Application of “Click” Chemistry to the Construction of Supramolecular Functional Systems

Growing Impact of Intramolecular Click Chemistry in Organic Synthesis

Click Chemistry, a modular, rapid, and one of the most reliable tool for the regioselective 1,2,3-triazole forming [3+2] reaction of organic azide and terimal alkyne is widely explored in various emerging domains of research ranging from chemical biology to catalysis and medicinal chemistry to material science. This regioselective reaction from a diverse range of azido-alkyne scaffolds has been well performed in both intermolecular as well as intramolecular fashions. In comparison to the intermolecular metal (Cu/Ru/Ni) variant of 'Click Chemistry', the intramolecular click tool is little addressed. The intramolecular click chemistry is exemplified as a mordern tool of cyclization which involves metal-catalyzed (CuAAC/RuAAC) cyclization, organo-catalyzed cyclization, and thermal-induced topochemical reaction. Thus, we report herein the recent approaches on intramolecular azide-alkyne cycloaddition 'Click Chemistry' with their wide-spread emerging applications in the developement of a diverse range of molecules including fused-heterocycles, well-defined peptidomemics, and macrocyclic architectures of various notable features.

Readily accessible azido-alkyne-functionalized monomers for the synthesis of cyclodextrin analogues using click chemistry

A set of linear and cyclic oligomers were synthesized starting from a suitable azido-alkyne monomer through click oligomerization. The synthesis of these monomers starting from bromobenzene features an enzymatic dihydroxylation...

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Conditionally Activatable Visible-Light Photocages

The proof of concept for conditionally activatable photocages is demonstrated on a new vinyltetrazine-derivatized coumarin. The tetrazine form is disabled in terms of light-induced cargo release, however, bioorthogonal transformation of the modulating tetrazine moiety results in fully restored photoresponsivity. Irradiation of such a “click-armed” photocage with blue light leads to fast and efficient release of a set of caged model species, conjugated via various linkages. Live-cell applicability of the concept was also demonstrated by the conditional release of a fluorogenic probe using mitochondrial pretargeting.

Living Supramolecular Polymerization of an Aza-BODIPY Dye Controlled by a Hydrogen-Bond-Accepting Triazole Unit Introduced by Click Chemistry

An aza-BODIPY dye 1 bearing two hydrophobic fan-shaped tridodecyloxybenzamide pendants through 1,2,3-triazole linkages was synthesized by a click reaction and characterized. ¹ H NMR studies indicated that dye 1 exhibited variable conformations through intramolecular H-bonding interaction, which is beneficial for the polymorphism of aggregation. The thermodynamic, structural, and kinetic aspect of the supramolecular polymerization of dye 1 was investigated by UV/Vis absorption spectroscopy, IR spectroscopy, AFM, TEM, and SEM. Biphasic aggregation pathways of dye 1, leads to the formation of off-pathway, metastable Agg. I and thermodynamically stable Agg. II with distinct H-aggregation spectra and nanoscale morphology. The living manner of the supramolecular polymerization of dye 1 was demonstrated in seeded polymerization experiments with temperature-modulated successive cooling-heating cycles.

Recent Advances in Bioorthogonal Click Chemistry for Efficient Synthesis of Radiotracers and Radiopharmaceuticals

In recent years, several catalyst-free site-specific reactions have been investigated for the efficient conjugation of biomolecules, nanomaterials, and living cells. Representative functional group pairs for these reactions include the following: (1) azide and cyclooctyne for strain-promoted cycloaddition reaction, (2) tetrazine and trans-alkene for inverse-electron-demand-Diels–Alder reaction, and (3) electrophilic heterocycles and cysteine for rapid condensation/addition reaction. Due to their excellent specificities and high reaction rates, these conjugation methods have been utilized for the labeling of radioisotopes (e.g., radiohalogens, radiometals) to various target molecules. The radiolabeled products prepared by these methods have been applied to preclinical research, such as in vivo molecular imaging, pharmacokinetic studies, and radiation therapy of cancer cells. In this review, we explain the basics of these chemical reactions and introduce their recent applications in the field of radiopharmacy and chemical biology. In addition, we discuss the significance, current challenges, and prospects of using bioorthogonal conjugation reactions.

Combretastatin A4-camptothecin micelles as combination therapy for effective anticancer activity

Cancer is a major worldwide health problem, for which chemotherapy is a common treatment option. However drug toxicity and the development of resistance to chemotherapy are two main challenges associated with the traditional anticancer drugs. Combined pharmacological therapy based on different mechanisms might be an effective strategy in cancer treatment, and could exhibit a synergistic therapeutic efficacy. Herein, we aim to combine combretastatin A4 (CA4) and camptothecin (Cpt) chemically into a codrug through two hydrophilic linkers utilizing click chemistry to improve their water solubility and anticancer activity. The synthesized amphiphilic structure could self-assemble into a micelle structure as confirmed by atomic force microscopy (AFM) and dynamic light scattering (DLS), which showed a high stability and improved water solubility at pH 7.4, with a low critical micelle concentration (CMC) value of 0.9 mM. Moreover, in vitro hydrolysis was observed upon incubation of the hybrid compound with an esterase enzyme, which suggested a complete disassembly into the starting active drugs. Finally, cytotoxicity studies on HeLa cancer cells showed that the codrug demonstrated an enhanced (five fold) cytotoxicity as compared with the free drugs. In addition the combination index (CI) was <1, which suggests a synergistic activity for the codrug. Moreover, the tested concentrations of the codrug were not significantly cytotoxic to a noncancerous fibroblast cell line. The imaging of HeLa cells treated with FITC-loaded micelles showed a rapid internalization. In conclusion, the codrug of CA4 and Cpt might be a potential novel anticancer drug as it demonstrated a synergistic cytotoxic activity that might spare noncancerous cells.

Metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides

In this review recent advances in the metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides are highlighted, covering references from the period 2000 until August 2018.

An eco-compatible strategy for the diversity-oriented synthesis of macrocycles exploiting carbohydrate-derived building blocks

An efficient, eco-compatible diversity-oriented synthesis (DOS) approach for the generation of library of sugar embedded macrocyclic compounds with various ring size containing 1,2,3-triazole has been developed. This concise strategy involves the iterative use of readily available sugar-derived alkyne/azide-alkene building blocks coupled through copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction followed by pairing of the linear cyclo-adduct using greener reaction conditions. The eco-compatibility, mild reaction conditions, greener solvents, easy purification and avoidance of hazards and toxic solvents are advantages of this protocol to access this important structural class. The diversity of the macrocycles synthesized (in total we have synthesized 13 macrocycles) using a set of standard reaction protocols demonstrate the potential of the new eco-compatible approach for the macrocyclic library generation.

Near-IR Absorbing J-Aggregate of an Amphiphilic BF2 -Azadipyrromethene Dye by Kinetic Cooperative Self-Assembly

A new amphiphilic BF₂ -azadipyrromethene (aza-BODIPY) dye 1 has been synthesized using a Cu^I -catalyzed "click" reaction. For this dye, two self-assembly pathways that lead to different type of J-aggregates with distinct near-infrared optical properties have been discovered. The metastable off-pathway product displays a broad, structureless absorption band while the thermodynamically stable on-pathway aggregate exhibits the characteristic spectral features of a J-aggregate, that is, red-shifted intense absorption band with significantly narrowed linewidth. The morphology and structure of the aggregates were studied by atomic force microscopy, transmission and scanning electron microscopy. The aggregation processes of 1 were investigated by temperature- and concentration-dependent UV/Vis spectroscopy and evaluated by models for cooperative self-assembly.

5,5′-Bistriazoles as axially chiral, multidentate ligands: synthesis, configurational stability and catalytic application of their scandium(iii) complexes

The design and development of 5,5′-bistriazoles featuring aminomethyl substituents is discussed.

Synthesis and catalytic applications of C3-symmetric tris(triazolyl)methanol ligands and derivatives

Recently introduced tris(1,2,3-triazol-4-yl)methanols and derivatives (TTM ligands) have become a valuable subclass of C3-symmetric tripodal ligands for transition metal-mediated reactions. TTM-based ligand architectures are modularly constructed through regioselective, one-pot triple [3+2] cycloaddition of azides and alkynes. Applications of homogeneous systems of this type and of heterogenised (polystyrene- and magnetic nanoparticle-supported) TTM ligands in synthesis and catalysis are compiled in this Feature Article.

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

A Click Chemistry Approach towards Flavin-Cyclodextrin Conjugates-Bioinspired Sulfoxidation Catalysts

A click chemistry approach based on the reaction between alkynylflavins and mono(6-azido-6-deoxy)-β-cyclodextrin has proven to be a useful tool for the synthesis of flavin-cyclodextrin conjugates studied as monooxygenase mimics in enantioselective sulfoxidations.

Click chemistry oligomerisation of azido-alkyne-functionalised galactose accesses triazole-linked linear oligomers and macrocycles that inhibit Trypanosoma cruzi macrophage invasion

Reaction of 2-(2-(2-azidoethoxy)ethoxy)ethyl 6-O-(prop-2-ynyl)-β-d-galactopyranoside (7) under CuAAC conditions gives rise to mixed cyclic and linear triazole-linked oligomers, with individual compounds up to d.p. 5 isolable, along with mixed larger oligomers. The linear compounds resolve en bloc from the cyclic materials by RP HPLC, but are separable by gel permeation chromatography. The triazole-linked oligomers—pseudo-galactooligomers—were demonstrated to be acceptor substrates for the multi-copy cell surface trans-sialidase of the human parasite Trypanosoma cruzi. In addition, these multivalent TcTS ligands were able to block macrophage invasion by T. cruzi.

Synthesis of glycotriazololipids and observations on their self-assembly properties

Various carbohydrate-anchored triazole-linked lipids prepared by solvent-free mechanochemical azide-alkyne click reaction, on analysis by TEM, have been found to spontaneously self-assemble in solvents leading to structures of interesting physicochemical attributes. Interestingly, analogous compounds based on different sugars (e.g., d-glucose, and d-galactose, as also d-lactose) assemble in patterns distinctly different from each other thus reiterating the fact that the structure of the sugar as well as that of the lipid are important factors that determine the size and shape of the supramolecular assembly formed. Besides, the molecular self-assembly was also found to be solvent-as well as temperature-dependent.

Synthesis of novel tryptamine-based macrocycles using an Ugi 4-CR/microwave assisted click-cycloaddition reaction protocol

A practical synthesis of novel tryptamine-based macrocycles using an Ugi 4-CR/click-cycloaddition sequential reaction protocol is described.

A fluorescent bistable [2]rotaxane molecular switch on SiO2 nanoparticles

A fluorescent bistable [2]rotaxane was immobilized onto the surface of SiO₂ nanoparticles through click reaction. The shuttling motion of the macrocycle component was driven by external acid–base stimuli both in solution and on SiO₂ nanoparticles, accompanied by visual fluorescence changes.