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Click chemistry has been employed to construct a library of the pyrrolidine-type asymmetric organocatalysts. The clicked organocatalysts were evaluated in asymmetric Michael addition of ketones to nitroolefins, showing good catalytic activity and stereoselectivity (up to 100% yield, syn:anti = 99:1, 96% ee).
The synthesis of star-shaped poly(epsilon-caprolactone) is described via azide-alkyne cycloaddition ('click' chemistry) and via self-assembly of polymeric ligands into [2 x 2] grid-like metal complexes ('supramolecular click' chemistry).
Examination of nature's favorite molecules reveals a striking preference for making carbon-heteroatom bonds over carbon-carbon bonds-surely no surprise given that carbon dioxide is nature's starting material and that most reactions are performed in water. Nucleic acids, proteins, and polysaccharides are condensation polymers of small subunits stitched together by carbon-heteroatom bonds. Even the 35 or so building blocks from which these crucial molecules are made each contain, at most, six contiguous C-C bonds, except for the three aromatic amino acids. Taking our cue from nature's approach, we address here the development of a set of powerful, highly reliable, and selective reactions for the rapid synthesis of useful new compounds and combinatorial libraries through heteroatom links (C-X-C), an approach we call "click chemistry". Click chemistry is at once defined, enabled, and constrained by a handful of nearly perfect "spring-loaded" reactions. The stringent criteria for a process to earn click chemistry status are described along with examples of the molecular frameworks that are easily made using this spartan, but powerful, synthetic strategy.
Click chemistry is a modular approach that uses only the most practical and reliable chemical transformations. Its applications are increasingly found in all aspects of drug discovery, ranging from lead finding through combinatorial chemistry and target-templated in situ chemistry, to proteomics and DNA research, using bioconjugation reactions. The copper-(I)-catalyzed 1,2,3-triazole formation from azides and terminal acetylenes is a particularly powerful linking reaction, due to its high degree of dependability, complete specificity, and the bio-compatibility of the reactants. The triazole products are more than just passive linkers; they readily associate with biological targets, through hydrogen bonding and dipole interactions.
Alkyne-azide cycloaddition ("click" chemistry) between two peptide strands derivatized with terminal azide and alkyne, respectively, provides an efficient convergent synthesis of triazole ring-based new beta-turn mimics.
The click-chemistry-derived formyl indolyl methyl triazole (FIMT) resin 1a was evaluated for the parallel solid-phase synthesis of a series of BP-897-type arylcarboxamides. By application of a five-step sequence (including loading by reductive amination, subsequent amide coupling, deprotection, palladium-catalyzed N-arylation, and acidic cleavage), a focused library of putative dopamine D3 receptor ligands was constructed. The final products revealed good to excellent purity and were screened for binding at monoaminergic G-protein-coupled receptors when selected library members proved to show excellent binding affinity, especially toward the dopamine D3 receptor subtype.
The "click chemistry" approach has been explored on the 2-(1H)-pyrazinone scaffold for the generation of pharmacologically interesting heterocyclic moieties. Huisgen 1,3-dipolar cycloaddition has been evaluated as the key step for the construction of the 1,2,3-triazole ring at the C-3 position of 2-(1H)-pyrazinones. Two different pathways have been successfully evaluated: (1) via C-C or C-O linkage of the acetylenic part to the C-3 position of the 2-(1H)-pyrazinone scaffold or (2) via azide introduction in the C-3 position. The subsequent application of "click chemistry" resulted in the formation of hitherto unknown skeletons. Microwave irradiation has successfully been applied in different steps of the sequence.
A methodology for the successive regiospecific "clicking" together of three components in one pot via two triazole linkages is reported. The protocol utilizes copper(I)-mediated alkyne-azide cycloaddition reactions combined with a silver(I)-catalyzed TMS-alkyne deprotection under mild hydroalcoholic conditions. We exemplify the approach with peptide-based components to illustrate its compatibility with polyfunctionalized biomolecules. The method constitutes a promising tool for peptide ligation. We also provide a procedure for directly using TMS-alkynes as the cycloaddition partner in classical "click" chemistry.
"Click chemistry" 1,3-dipolar cycloaddition between alkynyl 6-carboxyfluorescein (FAM) and azido-labeled single-stranded (ss) DNA was carried out under aqueous conditions to produce FAM-labeled ssDNA in quantitative yield. The FAM-labeled ssDNA was successfully used as a primer to produce DNA sequencing products with single-base resolution in a capillary electrophoresis DNA sequencer with laser-induced fluorescence detection.
An efficient and convenient chemoselective conjugation method based on "click chemistry" was developed for coupling ligands to the surface of preformed liposomes. It can be performed under mild conditions in aqueous buffers; the use of a water soluble Cu(I) chelator, such as bathophenanthrolinedisulfonate, was essential to obtain good yields in reasonable reaction times. A model reaction was achieved in which, in a single step, an unprotected alpha-D-mannosyl derivative carrying a spacer arm functionalized with an azide group was conjugated to the surface of vesicles presenting a synthetic lipid carrying a terminal alkyne function. When liposomes composed of saturated phospholipids were used, the reaction conditions developed in the present work did not damage the membranes as measured by the absence of leakage of entrapped 5,6-carboxyfluorescein. Moreover, as assessed by agglutination experiments using concanavalin A, the mannose residues were perfectly accessible on the surface of the targeted vesicles.
The combination of reversible chain transfer chemistry with highly orthogonal [2 + 3] cycloadditions ('click chemistry') allows for the synthesis of well-defined block copolymers of monomers with extremely disparate reactivities.
The in situ click chemistry approach to lead discovery employs the biological target itself for assembling inhibitors from complementary building block reagents via irreversible connection chemistry. The present publication discusses the optimization of this target-guided strategy using acetylcholinesterase (AChE) as a test system. The application of liquid chromatography with mass spectroscopic detection in the selected ion mode for product identification greatly enhanced the sensitivity and reliability of this method. It enabled the testing of multicomponent mixtures, which may dramatically increase the in situ screening throughput. In addition to the previously reported in situ product syn-TZ2PA6, we discovered three new inhibitors, syn-TZ2PA5, syn-TA2PZ6, and syn-TA2PZ5, derived from tacrine and phenylphenanthridinium azides and acetylenes, in the reactions with Electrophorus electricus and mouse AChE. All in situ-generated compounds were extremely potent AChE inhibitors, because of the presence of multiple sites of interaction, which include the newly formed triazole nexus as a significant pharmacophore.
New PEG-based hydrogel materials have been synthesized by Click chemistry and shown to result in well-defined networks having significantly improved mechanical properties; the selectivity of the azide/acetylene coupling reaction also allows for the incorporation of various additives and functional groups leading to chemical tailoring of the hydrogels.
Dendronized linear polymers are prepared from dendritic azides and poly(vinylacetylene) using "click chemistry." The Cu(I)-catalyzed Huisgen [2 + 3] cycloaddition is quantitative up to the third generation.
A series of G-quadruplex stabilizing compounds have been prepared via click chemistry employing the Cu(I)-catalyzed Huisgen reaction. These compounds were shown to bind tightly to G-quadruplex DNA even in the presence of competing high concentrations of duplex DNA. Furthermore, a modified TRAP assay has shown that some of these compounds also inhibit telomerase at low micromolar concentration.
[reaction: see text] A panel of 96 metalloprotease inhibitors was assembled using "click chemistry" by reacting eight zinc-binding hydroxamate warheads with 12 azide building blocks. Screens of the bidentate compounds against representative metalloproteases provided discerning inhibition fingerprints, revealing compounds with low micromolar potency against MMP-7. The relative ease and convenience of the strategy in constructing focused chemical libraries for rapid in situ screening of MMPs is thereby demonstrated.
The target-guided, in situ click chemistry approach to lead discovery has been successfully employed for discovering acetylcholinesterase (AChE) inhibitors by incubating a selected enzyme/tacrine azide combination with a variety of acetylene reagents that were not previously known to interact with the enzyme's peripheral binding site. The triazole products, formed by the enzyme, were identified by HPLC-mass spectrometry analysis of the crude reaction mixtures. The target-guided lead discovery search was also successful when performed with reagent mixtures containing up to 10 components. From 23 acetylene reagents, the enzyme selected two phenyltetrahydroisoquinoline (PIQ) building blocks that combined with the tacrine azide within the active center gorge to form multivalent inhibitors that simultaneously associate with the active and peripheral binding sites. These new inhibitors are up to 3 times as potent as our previous phenylphenanthridinium-derived compounds, and with dissociation constants as low as 33 femtomolar, they are the most potent noncovalent AChE inhibitors known. In addition, the new compounds lack a permanent positive charge and aniline groups and possess fewer fused aromatic rings. Remarkably, despite the high binding affinity, the enzyme displayed a surprisingly low preference for one PIQ enantiomer over the other.
Methods for profiling the activity of enzymes in vivo are needed to understand the role that these proteins and their endogenous regulators play in physiological and pathological processes. Recently, we introduced a tag-free strategy for activity-based protein profiling (ABPP) that utilizes the copper(I)-catalyzed azide-alkyne cycloaddition reaction ("click chemistry") to analyze the functional state of enzymes in living cells and organisms. Here, we report a detailed characterization of the reaction parameters that affect click chemistry-based ABPP and identify conditions that maximize the speed, sensitivity, and bioorthogonality of this approach. Using these optimized conditions, we compare the enzyme activity profiles of living and homogenized breast cancer cells, resulting in the identification of several enzymes that are labeled by activity-based probes in situ but not in vitro.