Advances of azide-alkyne cycloaddition-click chemistry over the recent decade

New Members of the Cinchona Alkaloids Family: Assembly of the Triazole Heterocycle at the 6' Position

The substance class of the well-known Cinchona alkaloids is widened by 6′-Amino-cinchonine and 6′-Amino-cinchonidine, novel compounds which incorporate a primary amino function in the quinolinic ring system. These key intermediates open the field for a range of fruitful chemistry. Here is described a short and direct pathway for the synthesis of triazole containing derivatives of the above-mentioned substances using the [3 + 2] Huisgen cycloaddition. For this purpose, the amines were first converted into the corresponding azides. Based on this, non-substituted and silyl-protected triazoles were synthesized as examples. Furthermore, didehydrated derivatives of quincorine and quincoridine were used as addition partners, resulting in compounds that carry the quinuclidine ring of the cinchona alkaloids at both ends. Some of these compounds were examined radiographically to investigate the position of the quinuclidine ring to the triazole. The solid-state structures of compounds 10, 11 and 28 were determined by X-ray diffraction analyses.

An inside look at a biofilm: Pseudomonas aeruginosa flagella biotracking

The opportunistic pathogen, Pseudomonas aeruginosa, a flagellated bacterium, is one of the top model organisms for biofilm studies. To elucidate the location of bacterial flagella throughout the biofilm life cycle, we developed a new flagella biotracking tool. Bacterial flagella were site-specifically labeled via genetic code expansion. This enabled us to track bacterial flagella during biofilm maturation. Live flagella imaging revealed the presence and synthesis of flagella throughout the biofilm life cycle. To study the possible role of flagella in a biofilm, we produced a flagella knockout strain and compared its biofilm to that of the wild-type strain. Results showed a one order of magnitude stronger biofilm structure in the wild type in comparison with the flagella knockout strain. This suggests a possible structural role for flagella in a biofilm, conceivably as a scaffold. Our findings suggest a new model for biofilm maturation dynamic which underscores the importance of direct evidence from within the biofilm.

Mechanistic Insight into the Light-Triggered CuAAC Reaction: Does Any of the Photocatalyst Go?

The attainment of transition-metal catalysis and photoredox catalysis has represented a great challenge over the last years. Herein, we have been able to merge both catalytic processes into what we have called "the light-triggered CuAAC reaction". Particularly, the CuAAC reaction reveals opposite outcomes depending on the nature of the photocatalyst (eosin Y disodium salt and riboflavin tetraacetate) and additives (DABCO, Et₃N, and NaN₃) employed. To get a better insight into the operating processes, steady-state, time-resolved emission, and laser flash photolysis experiments have been performed to determine reactivity and kinetic data. These results, in agreement with thermodynamic estimations based on reported data, support the proposed mechanisms. While for eosin Y (EY), Cu(II) was reduced by its triplet excited state; for riboflavin tetraacetate (RFTA), mainly triplet excited RFTA state photoreductions by electron donors as additives are mandatory, affording RFTA^•- (from DABCO and NaN₃) or RFTAH^• (from Et₃N). Subsequently, these species are responsible for the reduction of Cu(II). For both photocatalysts, photogenerated Cu(I) finally renders 1,2,3-triazole as the final product. The determined kinetic rate constants allowed postulating plausible mechanisms in both cases, bringing to light the importance of kinetic studies to achieve a strong understanding of photoredox processes.

Synthesis and Characterisation of a Boron-Rich Symmetric Triazine Bearing a Hypoxia-Targeting Nitroimidazole Moiety

Boron Neutron Capture Therapy (BNCT) is a binary therapy that promises to be suitable in treating many non-curable cancers. To that, the discovery of new boron compounds able to accumulate selectively in the tumour tissue is still required. Hypoxia, a deficiency of oxygen in tumor tissue, is a great challenge in the conventional treatment of cancer, because hypoxic areas are resistant to conventional anticancer treatments. 2-Nitroimidazole derivatives are known to be hypoxia markers due to their enrichment by bioreduction in hypoxic cells. In the present work, 2-nitroimidazole was chosen as the starting point for the synthesis of a new boron-containing compound based on a 1,3,5-triazine skeleton. Two o-carborane moieties were inserted to achieve a high ratio of boron on the molecular weight, exploiting a short PEG spacer to enhance the polarity of the compound and outdistance the active part from the core. The compound showed no toxicity on normal human primary fibroblasts, while it showed noteworthy toxicity in multiple myeloma cells together with a consistent intracellular boron accumulation.

1,2,3-Triazoles as leaving groups in SNAr-Arbuzov reactions: synthesis of C6-phosphonated purine derivatives

A new method for C-N bond transformations into C-P bonds was developed using 1,2,3-triazoles as leaving groups in SNAr-Arbuzov reactions. A series of C6-phosphonated 2-triazolylpurine derivatives was synthesized for the first time, with the isolated yields reaching up to 82% in the C-P-bond-forming event. The SNAr-Arbuzov reaction of 2,6-bistriazolylpurines follows the general regioselectivity pattern of the C6-position being more reactive towards substitution, which was unambiguously proved by X-ray analysis of diethyl (9-heptyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)-9H-purin-6-yl)phosphonate.

Synthesis of Silsesquioxanes with Substituted Triazole Ring Functionalities and Their Coordination Ability

A synthesis of a series of mono-T8 and difunctionalized double-decker silsesquioxanes bearing substituted triazole ring(s) has been reported within this work. The catalytic protocol for their formation is based on the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) process. Diverse alkynes were in the scope of our interest-i.e., aryl, hetaryl, alkyl, silyl, or germyl-and the latter was shown to be the first example of terminal germane alkyne which is reactive in the applied process' conditions. From the pallet of 15 compounds, three of them with pyridine-triazole and thiophenyl-triazole moiety attached to T8 or DDSQ core were verified in terms of their coordinating properties towards selected transition metals, i.e., Pd(II), Pt(II), and Rh(I). The studies resulted in the formation of four SQs based coordination compounds that were obtained in high yields up to 93% and their thorough spectroscopic characterization is presented. To our knowledge, this is the first example of the DDSQ-based molecular complex possessing bidentate pyridine-triazole ligand binding two Pd(II) ions.

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.

Synthesis and cytotoxicity of hybrids of 1,3,4- or 1,2,5-oxadiazoles tethered from ursane and lupane core with 1,2,3-triazole

Ursane and lupane type (1-((5-aryl-1,3,4-oxadiazol-2-yl)methyl)-1H-1,2,3-triazol-4-yl)methyl and (1-((4-methyl-2-oxido-1,2,5-oxadiazol-3-yl)methyl)-1H-1,2,3-triazol-4-yl)methyl hybrids were prepared by 1,3-cycloaddition reactions of azole-derived azides with alkyne esters connected to positions C-3 and C-28 of triterpene core and tested for cytotoxicity. Hybrid compounds of 1,3,4-oxadiazoles attached at positions 3- and 28- of triterpenoid frame via triazole spacer and combinations of 1,2,5-oxadiazole or 1,3,4-oxadiazole, tethered with succinate linker and 1,2,3-triazole at the position 3- of the ursane backbone, were inactive in relation to all the cancer cells tested. Eventually, combinations of furoxan fragment and 1,2,3-triazole linked to C-28 position of triterpene backbone demonstrated marked cytotoxic activity towards MCF-7 and HepG2 cells. The most active ester of ursolic acid with (1-((4-methyl-2-oxido-1,2,5-oxadiazol-3-yl)methyl)-1H-1,2,3-triazol-4-yl)methyl substituent and 3-O-acetyl group was superior in activity and selectivity over doxorubicin and ursolic acid on MCF-7 cells. The length of the carbon spacer group may be of crucial importance for cytotoxicity. The introduction of the additional ester linker between the C-28 of triterpenoid and triazole or changing triazole spacer between furoxan moiety and triterpenoid core resulted in activity decrease against all the tested cells. In accordance with molecular modeling results, the activity of new derivatives may be explained in terms of the interaction of the new hybrid molecules and Mdm2 binding sites.

2-Aminobenzothiazole-Containing Copper(II) Complex as Catalyst in Click Chemistry: An Experimental and Theoretical Study

The reaction of copper(II) acetate with the 2-aminobenzothiazole (abt) heterocycle affords the new copper(II) complex of formula [Cu(abt)2(OOCCH3)2] (1) in a straightforward manner. Compound 1 served as a precatalyst for azide/alkyne cycloaddition reactions (CuAAC) in water, leading to 1,4-disubstituted-1,2,3-triazole derivatives in a regioselective manner and with excellent yields at room temperature. The main advantages of the coordination of such a heterocyclic ligand in 1 are its strong σ-donating ability (N-Cu), nontoxicity and biological properties. In addition, the click chemistry reaction conditions using 1 allow the formation of a great variety of 1,2,3-triazole-based heterocyclic compounds that make this protocol potentially relevant from biological and sustainable viewpoints. A molecular electron density theory (MEDT) study was performed by using density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) (LANL2DZ for Cu) level to understand the observed regioselectivity in the CuAAC reaction. The intramolecular nature of this reaction accounts for the regioselective formation of the 1,4-regioisomeric triazole derivatives. The ionic nature of the starting copper-acetylide precludes any type of covalent interaction throughout the reaction, as supported by the electron localization function (ELF) topological analysis, reaffirming the zwitterionic-type (zw-type) mechanism of the copper(I)/aminobenzothiazole-catalysed azide-alkyne cycloaddition reactions.

Recent achievements in copper catalysis for C–N bond formation

A mini-review describes the development of the catalysis by Cu(I) complexes aimed at the formation of C–N bond at the Lomonosov MSU during 2010s. The main approach employs the amination of aryl and heteroaryl halides with the amines and polyamines, in this direction a great versatility of starting compounds was achieved: adamantane-containing amines, linear diamines, oxadiamines and polyamines, various aryl iodides and bromides, derivatives of pyridine, and quinoline were used for this purpose. In more peculiar cases, the copper catalysis was used for steroids transformations, including vinylation of azoles, wide-spread “click” reactions for the conjugate syntheses, and successful heterogenezation of the copper catalysts were also undertaken.

Computation-Guided Development of the "Click" ortho-Quinone Methide Cycloaddition with Improved Kinetics

We report here a deep mechanistic study of the "click" ortho-quinone methide (oQM) cycloaddition between ortho-quinolinone quinone methide (oQQM) and thio-vinyl ether (TV), named as TQ-ligation. DFT calculations revealed the unexpected fact that dehydration of oQQM precursors is the rate-determining step of this transformation, and two highly reactive oQQM precursors were predicted. Guided by the calculations, a new "click" oQM cycloaddition which shows significantly improved kinetics and remarkable efficiency on protein labeling was developed.

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 of Metallopolymers and Direct Visualization of the Single Polymer Chain

During the past few decades, the study of the single polymer chain has attracted considerable attention with the goal of exploring the structure-property relationship of polymers. It still, however, remains challenging due to the variability and low atomic resolution of the amorphous single polymer chain. Here, we demonstrated a new strategy to visualize the single metallopolymer chain with a hexameric or trimeric supramolecule as a repeat unit, in which Ru(II) with strong coordination and Fe(II) with weak coordination were combined together in a stepwise manner. With the help of ultrahigh-vacuum, low-temperature scanning tunneling microscopy (UHV-LT-STM) and scanning tunneling spectroscopy (STS), we were able to directly visualize both Ru(II) and Fe(II), which act as staining reagents on the repeat units, thus providing detailed structural information for the single polymer chain. As such, the direct visualization of the single random polymer chain is realized to enhance the characterization of polymers at the single-molecule level.

Nitrileimines as an alternative to azides in base-mediated click [3 + 2] cycloaddition with methylene active nitriles

Nitrileimines were implemented in practical click protocols with oxopropanenitriles for the straightforward 5-amino-1H-pyrazole synthesis via 1,3-dipolar cycloaddition. The reaction proceeds at room temperature in a short time with base catalysis and no chromatographic purification. High purity products were isolated by simple filtration. The selectivity of the reaction was observed.

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.

AB2 polymerization on hollow microporous organic polymers: engineering of solid acid catalysts for the synthesis of soluble cellulose derivatives

New post-synthetic functionalization of hollow microporous organic polymers was developed based on AB₂ polymerization and thiol–yne click reaction.

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.

Intermolecular fluoroamination of allenes towards substituted vinyl fluorides

Allene difunctionalization: Selectfluor mediated intermolecular aminofluorination of allenes with triazole and tetrazole nucleophiles towards vinyl fluorides.

Synthesis and Photophysical Behavior of a Highly Fluorescent Family of Unsymmetrical Organoboron Complexes Containing 5-(Pyridin-2-ylmethylene)imidazolidine-2,4-dione Moieties

A new and highly fluorescent family of unsymmetrical organoboron complexes containing 5-(pyridin-2-ylmethylene)imidazolidine-2,4-dione moieties has been synthesized in three steps. These compounds show strong absorptions covering a wide range of the UV-vis spectrum and are strongly emissive (ϕ_f of up to 0.92 in CH₃CN). Moreover, two fluorophores that include an alkyne or an azide group at the end of the alkyl chain and with potential utility in bioorthogonal chemistry have been developed. One of these, in which the glycol substituent provides an enhanced water solubility without compromising the fluorescence (ϕ_f = 0.85 in water), may be of particular importance.

Designing Microparticle-Impregnated Polyelectrolyte Composite: The Combination of ATRP, Fast Azidation, and Click Reaction Using a Single-Catalyst, Single-Pot Strategy

Polystyrene microparticles were covalently impregnated into the networks of functional polyelectrolyte chains designed via a tandem run of three reactions: (i) synthesis of water-soluble polyelectrolyte, (ii) fast azidation and (iii) a 'click' reaction, using the single-catalyst, single-pot strategy at room temperature in mild aqueous media. The model polyelectrolyte sodium polystyrenesulfonate (NaPSS) was synthesized via the well-controlled atom transfer radical polymerization (ATRP) whose halogen living-end was transformed to azide and subsequently coupled with an alkyne carboxylic acid through a 'click' reaction using the same ATRP catalyst, throughout. Halogen to azide transformation was fast and followed the radical pathway, which was explained through a plausible mechanism. Finally, the success of microparticle impregnation into the NaPSS network was evaluated through Kaiser assay and imaging. This versatile synthetic procedure, having a reduced number of discrete reaction steps and eliminated intermediate work-ups, has established a fast and simple pathway to design functional polymers required to fabricate stable polymer-particle composites where the particles are impregnated covalently and controllably.

The CuAAC: Principles, Homogeneous and Heterogeneous Catalysts, and Novel Developments and Applications

The copper-catalyzed azide/alkyne cycloaddition reaction (CuAAC) has emerged as the most useful "click" chemistry. Polymer science has profited enormously from CuAAC by its simplicity, ease, scope, applicability and efficiency. Basic principles of the CuAAC are reviewed with a focus on homogeneous and heterogeneous catalysts, ligands, anchimeric assistance, and basic chemical principles. Recent developments of ligand design and acceleration are discussed.

Intramolecular azavinyl carbene-triggered rearrangement of furans

An intramolecular rhodium-catalyzed reaction of 1-tosyl-1,2,3-triazoles with furans has been explored. The tosylimino functionality was found to play a significant chemical role participating in the subsequent domino-transformations of a key reaction intermediate. One of the reaction pathways leads to valuable 2-formyl- and 2-acetylpyridine building blocks, which could be obtained in one-pot starting from easily accessible (furan-2-ylmethyl)propargyl amines. An original method for the synthesis of highly functionalized indolizines from the obtained pyridines has been proposed.