Azide Component Research Articles

A click chemistry-based, free radical-initiated delivery system for the capture and release of payloads

Click chemistries are efficient and selective reactions that have been leveraged for multi-stage drug delivery. A multi-stage system allows independent delivery of targeting molecules and drug payloads, but targeting first-phase materials specifically to disease sites remains a challenge. Stimuli-responsive systems are an emerging strategy where common pathophysiological triggers are used to target payloads. Oxidative stress is widely implicated in disease, and we have previously demonstrated that reactive oxygen species (ROS) can crosslink and immobilize polyethylene glycol diacrylate (PEGDA) in tissue mimics. To build on these promising results, we present a two-step, catch-and-release system using azide-DBCO click chemistry and demonstrate the capture and eventual release of a fluorescent payload at defined times after the formation of a PEGDA capturing net. The azide component is included with radical-sensitive PEGDA, and the payload is conjugated to the DBCO group. In cell-free and cell-based tissue mimic models, azides were incorporated at 0–30% in the first-phase polymer net, and DBCO was delivered at 2.5–10 µM in the second phase to control payload delivery. The payload could be captured at multiple timepoints after initial net formation, yielding a flexible and versatile targeting system. Matrix metalloproteinase (MMP)-degradable peptides were incorporated into the polymer backbone to engineer fluorescent payload release by MMPs, which are broadly upregulated in diseases, through degradation of the capture net and directly from the DBCO. Taken together, this research demonstrates proof-of-principle for a responsive and clickable biomaterial to serve as a multi-potent agent for the treatment of diseases compounded by high free radicals.

Polyurethane polymers cured via azide-alkyne cycloaddition

Conventional thermoset polyurethane polymers are crosslinked by reaction of a polyisocyanate compound with a polyol. Herein are described alternative crosslinking polyurethanes (ACPUs) for coatings and related applications that cure by azide-alkyne cycloaddition. Commercial polyisocyanate resins including allophanate, isocyanurate, and biuret types were reacted with propargyl alcohol or 2-hydroxyethyl propiolate to yield polyurethane resins with terminal alkyne functionality. Various polyols, including polyether, polyester, and polyacrylic types were modified to convert their hydroxyl functionality to azide functionality. The best performance was obtained with an alkyne component based on Desmodur XP 2580 and an azidated polyol based on Setalux D A 870 BA. Clear, high-solids, two-component coatings were prepared with and without Cu(I) catalyst. The coating performance properties including pencil hardness, MEK double rubs, and glass transition temperature (Tg) were comparable to a conventional polyurethane control coating made from the precursor resins. Azide-alkyne formulations in the presence of copper catalyst exhibited faster curing kinetics than the polyurethane control. Propiolate-based systems showed significantly faster curing kinetics compared to the propargylated systems with or without Cu(I) catalyst. A study of azide:alkyne stoichiometry surprisingly showed that higher crosslink density of ACPUs may be obtained by formulating with 35–50 mol% excess azide component.

Synthesis of hybrid perillyl‐4H‐pyrans. Cytotoxicity evaluation against hepatocellular carcinoma HepG2/C3A cell line

AbstractA series of 15 new hybrid perillyl‐4H‐pyrans compounds was straightforwardly synthesized by a strategy combining the multicomponent reaction and the copper‐catalyzed alkyne‐azide cycloaddition (CuAAC). The 2‐amino‐4H‐pyrans‐3‐carbonitrile containing the alkyne moiety was prepared via multicomponent reaction between 1,3‐dicarbonyl, a propargyloxy aromatic aldehyde and malononitrile or ethyl α‐cyanoacetate. The alkyne derivative was sequentially reacted with the perillyl azide component through the copper‐catalyzed [3+2] Huisgen cycloaddition reaction. The antiproliferative activity of hybrid compounds were evaluated against the human hepatoma HepG2/C3A cell line.

Selective Synthesis of N-H and N-Aryl Benzotriazoles by the [3 + 2] Annulation of Sodium Azide with Arynes.

The synthetic utility of NaN3 as the azide component in the [3 + 2] annulation with arynes generated from 2-(trimethylsilyl)aryltriflates resulting in the transition-metal-free synthesis of N-H and N-aryl benzotriazoles has been demonstrated. Using CsF as the fluoride source in CH3CN, the N-H benzotriazoles are formed in high selectivity instead of the expected azidobenzene. Interestingly, N-aryl benzotriazoles are formed using KF and THF as solvent in an open-flask reaction. Moreover, a method for the N1-arylation of benzotriazole is also presented.

Nucleoside dimers analogs containing floxuridine and thymidine with unnatural linker groups: synthesis and cancer line studies. Part III

Two series of novel fluorinated nucleosides dimers with an unnatural 1,2,3-triazole linkage were synthesized. The obtained molecules were prepared using “click” chemistry approach based on copper(I) catalyzed Huisgen azide–alkyne cycloaddition. It was performed between 3′- and 5′-azido-nucleosides as the azide components, and the 3′-O- and 5′-O-propargyl-nucleosides as the alkyne components. Based on analysis of the 3JHH, 3JH1′C2 and 3JH1′C6 we estimated conformational preferences of sugar part and orientation around glycosidic bond. All described nucleosides dimers analogs were characterized by spectroscopic methods and evaluated for their in vitro cytotoxicity in three human cancer cell lines: cervical (HeLa), oral (KB) and breast (MCF-7).

High Boron-loaded DNA-Oligomers as Potential Boron Neutron Capture Therapy and Antisense Oligonucleotide Dual-Action Anticancer Agents.

Boron cluster-modified therapeutic nucleic acids with improved properties are of interest in gene therapy and in cancer boron neutron capture therapy (BNCT). High metallacarborane-loaded antisense oligonucleotides (ASOs) targeting epidermal growth factor receptor (EGFR) were synthesized through post-synthetic Cu (I)-assisted “click” conjugation of alkyne-modified DNA-oligonucleotides with a boron cluster alkyl azide component. The obtained oligomers exhibited increased lipophilicity compared to their non-modified precursors, while their binding affinity to complementary DNA and RNA strands was slightly decreased. Multiple metallacarborane residues present in the oligonucleotide chain, each containing 18 B-H groups, enabled the use of IR spectroscopy as a convenient analytical method for these oligomers based on the diagnostic B-H signal at 2400–2650 cm−1. The silencing activity of boron cluster-modified ASOs used at higher concentrations was similar to that of unmodified oligonucleotides. The screened ASOs, when used in low concentrations (up to 50 μM), exhibited pro-oxidative properties by inducing ROS production and an increase in mitochondrial activities in HeLa cells. In contrast, when used at higher concentrations, the ASOs exhibited anti-oxidative properties by lowering ROS species levels. In the HeLa cells (tested in the MTT assay) treated (without lipofectamine) or transfected with the screened compounds, the mitochondrial activity remained equal to the control level or only slightly changed (±30%). These findings may be useful in the design of dual-action boron cluster-modified therapeutic nucleic acids with combined antisense and anti-oxidant properties.

Fused Bicyclic Caspase-1 Inhibitors Assembled by Copper-Free Strain-Promoted Alkyne-Azide Cycloaddition (SPAAC).

Challenges exist in the development of potent and selective small-molecule inhibitors against caspase-1. Herein, by making use of the copper-free strain-promoted alkyne-azide cycloaddition (SPAAC) reaction between difluorinated cyclooctynes (DIFOs) and various azide-containing compounds, we showed for the first time that potential caspase-1 inhibitors could be rapidly synthesized. The resulting fused bicyclic compounds structurally resembled the central portion (P2 -P3 ) of Pralnacasan (a well-known small molecule caspase-1 inhibitor), with diversity at the P4 -position of the parental inhibitor conveniently installed from the azide component. Since our SPAAC-assembled inhibitor library was synthesized by using a copper-free bioorthogonal chemistry, the resulting 52-membered library (2 DIFOs×26 azides) was immediately ready for subsequent cell-based screening for rapid identification of potential cell-permeable hits capable of effectively inhibiting endogenous caspase-1 activities. C1FS, a recently reported fluorogenic two-photon probe, which possesses improved live-cell imaging sensitivity against endogenous caspase-1, was used both in vitro and in LPS/ATP-induced macrophages (a well-established caspase-1-activated cell model) to screen against selected compounds from the above-mentioned library, leading to subsequent discovery of a novel caspase-1 inhibitor named b7-b.

Stereo- and Regioselective Azide/Alkyne Cycloadditions in Carbonic Anhydrase II via Tethering, Monitored by Crystallography and Mass Spectrometry

The carbonic anhydrase II mutant His64Cys was prepared and applied to tethered alkyne/azide cycloaddition reactions. The azide component could be tethered to the enzyme surface through a disulfide bridge, while the alkyne component was reversibly coordinated through a sulfonamide anchor to the zinc ion in the original catalytic center of the enzyme. The incipient orientation of the reactants in the binding site and of the formed triazole product were characterized by crystallography. The reaction progression could be monitored by HPLC-MS analysis.

Synthesis of 3′-azido-3′-deoxythymidine (AZT)—Cinchona alkaloid conjugates via click chemistry: Toward novel fluorescent markers and cytostatic agents

Novel nucleoside-Cinchona alkaloid conjugates were synthesized using 'click' chemistry approach based on the copper(I) catalyzed Huisgen azide-alkyne cycloaddition. Two series of conjugates were prepared employing 3'-azido-3'-deoxythymidine (AZT) as the azide component and the four 10,11-didehydro Cinchona alkaloids as well as their 9-O-propargyl ethers as the alkyne components. All obtained conjugates showed strong fluorescence emission and some of them exhibited marked cytotoxic activity in vitro.

Fused tetrazoles as azide surrogates in click reaction: efficient synthesis of N-heterocycle-substituted 1,2,3-triazoles.

It has been shown that various pyrido-, quinolino-, pyrazino-, and quinoxalinotetrazoles can be used efficiently as azide components in Cu-catalyzed click reaction with alkynes. This method allows for efficient synthesis of a wide variety of N-heterocyclic derivatives of 1,2,3-triazoles.

Synthesis of an Antibacterial Compound Containing a 1,4-Substituted 1H-1,2,3-Triazole: A Scaleable Alternative to the “Click” Reaction

The copper-catalyzed “click” reaction of an azide with an alkyne has become a popular method to build up 1,4-substituted 1H-1,2,3-triazoles in medicinal chemistry and this approach was used on a laboratory scale during the preparation of novel macrolide 1. However, the manufacture of the key azide component, as well as its subsequent use in the presence of a copper catalyst on a large scale, was associated with potential safety concerns. Therefore, a sequence was developed in which construction of the 1,4-substituted 1H-1,2,3-triazole in 1 was accomplished via cyclocondensation of an α,α-dichloro tosyl hydrazone with an amine.

On-demand click functionalization of polyurethane films and foams

A novel platform of side-chain functionalized crosslinked polyurethane (PU) films and foams has been used to afford new classes of PU materials. Indeed, alkyne pendant groups located along the polyurethane backbone have been modified via the copper catalyzed Huisgen 1,3-dipolar cycloaddition with various azide components. These cycloaddition reactions were carried out in water or in a mixture of water/acetone under mild conditions (room temperature to 50°C) in the presence of low amounts of copper catalyst. The resulting functionalized films and foams were all characterized by FT-IR, contact angle measurements and confocal fluorescence microscopy. All these techniques successfully proved that the click chemistry process is valuable in functionalizing PU films and foams in an easy way. The post-functionalization of these alkyne-functionalized PUs afforded the creation of new classes of PU materials with easily adaptable physical properties by choosing the appropriate azide component.

Step-growth polymerization and ‘click’ chemistry: The oldest polymers rejuvenated

Since its discovery in 2001, copper catalyzed azide–alkyne ‘click’ chemistry has been extensively used in polymer chemistry to modify polymeric materials and create advanced polymer structures by efficient coupling reactions. Surprisingly, the contribution of this Huisgen cycloaddition reaction to industrially important commodity polymers, prepared by step-growth polymerization, was not existing until recently. Nevertheless, since many decades academic and industrial research was focused on finding attractive synthetic pathways to introduce large contents of different reactive functional groups in several polymer classes such as polyesters and polyurethanes. Because of the high tolerance of azide–alkyne coupling reactions to a wide variety of functional groups and to extreme reaction conditions often used in step-growth polymerizations, the straightforward synthesis of alkyne-containing building blocks created an ideal platform to modify and broaden the physico-chemical properties of step-growth polymers by choosing readily available low and high molecular weight azide components. This feature article provides a comprehensive review covering the strategies toward ‘click’-functionalization of several classes of industrially important step-growth polymers.

Surface Modification of Tobacco Mosaic Virus with “Click” Chemistry

Surface Modification of Tobacco Mosaic Virus with “Click” Chemistry

Donor−Acceptor Triazenes: Synthesis, Characterization, and Study of Their Electronic and Thermal Properties

A new class of 1,3-disubstituted-triazenes were synthesized by coupling functionalized benzimidazol-2-ylidenes, as their free N-heterocyclic carbenes or generated in situ from their respective benzimidazolium precursors, to various aryl azides in modest to excellent isolated yields (36-99%). Electron delocalization between the two coupled components was studied using UV-vis spectroscopy, NMR spectroscopy, and X-ray crystallography. Depending on the complementarity of the functional groups on the N-heterocyclic carbenes and the organic azides, the respective triazenes were found to exhibit lambda(max) values ranging between 364 and 450 nm. X-ray crystallography revealed bond alteration patterns in a series of triazenes characteristic of donor-acceptor compounds. Triazene thermal stabilities were studied using thermogravimetric analysis and found to be strongly dependent on the sterics of the benzimidazol-2-ylidene component and the electronics of the azide component. Triazenes possessing bulky N-substituents (e.g., neo-pentyl, tert-butyl, etc.) were stable in the solid-state to temperatures exceeding 150 degrees C, whereas analogues with small N-substituents (e.g., methyl) were found to slowly decompose at room temperature. Triazenes featuring electron-rich phenyl azide components decomposed at higher temperatures than their electron-deficient analogues. Products of the thermally induced triazene decomposition reaction were identified as molecular nitrogen and the respective guanidine. Using an isotopically labeled triazene, the mechanism of the decomposition reaction was found to be analogous to the Staudinger reaction.

Mechanism of Thio Acid/Azide Amidation

A combined experimental and computational mechanistic study of amide formation from thio acids and azides is described. The data support two distinct mechanistic pathways dependent on the electronic character of the azide component. Relatively electron-rich azides undergo bimolecular coupling with thiocarboxylates via an anion-accelerated [3+2] cycloaddition to give a thiatriazoline. Highly electron-poor azides couple via bimolecular union of the terminal nitrogen of the azide with sulfur of the thiocarboxylate to give a linear adduct. Cyclization of this intermediate gives a thiatriazoline. Decomposition to amide is found to proceed via retro-[3+2] cycloaddition of the neutral thiatriazoline intermediates. Computational analysis (DFT, 6-31+G(d)) identified pathways by which both classes of azide undergo [3+2] cycloaddition with thio acid to give thiatriazoline intermediates, although these paths are higher in energy than the thiocarboxylate amidations. These studies also establish that the reaction profile of electron-poor azides is attributable to a prior capture mechanism followed by intramolecular acylation.