Biocompatible Azide–Alkyne “Click” Reactions for Surface Decoration of Glyco‐Engineered Cells

Bio‐orthogonal copper (I)‐catalyzed azide–alkyne cycloaddition (CuAAC) has been widely used to modify azide‐ or alkyne‐bearing monosaccharides on metabolic glyco‐engineered mammalian cells. Here, we present a systematic study to elucidate the design space for the cytotoxic effects of the copper catalyst on NIH 3T3 fibroblasts and on HEK 293‐F cells. Monitoring membrane integrity by flow cytometry and RT‐PCR analysis with apoptotic and anti‐apoptotic markers elucidated the general feasibility of CuAAC, with exposure time of the CuAAC reaction mixture having the major influence on biocompatibility. A high labeling efficiency of HEK 293‐F cells with a fluorescent alkyne dye was rapidly achieved by CuAAC in comparison to copper free strain‐promoted azide–alkyne cycloaddition (SPAAC). The study details effective and biocompatible conditions for CuAAC‐based modification of glyco‐engineered cells in comparison to its copper free alternative.     

Fine‐Tunable Tris(triazolyl)methane Ligands for Copper(I)‐ Catalyzed Azide–Alkyne Cycloaddition Reactions

The preparation of a small library of modular tris(triazolyl)methane ligands for copper‐catalyzed azide–alkyne cycloaddition (CuAAC) reactions is reported. The synthesis of the first generation ligand, tris(1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl)methanol ( 1a ), suitable for work in aqueous systems, is reported at the 50–100 mmol scale through a one‐stage, environmentally benign procedure. One‐stage procedures for the synthesis of tris(aryltriazolyl)methanol structures ( 1b , phenyl; 1c , para‐trifluoromethylphenyl; 1d , para‐methoxyphenyl) designed for electronic fine‐tuning of catalytic properties, and of 1a ‐derived ethers 2c (OBn) and 2d (OMe), designed for CuAAC reactions in organic solvents, are also reported. The complete set of ligands ( 1a–d , 2c–d ) has been tested in the reaction of phenylacetylene with benzyl azide in six different solvents (water, hexane, toluene, dichloromethane, tetrahydrofuran, and acetonitrile), and this has allowed the identification of 1b , 1c and 2c as the ligands depicting the highest tolerance to changes in solvent polarity within the considered family. The comparative performance of ligands 1b–d and 2c in the cycloaddition of a small family of alkynes with benzyl azide in two very different reaction media (1:1 t‐BuOH/H₂O and toluene) has been studied as a guide for catalyst selection in specific applications. The applicability of 1c in CuAAC reactions involving functional substrates in toluene has been explored under thermal and microwave‐accelerated (tandem azide formation plus CuAAC reaction) reaction conditions.

     

Multimeric Lactoside “Click Clusters” as Tools to Investigate the Effect of Linker Length in Specific Interactions with Peanut Lectin,Galectin‐1, and ‐3

Multimeric lactosides based on carbohydrate scaffolds with valencies ranging from 1 to 4 and different linker lengths were synthesized by a copper‐catalyzed azide–alkyne cycloaddition (CuAAC). The binding affinities and crosslinking abilities of the new “click clusters” toward biologically relevant galectins (gal‐1, gal‐3) and peanut lectin were evaluated by fluorescent polarization assay (FPA) and enzyme‐linked lectin assay (ELLA), respectively. FPA indicated that the binding affinities of the synthetic multilactosides towards the galectins increased proportionally with their lactosyl content, without significant differences due to the spacer length. ELLA evidenced a modest cluster effect for the multivalent conjugates, with a relative potency per lactoside ranging from 2.1 to 3.2. Nearly identical binding affinities were recorded for derivatives differing in the length of the linkers, in agreement with the FPA data. These results demonstrate that this parameter does not significantly influence the recognition process when interactions occur at a single lectin site. Molecular dynamics revealed that glycoconjugates adopt a pseudoglobular structure with a random localization of the lactoside residues. These spatial distributions were observed irrespective of the linker length; this explains the virtually equal affinities recorded by ELLA. In contrast, two‐site “sandwich” ELLA clearly revealed that multivalent derivatives bearing the longest spacers were more efficient for crosslinking lectins. Intrinsic affinities, devoid of aggregation effects, and crosslinking capabilities are, therefore, not directly related phenomena that must be taking into consideration in neoglycoconjugate design for specific applications.     

In situ crosslinked hydrogels formed using Cu(I)‐free Huisgen cycloaddition reaction

BACKGROUND: ‘Click’ chemistry, or the 1,3‐dipolar cycloaddition of organic azides with alkynes, has been evaluated for many biomedical purposes; however, its utility in crosslinking hydrogels in situ is limited by the toxicity of the requisite copper(I) catalyst. We report the first use of catalyst‐free Huisgen cycloaddition to generate crosslinked hydrogels under physiological conditions using multivalent azide‐functionalized polymers and an electron‐deficient dialkyne crosslinker. RESULTS: Water‐soluble azide‐functionalized polymers were crosslinked with an electron‐deficient dialkyne crosslinker to form hydrogels at physiological temperature without the addition of copper(I) catalyst. Crosslinking was confirmed using scanning electron microscopy, Fourier transform infrared and ¹H NMR analyses. Flow by vial inversion and dynamic rheological methodologies were implemented to evaluate gelation kinetics at 37 °C of variable polymer compositions, concentrations and stoichiometric ratios. Kinetic studies revealed gelation in as little as 12 h at 37 °C, although strong gels that withstand inversion were observed by 1–8 days. CONCLUSION: The ability to form hydrogel networks under mild conditions demonstrates the potential viability of the catalyst‐free ‘click’ crosslinking chemistry for in situ gelling and other biological applications. Further chemical modifications in the crosslinking moieties, as well as polymer and crosslinker conformations, are expected to enhance gelation kinetics to a more biomedically practical rate. Copyright © 2009 Society of Chemical Industry     

A Tailor‐Made “Tag–Receptor” Affinity Pair for the Purification of Fusion Proteins

A novel affinity “tag–receptor” pair was developed as a generic platform for the purification of fusion proteins. The hexapeptide RKRKRK was selected as the affinity tag and fused to green fluorescent protein (GFP). The DNA fragments were designed, cloned in Pet‐21c expression vector and expressed in E. coli host as soluble protein. A solid‐phase combinatorial library based on the Ugi reaction was synthesized: 64 affinity ligands displaying complementary functionalities towards the designed tag. The library was screened by affinity chromatography in a 96‐well format for binding to the RKRKRK‐tagged GFP protein. Lead ligand A7C1 was selected for the purification of RKRKRK fusion proteins. The affinity pair RKRKRK‐tagged GFP with A7C1 emerged as a promising solution (K_a of 2.45×10⁵ M ^?1). The specificity of the ligand towards the tag was observed experimentally and theoretically through automated docking and molecular dynamics simulations.     

Multicomponent Synthesis of 1,2,3‐Triazoles in Water Catalyzed by Copper Nanoparticles on Activated Carbon

Copper nanoparticles on activated carbon have been found to effectively catalyze the multicomponent synthesis of 1,2,3‐triazoles from different azide precursors, such as organic halides, diazonium salts, anilines and epoxides in water. The first one‐pot transformation of an olefin into a triazole is also described. The catalyst is easy to prepare, very versatile and reusable at a low copper loading.     

Copper(I) Acetate: A Structurally Simple but Highly Efficient Dinuclear Catalyst for Copper‐Catalyzed Azide‐Alkyne Cycloaddition

In this article, the structurally well‐defined dinuclear complex copper(I) acetate was studied in detail and was developed as a highly practical and efficient catalyst for the copper(I)‐catalyzed azide‐alkyne cycloaddition. The “bare” phenylethynylcopper(I) (i.e., with no exogeneous ligands) was isolated as an intermediate, which can be converted into an active catalytic species by treatment with acetic acid (in situ produced in the reaction) to efficiently catalyze the azide‐alkyne cycloaddition under mild conditions.     

Insights into Supported Copper(II)‐Catalyzed Azide‐Alkyne Cycloaddition in Water

Cross‐linked polymeric ionic liquid material‐supported copper (Cu‐CPSIL), imidazolium‐loaded Merrifield resin‐supported copper (Cu‐PSIL) and silica dispersed CuO (CuO/SiO₂), were prepared and proved to be efficient catalysts for the one‐pot synthesis of 1,4‐disubsituted‐1,2,3‐triazoles by the reaction of alkyl halides with sodium azide and terminal alkynes in water at room temperature. Moreover, these supported copper catalysts were recovered quantitatively from the reaction mixture by simple filtration and reused for five consecutive recycles without significant loss of catalytic activity. Among the three immobilized copper catalysts, Cu‐CPSIL exhibited excellent catalytic activity for the reaction of aliphatic bromides, sodium azide and terminal alkynes. The differences in the catalytic performances of the catalysts could be ascribed to the copper dispersion and the interaction between copper and the supports. In addition, water was used as the reaction media and the proton provider, the latter was found to be very important for the reaction. The XPS results suggested that the supported Cu(II) catalysts were reduced to catalytic Cu(I) species via alkynes homocoupling reaction. By means of IR and ESI‐MS studies, a possible mechanism of cycloaddition based on the reduction of Cu(II) to Cu(I) species was proposed.     

The Efficient Copper(I) (Hexabenzyl)tren Catalyst and Dendritic Analogues for Green “Click” Reactions between Azides and Alkynes in Organic Solvent and in Water: Positive Dendritic Effects and Monometallic Mechanism

An easily synthesized, copper(I) (hexabenzyl)tren complex 1 is an efficient catalyst for the copper(I)‐catalyzed Huisgen‐type 1,3‐cycloaddition between azides and alkynes (CuAAC) reaction in toluene. Alternatively, a convenient procedure involves mixing copper(I) bromide (CuBr) with hexabenzyltren and the substrates in toluene which gives, for instance, 100% yield of triazole in 10 min using 0.1 equiv. catalyst with phenylacetylene and benzyl azide at room temperature. The toluene‐soluble catalyst 1 is recyclable and is applied, for example, to the CuAAC synthesis of an 81‐branched dendrimer that previously required the use of a stoichiometric amount of copper(II) sulfate (CuSO₄)+sodium ascorbate “catalyst”. Dendritic copper(I)‐centered analogues 2 and 3 of the first and second generations (G1 and G2, respectively) containing respectively 18 and 54 branch termini, including a 54‐branched water‐soluble metallodendrimer 5 , are also very efficient catalysts for the CuAAC reaction. With the metallodendritic Cu(I) derivatives 2 and 3 , the dendritic frame brings about steric protection against the well‐known inner‐sphere aerobic oxidation of Cu(I) to bis(μ‐oxo)‐bis‐Cu(II). The metallodendrimers 2 and 3 are also sometimes more efficient than the parent catalyst, as shown by kinetic studies. Catalysis in water without co‐solvent of the CuAAC reactions of water‐insoluble substrates was achieved under ambient conditions in good yields with the recyclable catalyst 5 . Efficient catalysis of the CuAAC reaction by these bulky Cu(I) metallodendrimers emphasizes the monometallic mechanism. The difference of kinetic behavior between 1 and CuBr+hexabenzyltren suggests, however, that whereas a monometallic mechanism is working for 1 , the mixture of CuBr+hexabenzyltren might involve the bimetallic mechanism proposed by Fokin and Finn.     

Preparation of 1,4,5‐Trisubstituted 5‐Acyl‐1,2,3‐triazoles by Selective Acylation between Copper(I)‐Carbon(sp) and Copper(I)‐Carbon(sp2) Bonds with Acyl Chlorides

A one‐pot, three‐component method for preparation of 1,4,5‐trisubstituted 5‐acyl‐1,2,3‐triazoles has been developed based on a highly selective acylation of the copper(I)‐carbon(sp²) bond with acyl chlorides in the presence of the copper(I)‐carbon(sp) bond. The procedure is characterized by high efficiency and simple conditions.

     

General Cycloaddition Between a Trimethylsilyl‐Capped Alkyne and an Azide Catalyzed by an N‐Heterocyclic Carbene‐Copper Complex and Pyridine‐Biscarboxamide

With an externally provided catalytic amide facilitator and an N‐heterocyclic carbene‐copper (NHC‐Cu) complex, the cycloaddition of an azide and a trimethylsilyl (TMS)‐capped alkyne can proceed smoothly. This protocol can be applied to a variety of TMS‐capped substrates, with electron‐rich alkynes generally giving higher yields and nitroaromatic alkynes giving lower yields. For special applications of this protocol, a substrate containing both a terminal alkyne and a TMS‐capped alkyne can sequentially react with different azides without isolation of intermediates; and a macrocyclic product can also be formed efficiently without the complication of polymer formation.

     

A “Clickable” MTX Reagent as a Practical Tool for Profiling Small‐Molecule–Intracellular Target Interactions via MASPIT

We present a scalable synthesis of a versatile MTX reagent with an azide ligation handle that allows rapid γ‐selective conjugation to yield MTX fusion compounds (MFCs) appropriate for MASPIT, a three‐hybrid system that enables the identification of mammalian cytosolic proteins that interact with a small molecule of interest. We selected three structurally diverse pharmacologically active compounds (tamoxifen, reversine, and FK506) as model baits. After acetylene functionalization of these baits, MFCs were synthesized via a CuAAC reaction, demonstrating the general applicability of the MTX reagent. In analytical mode, MASPIT was able to give concentration‐dependent reporter signals for the established target proteins. Furthermore, we demonstrate that the sensitivity obtained with the new MTX reagent was significantly stronger than that of a previously used non‐regiomeric conjugate mixture. Finally, the FK506 MFC was explored in a cellular array screen for targets of FK506. Out of a pilot collection of nearly 2000 full‐length human ORF preys, FKBP12, the established target of FK506, emerged as the prey protein that gave the highest increase in luciferase activity. This indicates that our newly developed synthetic strategy for the straightforward generation of MFCs is a promising asset to uncover new intracellular targets using MASPIT cellular array screening.     

Efficient Diastereo‐ and Enantioselective Synthesis of exo‐Nitroprolinates by 1,3‐Dipolar Cycloadditions Catalyzed by Chiral Phosphoramidite⋅Silver(I) Complexes

Chiral complexes formed by privileged phosphoramidites and silver triflate or silver benzoate are excellent catalysts for the general 1,3‐dipolar cycloaddition between azomethine ylides generated from α‐amino acid‐derived imino esters and nitroalkenes affording with high dr the exo‐cycloadducts 4,5‐trans‐2,5‐cis‐4‐nitroprolinates in high ee at room temperature. In general, better results are obtained using silver rather than copper(II) complexes. In many cases the exo‐cycloadducts can be obtained in enantiomerically pure form just after simple recrystallization. The mechanism and the justification of the experimentally observed stereodiscrimination of the process are supported by DFT calculations. These enantiomerically enriched exo‐nitroprolinates can be used as reagents for the synthesis of nitropiperidines, by ester reduction and ring expansion, which are inhibitors of farnesyltransferase.

     

Synthesis of 4‐Aryl‐2(5H)‐furanones by Gold(I)‐Catalyzed Intramolecular Annulation

A novel method that involves intramolecular annulation and a new type of rearrangement has been developed for the synthesis of 4‐aryl‐2(5H)‐furanones. A variety of prop‐2‐ynyl 3‐oxo‐3‐phenylpropanoates undergo annulation cyclization in the presence of chloro(triphenylphosphine)gold and trifluoromethanesulfonic to afford the desired products in moderate to high yields.     

Stabilized Copper(I) Oxide Nanoparticles Catalyze Azide‐Alkyne Click Reactions in Water

A novel form of polyvinylpyrrolidone (PVP) coated copper(I) oxide nanoparticle (Cu₂O‐NP) was prepared and used to catalyze azide‐alkyne click reactions in water under aerobic conditions. The nanoparticles were well dispersed in aqueous solutions and have a size of 20±10 nm, as determined by transmission electron microscope (TEM). Inductively coupled plasma (ICP), X‐ray powder diffraction (XRD), and X‐ray photoelectron spectroscopy (XPS) analyses demonstrated that the main content of Cu₂O‐NP is copper(I). The cytotoxicity of it was evaluated by an in vitro 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) assay and its catalytic efficiency for azide‐alkyne click reactions was studied in water and organic solvents at physiological temperatures. Our results indicate that Cu₂O‐NP is more efficient in catalytic reactions in water for both aliphatic and aromatic azides and alkynes and less toxic than the commonly used CuSO₄/reductant catalyst systems.     

Carbonylative Sonogashira Coupling in the Synthesis of Ynones: A Study of “Boomerang” Phenomena

The behaviour of several organophosphino‐palladium complexes immobilized on mesoporous silica during the palladium‐catalyzed synthesis of propynone by carbonylative Sonogashira coupling was studied, particularly concerning leaching/redeposition phenomena. The results demonstrated that this cross‐coupling reaction is catalyzed by soluble species. Furthermore, it is shown that the palladium leaching is not initiated by the oxidative addition step but rather by palladium‐decoordination from grafted ligand. Despite this decoordination, catalyst performance after recycling is adequate. Additionally, several parameters linked either to catalyst preparation or reaction procedures were shown to reduce leaching allowing one to achieve metal contamination levels close to the recommendation of the European Agency for the Evaluation of Medicinal Products. Interestingly, this heterogeneous palladium‐catalyzed procedure is fully selective toward the formation of ynones, allowing the preparation of various target compounds.

     

Dissecting Anion Effects in Gold(I)‐Catalyzed Intermolecular Cycloadditions

From a series of gold complexes of the type [t‐BuXPhosAu(MeCN)]X (X=anion), the best results in intermolecular gold(I)‐catalyzed reactions are obtained with the complex with the bulky and soft anion BAr₄^F− [BAr₄^F−=3,5‐bis(trifluoromethyl)phenylborate] improving the original protocols by 10–30% yield. A kinetic study on the [2+2] cycloaddition reaction of alkynes with alkenes is consistent with an scenario in which the rate‐determining step is the ligand exchange to generate the (η²‐phenylacetylene)gold(I) complex. We have studied in detail the subtle differences that can be attributed to the anion in this formation, which result in a substantial decrease in the formation of unproductive σ,π‐(alkyne)digold(I) complexes by destabilizing the conjugated acid formed.