Click Products Research Articles

A novel Pt(iv) mono azido mono triazolato complex evolves azidyl radicals following irradiation with visible light.

The platinum(iv) azido complex trans,trans,trans-[PtIV(N3)2(OH)2(py)2] (1) undergoes cycloaddition with 1,4-diphenyl-2-butyne-1,4-dione (2) under mild, catalyst-free conditions, affording a number of mono and bis click products. The major mono click product (3) exists in MeCN as an equilibrium mixture between two species; 3a and 3b rapidly interconvert through nucleophilic attack of the axial Pt-OH group at the adjacent Ph-CO group. The kinetic and thermodynamic parameters for this interconversion have been measured by selective saturation-transfer NMR spectroscopic experiments and are consistent with cyclisation at the Pt centre. Complex 3b was also characterised by X-ray crystallography. Visible light irradiation (440-480 nm) of 3 in d3-MeCN produces azidyl radicals (N3˙), as demonstrated by EPR spin-trapping with DMPO; no generation of hydroxyl radicals was observed. 1H-195Pt HMBC NMR confirmed that the photoproducts were PtIV rather than PtII species, and HPLC was consistent with these being [3-N3]+ species; no facile photoejection of the triazolato ligand was observed, consistent with MS/MS fragmentation of 3. When 3 was irradiated in the presence of 5'-GMP, no 5'-GMP photoproducts were observed, suggesting that complex 3 is likely to exhibit significantly simplified biological activity (release of azidyl radicals but not DNA binding) compared with complex 1.

Effective and simple one-step strategy for preparation of “clickable” cellulose modules: support to build antibacterial materials

An efficient and simple way to generate “clickable” cellulose modules called alkyne-functionalized cellulose fibers (ACFs) for click reaction was presented. This method is only based on one step reaction: propargyl methacrylate (PMA) with pendant alkynyl groups was grafted onto the surface of cellulose fibers (CFs) using ceric ammonium nitrate as the initiator in aqueous medium. This has been providing a new and versatile platform for the wide utilization of cellulose resources and creation of novel functional materials. The effect of process variables on grafting copolymerization of PMA was investigated and suitable preparation conditions were identified. The clickability of ACFs was evaluated by reacting with two azides. The antibacterial cellulose functional material was achieved through incorporation of a “clickable” handle (alkynyl groups) to cellulose fibers followed by rapid attachment of β-cyclodextrin with an azide tag using copper-catalyzed click reaction and loading with ciprofloxacin hydrochloride (CipHCl). Pure CFs and ACFs showed a little affinity to CipHCl, whereas click products (ACFs@Azide-β-CD) increased the loading amount dramatically because the equilibrium amount was higher than pure CFs and ACFs. Meanwhile, the release process of CipHCl from ACFs@Azide-β-CD was prolonged to reaches the equilibrium. The ACFs@Azide-β-CD loaded with CipHCl showed excellent antibacterial activity against Escherichia coli. The ACFs@Azide-β-CD loading CipHCl have considerably longer bacterial activity against E. coli compared to CFs and ACFs.

'Second-generation' 1,2,3-triazole-based inhibitors of Porphyromonas gingivalis adherence to oral streptococci and biofilm formation.

Several 'second-generation' click inhibitors of the multi-species biofilm propagated by the adherence of the oral pathogen Porphyromonas gingivalis to Streptococcus gordonii were synthesized and evaluated. The design of the structures was based on the results obtained with the first-generation diphenyloxazole 'click' inhibitors which bear suitable hydrophobic and polar groups within a dual scaffold molecule bearing a 1,2,3-triazole spacer. The structures of the synthetic targets reported herein now consist of a triazolyl(phenylsulfonylmethyl) and a triazolyl(phenylsulfinylmethyl) spacer which joins a 4,5-diphenyloxazole with both phenyl rings bearing lipophilic substituents. The triazolyl "linker" group is formed by a click reaction between the 4-azido(phenylsulfonyl/sulfinylmethyl) oxazoles and acetylenic components having aryl groups bearing hydrophobic substituents. The 1,3,5-trisubstituted-2,4,6-triazine scaffold of the most active click compounds were modeled after the structural motif termed the VXXLL nuclear receptor (NR) box. When substituted at the 3- and 5-positions with 2- and 4-fluorophenylamino and N,N-diethylamino units, the candidates bearing the 1,3,5-trisubstituted-2,4,6-triazine scaffold formed a substantial subset of the second-generation click candidates. Four of the click products, compounds 95, 111, 115 and 122 showed inhibition of the adherence of P. gingivalis to S. gordonii with an IC50 range of 2.3-4.3 μM and only 111 exhibited cytotoxic activity against telomerase immortalized gingival keratinocytes at 60 μM. These results suggest that compounds 95, 115, 122, and possibly 111 represent the most suitable compounds to evaluate for activity in vivo.

Alternate pathway for the click reaction of 2-(2-azidophenyl)-4,5-diaryloxazoles

The CuSO4/ascorbate-mediated ‘click’ reaction of 2-(2-azidophenyl)-4,5-diaryloxazoles and arylacetylenes proceeded through an alternate pathway whereby reduction of the azide predominated over formation of the 1,2,3-triazole-forming cycloaddition. The unimolecular product, 2-(2-aminophenyl)-4,5-diphenyloxazole, was isolated which appears to be a formal reduction of the arylazide to the corresponding arylamine. A series of oxazoles which possessed various substituents (F, Cl, Br, OCH3) on the 4,5-diaryl rings and having the 2-azido group on the 2-oxazolylphenyl position were submitted to the same ‘click’ conditions and gave the corresponding arylamine products (73–99%). The reaction appears to be specific toward the ortho-azido substitution of the polycyclic system, as the corresponding azidomethyl-substituted phenyl oxazoles do not give the ‘reduction’ products but gave the expected click products with the acetylenic co-reactants.

Grafting polycaprolactone diol onto cellulose nanocrystals via click chemistry: Enhancing thermal stability and hydrophobic property

Hydrophobic and thermally-stable cellulose nanocrystals (CNCs) were synthesized by polycarpolactone diol (PCL diol) grafting via click chemistry strategy. The synthesis was designed as a three-step procedure containing azide-modification of CNCs, alkyne-modification of PCL diol and sequent copper(I)-catalyzed azide–alkyne cycloaddition reaction. The structure of azide-modified CNCs and alkyne-modified PCL diol, the structure, hydrophobic ability and thermal stability of click products CNC-PCL were characterized. FTIR, XPS and H1 NMR results indicated a successful grafting of the N3 groups onto the CNCs, synthesis of PCL diol-CCH, and formation of the CNC-PCL material. CNC-PCL had enhanced dispersion in the non-polar solvent chloroform owing to the well-maintained microscale size and PCL-induced hydrophobic surface. The thermal stability of CNC-PCL was largely increased due to the grafting of thermally-stable PCL. This work demonstrates that click chemistry is an attractive modification strategy to graft CNCs with polyester chains for further potential application in polymer composites.

Strategy for Conditional Orthogonal Sequential CuAAC Reactions Using a Protected Aromatic Ynamine.

A method for conditional control of orthogonal sequential Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reactions is reported. The inherent reactivity of an aromatic ynamine is controlled by a silyl protecting group that allows the selective CuAAC reaction of less reactive alkynes. Alternatively, the same protected ynamine undergoes selective CuAAC reaction via silyl deprotection in situ to give the ynamine click products. This allows complete orthogonal control of dialkyne systems and provides a unifying strategy for chemoselective CuAAC ligations in multialkyne/azide systems.

The intramolecular click reaction using ‘carbocontiguous’ precursors

The synthesis and utilization of all carbon-chain ‘carbocontiguous’ azidoalkynyl precursors for an intramolecular click reaction is described. The substrates contain both azidoalkyl and ethynylmethyl groups which are conjoined by a 2-(phenylsulfonylmethyl)-4,5-diphenyloxazole lynchpin and are suitably disposed for ring closure. On promotion by copper salts, a number of cyclic click products having the 1,4-disubstituted endo-fused triazole component and the 4,5-diphenyloxazole component are obtained. In one case, removal of the phenylsulfonylmethyl group from the substrate prior to cyclization gave the 1,5-disubstituted exo-fused triazole. The utilization of CuSO4/sodium ascorbate system appears to be the optimal conditions for closure/cyclization and afforded the cyclized products in yields of 84–95%.

Ribosome-Templated Azide-Alkyne Cycloadditions: Synthesis of Potent Macrolide Antibiotics by In Situ Click Chemistry.

Over half of all antibiotics target the bacterial ribosome-nature's complex, 2.5 MDa nanomachine responsible for decoding mRNA and synthesizing proteins. Macrolide antibiotics, exemplified by erythromycin, bind the 50S subunit with nM affinity and inhibit protein synthesis by blocking the passage of nascent oligopeptides. Solithromycin (1), a third-generation semisynthetic macrolide discovered by combinatorial copper-catalyzed click chemistry, was synthesized in situ by incubating either E.coli 70S ribosomes or 50S subunits with macrolide-functionalized azide 2 and 3-ethynylaniline (3) precursors. The ribosome-templated in situ click method was expanded from a binary reaction (i.e., one azide and one alkyne) to a six-component reaction (i.e., azide 2 and five alkynes) and ultimately to a 16-component reaction (i.e., azide 2 and 15 alkynes). The extent of triazole formation correlated with ribosome affinity for the anti (1,4)-regioisomers as revealed by measured Kd values. Computational analysis using the site-identification by ligand competitive saturation (SILCS) approach indicated that the relative affinity of the ligands was associated with the alteration of macrolactone+desosamine-ribosome interactions caused by the different alkynes. Protein synthesis inhibition experiments confirmed the mechanism of action. Evaluation of the minimal inhibitory concentrations (MIC) quantified the potency of the in situ click products and demonstrated the efficacy of this method in the triaging and prioritization of potent antibiotics that target the bacterial ribosome. Cell viability assays in human fibroblasts confirmed 2 and four analogues with therapeutic indices for bactericidal activity over in vitro mammalian cytotoxicity as essentially identical to solithromycin (1).

Ratiometric Fluorescence Azide–Alkyne Cycloaddition for Live Mammalian Cell Imaging

Click chemistry with metabolic labeling has been widely used for selectively imaging biomacromolecules in cells. The first example of azide-alkyne cycloaddition for ratiometric fluorescent imaging of live cells is reported. The precursor of the azido fluorophore (cresyl violet) has a fluorescence emission peak at 620 nm. The electron-rich nitrogen of the azido group blue-shifts the emission peak to 566 nm. When the click reaction occurs, an emission peak appears at 620 nm due to the lower electronic density of the newly formed triazole ring, which allows us to ratiometrically record fluorescence signals. This emission shift was applied to ratiometric imaging of propargylcholine- and dibenzocyclooctyne-labeled human breast cancer cells MCF-7 under laser confocal microscopy. Two typical triazole compounds were isolated for photophysical parameter measurements. The emission spectra presented a fluorescence emission peak around 620 nm for both click products. The results further confirmed the emission wavelength change was the result of azide-alkyne cycloaddition reaction. Since nearly all biomolecules can be metabolically labeled by reported alkyne-functionalized derivatives of native metabolites, our method can be readily applied to image these biomacromolecules.

Modification of chemical and conformational properties of natural organic matter by click chemistry as revealed by ESI-Orbitrap mass spectrometry.

A click reaction is reported here for the first time as a useful technique to control the conformational stability of natural organic matter (NOM) suprastructures. Click conjugates were successfully formed between a previously butynylated NOM hydrophobic fraction and a hydrophilic polyethylene glycol (PEG)-amino chain. The click products were shown by size exclusion chromatography (HPSEC) hyphenated with Orbitrap mass spectrometry (MS) in electrospray ionization (ESI) (+), while precursors were visible in ESI (-). Despite their increase in molecular weight, HPSEC elution of click conjugates occurred after that of precursors, thus showing their departure from the NOM supramolecular association. This indicates that the click-conjugated NOM molecules were varied in their hydrophilic and cationic character and lost the capacity to accommodate in the original hydrophobic suprastructures. The most abundant product had the C16H30O5N4 formula, a click conjugate of butanoic acid, while other products were short-chained (C4-C8) linear unsaturated and hydroxylated carboxylic acids. Tandem MS revealed formation of triazole rings in clicked conjugates and their two fragmentations at the ester and the C-N alkyl-aryl bonds. The behavior of NOM molecules modified by click chemistry confirms that hydrophobicity and ionic charge of humic molecules play a pivotal role in stabilizing intermolecular forces in NOM. Moreover, the versatility of the click reaction may become useful to decorate NOM molecules with a variety of substrates, in order to alter NOM conformational and chemical properties and diversify its applications in the environment.

Strain Promoted Click Chemistry of 2- or 8-Azidopurine and 5-Azidopyrimidine Nucleosides and 8-Azidoadenosine Triphosphate with Cyclooctynes. Application to Living Cell Fluorescent Imaging.

Strain-promoted click chemistry of nucleosides and nucleotides with an azido group directly attached to the purine and pyrimidine rings with various cyclooctynes in aqueous solution at ambient temperature resulted in efficient formation (3 min to 3 h) of fluorescent, light-up, triazole products. The 2- and 8-azidoadenine nucleosides reacted with fused cyclopropyl cyclooctyne, dibenzylcyclooctyne, or monofluorocyclooctyne to produce click products functionalized with hydroxyl, amino, N-hydroxysuccinimide, or biotin moieties. The 5-azidouridine and 5-azido-2'-deoxyuridine were similarly converted to the analogous triazole products in quantitative yields in less than 5 min. The 8-azido-ATP quantitatively afforded the triazole product with fused cyclopropyl cyclooctyne in aqueous acetonitrile (3 h). The novel triazole adducts at the 2- or 8-position of adenine or 5-position of uracil rings induce fluorescence properties which were used for direct imaging in MCF-7 cancer cells without the need for traditional fluorogenic reporters. FLIM of the triazole click adducts demonstrated their potential utility for dynamic measuring and tracking of signaling events inside single living cancer cells.

Synthesis and characterization of redox active cyrhetrene–triazole click products

We report the synthesis and characterization of two new cyclopentadienyl tricarbonyl rhenium(I) (cyrhetrene) complexes modified with a 1,4-disubstituted 1,2,3-triazole moiety. The two compounds, (η5-[4-phenyltriazol-1-yl]cyclopentadienyl) tricarbonyl rhenium(I), and (η5-[4-(4-aminophenyl)triazol-1-yl]cyclopentadienyl) tricarbonyl rhenium(I), were structurally characterized using 1H and 13C NMR, ATR-IR spectroscopy, UV–vis spectroscopy, mass spectrometry and X-ray crystallography where appropriate. The electrochemical behaviour of these two cyrhetrene–triazole complexes was explored using cyclic voltammetry, whereupon we observed that irreversible oxidation of the pendant 4-substituted-triazole moiety occurs before any electron transfer at the metal centre. This redox behaviour is in stark contrast to that of the analogous manganese(I) cymantrene–triazole derivatives, recently reported by our group.

Facile green synthesis and potent antimicrobial efficacy of β-aminoheteronapthol via tailored Betti’s protocol and their bis-aryl hydrazone click products

A series of novel Mannich base and their hydrazone derivatives were synthesized by highly resourceful, chic, simple and green technique with exceptionally facile reaction conditions of one-pot, three component reaction with an array of biologically and pharmaceutically active novel heterocycles. The protocol offers a valuable alternative to known methods and will find applications in the field of green synthesis and antimicrobial study against pathogenic microbes, supporting the development of bioinformatical database of novel and derelict heterocycles. These data indicate their potential to become antifungal agents. The novel products were established by elemental, IR, mass spectroscopic and NMR analysis. The environmental advantages of the method include short reaction time, excellent yield, easy work-up, absence of extraction and chromatographic purification steps.

Traceless Tosylhydrazone‐Based Triazole Formation: A Metal‐Free Alternative to Strain‐Promoted Azide–Alkyne Cycloaddition

Triple-T trick! Traceless tosylhydrazone-based triazole formation is readily achieved by reacting primary amines with functional α,α-dichlorotosylhydrozones under ambient conditions. This fast and efficient alternative affords exclusively 1,4-substituted triazole “click products” with complete retention of configuration. Primary amines, inherent to many natural products, can be modified in this way without protecting group manipulations. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Synthesis of luminescent homo-dinuclear cationic lanthanide cyclen complexes bearing amide pendant arms through the use of copper catalysed (1,3-Huisgen, CuAAC) click chemistry

The design and synthesis of dinuclear-lanthanide complexes possessing triazole-based bridges, formed by using copper catalysed 1,3-cycloaddition reactions between heptadentate alkyne functionalised cyclen europium or terbium complexes and di-azides (CuAAC reactions), are described. While this click reaction worked well for the formation of the homo-Eu(III) and Tb(III) bis-tri-arm cyclen N,N-dimethyl acetamide complexes, 2Eu and 2Tb, and for the homo-Eu(III) chiral N-methylnaphthalene based complexes 3Eu (S,S,S) and 4Eu (R,R,R), the formation of the Eu(III) complex of the primary amide analogue of 2, namely 1Eu, was not successful, clearly demonstrating the effect that the nature of the pendant arms has on this reaction. Furthermore, the click reactions between the free alkyne cyclen bis-derivatives (5-8) and the di-azide were unsuccessful, most likely due to the high affinity of the cyclen macrocycles for Cu(II). The Eu(III) complexes of 2-4 and 2Tb all gave rise to sensitised metal ion centred emission upon excitation of the triazole or the naphthalene antennae in methanol solution, and their hydration states were determined, which showed that while the Eu(III) mono-nuclear complexes had q ∼ 2, the click products all had q ∼ 1. In the case of 3Eu (S,S,S) and 4Eu (R,R,R), the circular polarised emission (CPL) was also observed for both, demonstrating the chiral environment of the lanthanide centres.

2-Dialkynyl derivatives of (N)-methanocarba nucleosides: 'Clickable' A(3) adenosine receptor-selective agonists.

We modified a series of (N)-methanocarba nucleoside 5'-uronamides to contain dialkyne groups on an extended adenine C2 substituent, as synthetic intermediates leading to potent and selective A(3) adenosine receptor (AR) agonists. The proximal alkyne was intended to promote receptor recognition, and the distal alkyne reacted with azides to form triazole derivatives (click cycloaddition). Click chemistry was utilized to couple an octadiynyl A(3)AR agonist to azido-containing fluorescent, chemically reactive, biotinylated, and other moieties with retention of selective binding to the A(3)AR. A bifunctional thiol-reactive crosslinking reagent was introduced. The most potent and selective novel compound was a 1-adamantyl derivative (K(i) 6.5nM), although some of the click products had K(i) values in the range of 200-400nM. Other potent, selective derivatives (K(i) at A(3)AR innM) were intended as possible receptor affinity labels: 3-nitro-4-fluorophenyl (10.6), alpha-bromophenacyl (9.6), thiol-reactive isothiazolone (102), and arylisothiocyanate (37.5) derivatives. The maximal functional effects in inhibition of forskolin-stimulated cAMP were measured, indicating that this class of click adducts varied from partial to full A(3)AR agonist compared to other widely used agonists. Thus, this strategy provides a general chemical approach to linking potent and selective A(3)AR agonists to reporter groups of diverse structure and to carrier moieties.

Cu(I)-Catalyzed Intramolecular Cyclization of Alkynoic Acids in Aqueous Media: A “Click Side Reaction”

Alkynoic acids, in particular, 4-pentynoic acid derivatives, undergo intramolecular cyclizations to enol lactones under reaction conditions typically applied for the Cu(I)-catalyzed cycloaddition of terminal alkynes and azides (click chemistry). Starting from appropriate alkynoic acid derivatives, either enol lactones or 1,2,3-triazole click products can be obtained selectively by Cu(I) catalysis in aqueous media.

Covalent Modification of Diruthenium Alkynyl Compounds: Novel Application of Click Reactions in Organometallic Chemistry

Diruthenium alkynyl compounds containing one or two 1,2,3-triazoles were prepared via the Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction (click reaction). Electrochemical, magnetic, and molecular structural features of the click products indicate that peripheral modification with the 1,2,3-triazole substituent induces minimal changes in both the molecular structure and electronic properties of Ru2 alkynyl compounds.