite membranes and benchmarked them in a crossflow permeation system with standard styrene oligomers dissolved in a variety of organic solvents as well as an exemplar organic solvent reverse osmosis separation. We found that the presence of ASPOC increased both permeance and styrene dimer separation factor by up to 79% and 154%, respectively, over crosslinked Matrimid depending on the solvent, although the separation factor decreased at higher ASPOC loadings. The crosslinked MMCMs were challenged with a solvent-solvent separation in an organic solvent reverse osmosis modality and were able to effectively purify toluene from triisopropylbenzene. This work provides experimental observations needed to understand the mass transport processes occurring in MMCMs and highlights their separation performance and scale-up potential.

Novel molecules containing structural features of NSAIDs and 1,2,3-triazole ring: Design, synthesis and evaluation as potential cytotoxic agents

For the first time the template containing structural features of more than one NSAIDs and the 1,2,3-triazole ring was explored for the identification of potential cytotoxic agents. These new and complex molecules were predicted to be effective inhibitors of PDE4B by molecular modelling studies in silico. The multi-step synthesis of these compounds were carried out starting from the well-known drug nimesulide and involved the use of copper-catalyzed azide-alkyne cycloaddition (CuAAC) approach as the key step. Mainly two types of compounds e.g. 1-aryl-1H-1,2,3-triazoles and N-aryl-2-(1H-1,2,3-triazol-1-yl)acetamide derivatives were synthesized by using this method in good yields. The in vitro screening of these compounds against two cancer cell lines e.g. HCT-15 (human colon cancer cell line) and NCI-H226 (human lung cancer cell line) using a colorimetric MTT assay allowed identification of two preliminary hit molecules i.e. 8a and 8f. The SAR (Structure Activity Relationship) analysis indicated that the presence of an amide linker between the aryl ring and the 1,2,3-triazole moiety was favorable for the activities. The compound 8a and 8f showed significant inhibition of PDE4B in vitro and good interactions with this protein in silico suggesting PDE4B as their potential target. The usefulness and concerns of these molecules in the light of computational ADME prediction were analyzed. Overall, novel molecules were identified as potential cytotoxic agents for further study.

Use of NAD-Seq to Profile NAD+-Capped RNAs in Plants.

A 7-methylguanosine (m7G) cap in the 5′ end of eukaryotic mRNA transcript to protect mRNAs from degradation and regulate gene expression has been well characterized. Recently, some RNAs were found to contain a NAD+ cap, which is a different type of 5′ cap structure in eukaryotic organisms, including bacteria, yeast, mammalian cells, and Arabidopsis thaliana (arabidopsis) plant. NAD captureSeq was the first method to label NAD+-capped RNAs (NAD-RNAs) and uses ADP ribose cyclase (ADPRC)-catalyzed replacement of the nicotinamide of NAD+ with an alkynyl alcohol followed by copper-catalyzed azide-alkyne cycloaddition (CuAAC) to biotinylate NAD-RNA. In the next step, the eluted RNAs are subjected to library preparation and sequencing, followed by determination of NAD+-capped RNA in the transcriptome. Improved methods are emerging to determine NAD-RNAs, such as NAD tagSeq and strain-promoted azide–alkyne cycloaddition reaction (SPAAC)-NAD-Seq. Commonly, those methods use an enzymatic reaction followed by a click chemistry reaction to label NAD-RNAs with biotin or synthetic RNA tag (TagRNA).

Recent syntheses of steroid derivatives using the CuAAC “click” reaction

The introduction of heterocycles into steroids is often at the origin of a modification of their physiological activity and thus allows the formation of new biologically interesting molecules. Recent advances in the field of steroid synthesis using the copper-catalyzed azide-alkyne cycloaddition (CuAAC) are presented here. Compounds exhibiting known biological activities are also reported in this review. This approach holds great promise and allows us to consider preparing new, highly functionalized complex molecules using the CuAAC click reaction.

Click chemistry: An efficient tool to control the functionalization of metallic surfaces with alkyl chains possessing two reactive end groups

Major challenges in the design of devices based on self-assembled monolayers (SAMs) standing in sandwich between two electrodes rely in SAMs of high quality and to simultaneously self-assemble them between two different electrodes, especially in case of Metal-Molecule-Metal junction devices. In this study, we demonstrate the synthesis and the characterization of two ferrocene-based functional molecules with two active thiol/thiol-acetate anchoring groups which could be used to bind with two different electrodes. To control the orientation of the molecules at the surface of the bottom electrode, a Copper catalysed Alkyne Azide Cycloaddition (CuAAC) (click chemistry) approach is applied on ultra-smooth template stripped gold substrates. Self-assembly of the two molecules could be obtained in two consecutive steps enabling us to control the orientation and the regularity of the SAMs. Cyclic voltammetry results shows that the SAMs obtained by mean of the two-step procedure are of high quality with more than 60% of the gold surface covered by molecules. We further investigated the SAMs by PM-IRRAS, XPS, ellipsometry and contact angle measurements to investigate the presence of functional groups, individual elements and succession of the click reaction on surface.

Differential Labeling of Chemically Modified Peptides and Lipids among Cyanobacteria Planktothrix and Microcystis.

The cyanoHAB forming cyanobacteria Microcystis and Planktothrix frequently produce high intracellular amounts of microcystins (MCs) or anabaenopeptins (APs). In this study, chemically modified MCs and APs have been localized on a subcellular level in Microcystis and Planktothrix applying copper-catalyzed alkyne-azide cycloaddition (CuACC). For this purpose, three different non-natural amino acids carrying alkyne or azide moieties were fed to individual P. agardhii strains No371/1 and CYA126/8 as well as to M. aeruginosa strain Hofbauer showing promiscuous incorporation of various amino acid substrates during non-ribosomal peptide synthesis (NRPS). Moreover, CYA126/8 peptide knock-out mutants and non-toxic strain Synechocystis PCC6803 were processed under identical conditions. Simultaneous labeling of modified peptides with ALEXA405 and ALEXA488 and lipid staining with BODIPY 505/515 were performed to investigate the intracellular location of the modified peptides. Pearson correlation coefficients (PCC) obtained from confocal images were calculated between the different fluorophores and the natural autofluorescence (AF), and between labeled modified peptides and dyed lipids to investigate the spatial overlap between peptides and the photosynthetic complex, and between peptides and lipids. Overall, labeling of modified MCs (M. aeruginosa) and APs (P. agardhii) using both fluorophores revealed increased intensity in MC/AP producing strains. For Synechocystis lacking NRPS, no labeling using either ALEXA405 or ALEXA488 was observed. Lipid staining in M. aeruginosa and Synechocystis was intense while in Planktothrix it was more variable. When compared with AF, both modified peptides and lipids showed a heterologous distribution. In comparison, the correlation between stained lipids and labeled peptides was not increased suggesting a reduced spatial overlap.

Tannic Acid: A green and efficient stabilizer of Au, Ag, Cu and Pd nanoparticles for the 4-Nitrophenol Reduction, Suzuki–Miyaura coupling reactions and click reactions in aqueous solution

Due to the good electrical, optical, magnetic, catalytic properties, transition metal nanoparticles (TMNPs) have been becoming more and more interesting in the fileds of environment, material, biomedicine, catalysis, and so on. Here, tannic acid (TA) is used as a green and efficient stabilizer to fabricate all kinds of TMNPs including AuNPs, AgNPs, CuNPs and PdNPs. These TMNPs possess small sizes ranging from 1 nm to 6 nm, which is conducive to several catalytic reactions in aqueous solution, such as 4-nitrophenol (4-NP) reduction, CuAAC reactions and Suzuki–Miyaura coupling reactions. AuNPs and PdNPs are found to have distinctly higher catalytic activities than AgNPs and CuNPs in the 4-NP reduction process. Especially, PdNPs show the highest catalytic activities with TOF up to 7200 h−1 in the 4-NP reduction. Furthermore, PdNPs also exhibit satisfying catalytic performance in the Suzuki–Miyaura coupling process, and CuNPs are catalytically active in the copper-catalyzed azide alkyne cycloaddition (CuAAC) reactions. The applicability and generality of PdNPs and CuNPs are respectively confirmed via the reaction between different substrates in the Suzuki–Miyaura coupling reactions and the CuAAC reactions. This work present a simple, fast, green and efficient strategy to synthesize TMNPs for multiple catalysis.

Thymol as an Interesting Building Block for Promising Fungicides against Fusarium solani.

The semisynthesis of 15 new thymol derivatives was achieved through Williamson synthesis and copper-catalyzed azide-alkyne cycloaddition (CuAAC) approaches. The reaction of CuAAC using the "Click Chemistry" strategy, in the presence of an alkynyl thymol derivative and commercial or prepared azides, provided nine thymol derivatives under microwave irradiation. This procedure reduces reaction time and cost. All molecular entities were elucidated by 1H and 13C NMR, IR, and HRMS data. These derivatives were evaluated in vitro for their fungicidal activity against Fusarium solani sp. Among the nine triazolic thymol derivatives obtained, seven of them were found to have moderated antifungal activity. In contrast, naphthoquinone/thymol hybrid ether 2b displayed activity comparable with that of the commercial fungicide thiabendazole. The structure-activity relationship for the most active compound 2b was discussed, and the mode of action was predicted by a possible binding to the fungic ergosterol and interference of osmotic balance of K+ into the extracellular medium.

A promoted copper-catalysed Azide-alkyne cycloaddition (CuAAC) for broad spectrum peptide-engineered implants

As an effective strategy to develop novel implants with a biomolecule, the efficiency of a copper-catalysed azide-alkyne cycloaddition (CuAAC) urgently requires improvement. Herein, we constructed a facile CuAAC system with catalytic CuSO4/sodium borohydride (CuAAC-Bor). As Cu nanoclusters (CuNCs) formed in situ, they showed stronger efficiency in immobilizing 41 types of peptides on Ti implants than traditional CuAAC with CuSO4/sodium ascorbate. Then, we mechanically revealed the significance of peptide size on CuAAC efficiency by all-atom molecular dynamics simulation and machine learning, and found that larger peptides preferred to adhere to Ti and CuNCs to increase their reaction opportunity. With CuAAC-Bor, HHC36-engineered implants strongly inhibited 5 types of clinical bacteria in vitro and prevented infection in vivo, and RGD-, DGEA-, QK- or YGFGG-engineered implants had high biocompatibilities with HUVECs/hBMSCs in vitro. Moreover, we demonstrated that QK-engineered implants prepared with CuAAC-Bor promoted the osteogenic differentiation of hBMSCs via the PI3K/Akt signalling pathway and triggered angiogenesis and osteogenesis in vivo. Our study shows that this facile and highly efficient CuAAC-Bor system holds high promise for the preparation of novel biomaterial surfaces.

The Copper-Catalyzed Reaction of 2-(1-Hydroxyprop-2-yn-1-yl)phenols with Sulfonyl Azides Leading to C3-Unsubstituted N-Sulfonyl-2-iminocoumarins.

An operationally simple synthesis of Z-configured and C3-unsubstituted N-sulfonyl-2-iminocoumarins (e.g., 8a) that proceeds under mild conditions is achieved by reacting 2-(1-hydroxyprop-2-yn-1-yl)phenols (e.g., 6a) with sulfonyl azides (e.g., 7a). The cascade process involved likely starts with a copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction. This is followed by ring-opening of the resulting metalated triazole (with accompanying loss of nitrogen), reaction of the ensuing ketenimine with the pendant phenolic hydroxyl group, and finally dehydration of the (Z)-N-(4-hydroxychroman-2-ylidene)sulfonamide so formed.

Bioorthogonal Azide-Thioalkyne Cycloaddition Catalyzed by Photoactivatable Ruthenium(II) Complexes.

Tailored ruthenium sandwich complexes bearing photoresponsive arene ligands can efficiently promote azide–thioalkyne cycloaddition (RuAtAC) when irradiated with UV light. The reactions can be performed in a bioorthogonal manner in aqueous mixtures containing biological components. The strategy can also be applied for the selective modification of biopolymers, such as DNA or peptides. Importantly, this ruthenium‐based technology and the standard copper‐catalyzed azide–alkyne cycloaddition (CuAAC) proved to be compatible and mutually orthogonal.

Fabrication and Characterization of a Biomaterial Based on Extracellular-Vesicle Functionalized Graphene Oxide

Mesenchymal stem cell (MSC) derived extracellular vesicles (EV) are emerging as acellular therapeutics for solid organ injury and as carriers for drug delivery. Graphene-based materials are novel two-dimensional crystal structure-based materials with unique characteristics of stiffness, strength and elasticity that are being explored for various structural and biological applications. We fabricated a biomaterial that would capture desirable properties of both graphene and stem cell derived EV. Metabolically engineered EV that express azide groups were cross-linked with alkyne-functionalized graphene oxide (GO) via a copper catalyzed alkyne-azide cycloaddition (CuAAC) reaction. The crosslinking between EV and GO was accomplished without the need for ligand expression on the metal. Scanning electron and fluorescence microscopy demonstrated excellent cross-linking between EV and GO. Biological effects were assessed by phagocytosis studies and cell viability studies. The uptake of GO or sonicated GO (sGO) resulted in a durable pro-inflammatory immune response. Cell studies further showed that crosslinked GO-EV scaffolds exhibited cell-type dependent cytotoxicity on liver cancer cells whereas there was minimal impact on healthy hepatocyte proliferation. In vitro, neither GO-EV nor sGO-EV induced DNA strand breaks. In vivo studies in zebrafish revealed gross developmental malformations but treatment-induced mortality was only seen with the highest doses of GO-EV and sGO-EV. With these advantages, this engineered biomaterial combining the versatility of graphene with the therapeutic effects of MSC-EV has potential for applications in tissue engineering and regenerative medicine.

Synthesis and post-polymerization modification of poly(propargyl 2-ylidene-acetate)

Rhodium(I)-catalyzed C1 polymerization of propargyl 2-diazoacetate was conducted for the first time resulting in a novel functional polymethylene derivative, poly(propargyl 2-ylidene-acetate), which is suitable for a subsequent post-polymerization modification under mild reaction conditions via copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The post-modification reaction results in quantitative conversions, and provides an efficient and general route for the selective post-modification of C1 polymers with a variety of azide compounds. The structure of the poly(propargyl 2-ylidene-acetate) was confirmed and characterized by NMR, FT-IR, and GPC techniques. Upon comparison with results on post-polymerization modification of the corresponding C2 polymer, poly(propargyl acrylate), no obvious influence on the reactivity towards the post-modification of acetylene group on the C1 polymer precursor was observed. Noteworthy, the introduction of pentafluorophenyl azide facilitated a subsequent second modification via the Para-Fluoro-Thiol reaction (PFTR). Accordingly, cascade post-modifications of C1 polymers yielded novel polymethylenes that are not accessible by direct C1 polymerization.

The flexibility of periphery enhances the electrochemical reversibility of ferrocenyl-terminated polyphenylene dendrimers

Four dendrimers of the first and second generations containing rigid polyphenylene interior and flexible ferrocenyl-terminated alkyl arms (CH2)m(C5H4)Fe(C5H5) (m = 6, 11) were synthesized by combining two types of “click” reactions: Diels-Alder and copper-catalyzed azide-alkyne cycloaddition reactions. The resulting compounds were characterized by NMR spectroscopy, gel permeation chromatography, and cyclic voltammetry (CV). CVs of the dendrimers show a single chemically reversible wave. In contrast to wholly aromatic redox-active dendrimers, a combination of rigid polyphenylene interior with flexible alkylferrocenyl periphery provides an electrochemically reversible redox process. The dendrimers exhibit strong adsorption onto the Pt electrode upon oxidation, thus providing fabrication of stable modified electrodes coated with a second-generation dendrimer.