Inverse Electron Demand Diels-Alder Reaction Research Articles

De Novo Synthesis of Dihydrobenzofurans and Indolines and Its Application to a Modular, Asymmetric Synthesis of Beraprost

Dihydrobenzofurans and indolines are important constituents of pharmaceuticals. Herein, we describe a novel strategy for their construction in which the aromatic ring is created de novo through an inverse-electron demand Diels-Alder reaction and cheletropic extrusion sequence of a 2-halothiophene-1,1-dioxide with an enol ether/enamide, followed by aromatization. Unusually, the aromatization process proved to be highly challenging, but it was discovered that treatment of the halocyclohexadienes with a base effected an α-elimination-aromatization reaction. Mechanistic investigation of this step using deuterium-labeling studies indicated the intermediacy of a carbene which undergoes a 1,2-hydrogen shift and subsequent aromatization. The methodology was applied to a modular and stereoselective total synthesis of the antiplatelet drug beraprost in only 8 steps from a key enal-lactone. This lactone provided the core of beraprost to which both its sidechains could be appended through a 1,4-conjugate addition process (lower ω-sidechain), followed by de novo construction of beraprost's dihydrobenzofuran (upper α-sidechain) using our newly developed methodology. Additionally, we have demonstrated the breadth of our newly established protocol in the synthesis of functionalized indolines, which occurred with high levels of regiocontrol. According to density-functional theory (DFT) calculations, the high selectivity originates from attractive London dispersion interactions in the TS of the Diels-Alder reaction.

Reversible control of tetrazine bioorthogonal reactivity by naphthotube-mediated host-guest recognition

•Naphthotubes can recognize phenyltetrazine-modified biomolecules with high affinity •Reversible control of phenyltetrazine reactivity through molecular recognition •Selective tetrazine residue caging enables sequential IEDDA reactions on proteins •Reversible caging of phenyltetrazine activity in living cells and animals Tetrazine-mediated inverse electron demand Diels-Alder (IEDDA) reactions are widely used ultrafast bioorthogonal reactions, and methods to control such reactions would be extremely useful. Here, we identified a high-affinity host-guest pair between synthetic macrocyclic naphthotubes and phenyltetrazine and developed a molecular-recognition strategy for controlling tetrazine reactions by host caging. Naphthotubes can recognize phenyltetrazine on various biomolecules with low-micromolar and sub-micromolar binding affinities and thus cage their IEDDA reactivity efficiently in a reversible manner. For tetrazine residues on proteins encoded by non-canonical amino acid mutagenesis, their positions could be computationally designed such that exposed residues could be reversibly caged, whereas semiburied residues that were resistant to caging retained their reactivity, thus allowing preparation of site-specific protein dual conjugates and dual protein labeling on living cells with IEDDA reactions. The reactivity of tetrazine can be further regulated in living animals. Our strategy is thus expected to expand the applications of tetrazine chemistry. Tetrazine-mediated inverse electron demand Diels-Alder (IEDDA) reactions are widely used ultrafast bioorthogonal reactions, and methods to control such reactions would be extremely useful. Here, we identified a high-affinity host-guest pair between synthetic macrocyclic naphthotubes and phenyltetrazine and developed a molecular-recognition strategy for controlling tetrazine reactions by host caging. Naphthotubes can recognize phenyltetrazine on various biomolecules with low-micromolar and sub-micromolar binding affinities and thus cage their IEDDA reactivity efficiently in a reversible manner. For tetrazine residues on proteins encoded by non-canonical amino acid mutagenesis, their positions could be computationally designed such that exposed residues could be reversibly caged, whereas semiburied residues that were resistant to caging retained their reactivity, thus allowing preparation of site-specific protein dual conjugates and dual protein labeling on living cells with IEDDA reactions. The reactivity of tetrazine can be further regulated in living animals. Our strategy is thus expected to expand the applications of tetrazine chemistry. Bioorthogonal chemistry: Bridging chemistry, biology, and medicineHartung et al.ChemJune 5, 2023In BriefBioorthogonal chemistry has become a flourishing toolbox of approaches for performing selective reactions in complex biological systems without perturbing natural processes. Through the implementation of this chemistry, chemical biologists have answered previously unanswerable questions, made new and exciting discoveries, and even begun to have an impact on important therapies, including implementing a bioorthogonal reaction at the center of a new cancer-treatment clinical trial. We highlight some key advances and anticipate next steps within this latest Nobel Prize-winning field. Full-Text PDF

Reduction-Responsive Chitosan-Based Injectable Hydrogels for Enhanced Anticancer Therapy.

Selective delivery of anticancer drug molecules to the tumor site enhances local drug dosages, which leads to the death of cancer cells while simultaneously minimizing the negative effects of chemotherapy on other tissues, thereby improving the patient's quality of life. To address this need, we developed reduction-responsive chitosan-based injectable hydrogels via the inverse electron demand Diels-Alder reaction between tetrazine groups of disulfide-based cross-linkers and norbornene groups of chitosan derivatives, which were applied to the controlled delivery of doxorubicin (DOX). The swelling ratio, gelation time (90-500 s), mechanical strength (G'~350-850 Pa), network morphology, and drug-loading efficiency (≥92%) of developed hydrogels were investigated. The in vitro release studies of the DOX-loaded hydrogels were performed at pH 7.4 and 5.0 with and without DTT (10 mM). The biocompatibility of pure hydrogel and the in vitro anticancer activity of DOX-loaded hydrogels were demonstrated via MTT assay on HEK-293 and HT-29 cancer cell lines, respectively.

Bioorthogonal Tethering Enhances Drug Fragment Affinity for G Protein-Coupled Receptors in Live Cells.

G protein-coupled receptors (GPCRs) modulate diverse cellular signaling pathways and are important drug targets. Despite the availability of high-resolution structures, the discovery of allosteric modulators remains challenging due to the dynamic nature of GPCRs in native membranes. We developed a strategy to covalently tether drug fragments adjacent to allosteric sites in GPCRs to enhance their potency and enable fragment-based drug screening in cell-based systems. We employed genetic code expansion to site-specifically introduce noncanonical amino acids with reactive groups in C-C chemokine receptor 5 (CCR5) near an allosteric binding site for the drug maraviroc. We then used molecular dynamics simulations to design heterobifunctional maraviroc analogues consisting of a drug fragment connected by a flexible linker to a reactive moiety capable of undergoing a bioorthogonal coupling reaction. We synthesized a library of these analogues and employed the bioorthogonal inverse electron demand Diels-Alder reaction to couple the analogues to the engineered CCR5 in live cells, which were then assayed using cell-based signaling assays. Tetherable low-affinity maraviroc fragments displayed an increase in potency for CCR5 engineered with reactive unnatural amino acids that were adjacent to the maraviroc binding site. The strategy we describe to tether novel drug fragments to GPCRs should prove useful to probe allosteric or cryptic binding site functionality in fragment-based GPCR-targeted drug discovery.

Double Click: Unexpected 1:2 Stoichiometry in a Norbornene-Tetrazine Reaction.

We report a new reactivity for the inverse electron demand Diels-Alder (iEDDA) reaction between norbornene and tetrazine. Instead of simple 1:1 condensation between norbornene- and tetrazine-conjugated biomolecules, we observed that dimeric products were preferentially formed. As such, an olefinic intermediate formed after the addition of the first tetrazine unit to norbornene rapidly undergoes a consecutive cycloaddition reaction with a second tetrazine unit to result in a conjugate with a 1:2 stoichiometric ratio. This unexpected dimer formation was consistently observed in the reactions of both small-molecule norbornenes and tetrazines, as well as oligonucleotide conjugates. When norbornene was replaced with bicyclononyne to bypass the formation of this olefinic reaction intermediate, the reactions resulted exclusively in rapid formation of the expected 1:1 stoichiometric conjugates.

The effect of molecular weight and chemical structure of cross-linkers on the properties of redox-responsive hyaluronic acid hydrogels

In this work, we investigated the effect of the size and the chemical structure of crosslinkers on the properties of hyaluronic acid-based hydrogels prepared via an inverse electron demand Diels-Alder reaction. Hydrogels having loose and dense networks were designed by cross-linkers with and without polyethylene glycol (PEG) spacers of different molecular weights (1000 and 4000 g/mol). The study showed that the properties of hydrogels such as swelling ratios (20–55 times), morphology, stability, mechanical strength (storage modulus in the range 175–858 Pa), and drug loading efficiency (87 % ~ 90 %) were greatly influenced by the addition of PEG and changing its molecular weight in the cross-linker. Particularly, the presence of PEG chains in redox- responsive crosslinkers increased the doxorubicin release (85 %, after 168 h) and the degradation rate (96 %, after 10 d) of hydrogels in the simulated reducing medium (10 mM DTT). The in vitro cytotoxicity experiments conducted for HEK-293 cells revealed that the formulated hydrogels were biocompatible, which could be a promising candidate for drug delivery applications.

Ligation to Scavenging Strategy Enables On-Demand Termination of Targeted Protein Degradation.

Event-driven bifunctional molecules, typified by proteolysis targeting chimera (PROTAC) technology, have been successfully applied in degrading many proteins of interest (POI). Due to the unique catalytic mechanism, PROTACs will induce multiple cycles of degradation until the elimination of the target protein. Here, we propose a versatile "Ligation to scavenging" approach to terminate event-driven degradation for the first time. Ligation to the scavenging system consists of a TCO-modified dendrimer (PAMAM-G5-TCO) and tetrazine-modified PROTACs (Tz-PROTACs). PAMAM-G5-TCO can rapidly scavenge intracellular free PROTACs via an inverse electron demand Diels-Alder reaction and terminate the degradation of certain proteins in living cells. Thus, this work proposes a flexible chemical knockdown approach to adjust the levels of POI on-demand in living cells, which paves the way for controlled target protein degradation.

Injectable Anhydrous Poly(ethylene glycol) Polymer Liquids Form Protein Depot for Extended Controlled Release Applications Via Rapid In Situ Gelation.

Water-free preparation of protein delivery systems has the potential to overcome the limitations of hydrogel depot systems such as off-target reactions, functional group hydrolysis, and limited loading capacity. However, a major roadblock in the development and use of these systems is administration as implantation is often required. In this study, we developed a biodegradable and water-free injectable protein delivery system via inverse electron demand Diels-Alder reaction between norbornene- and tetrazine-functionalized four-armed poly(ethylene glycol) macromonomers. 1:1 mixtures of these precursors gelled rapidly in situ, taking less than 11 s to reach their gelation point. Methyl substitution of tetrazine slowed the gelation time and increased the cross-linking density, whereas oxygen incorporation into norbornene changed the mechanical properties. Introduction of hydrolytically cleavable groups enabled biodegradability. Using phenyl carbamate and phenyl carbonate ester groups, we could tune the stability. Controlled release of the protein surrogate glucose oxidase was achieved over a period of 500 days. The novel preparation method presented here is a promising step toward the development of water-free injectable protein depots for controlled drug delivery.

Inverse Electron Demand Diels-Alder-Type Heterocycle Syntheses with 1,2,3-Triazine 1-Oxides: Expanded Versatility.

1,2,3-Triazine 1-oxides are remarkably effective substrates for inverse electron demand Diels-Alder reactions. Formed from vinyldiazoacetates via reaction with tert-butyl nitrite, these stable heterocyclic compounds undergo clean nucleophilic addition with amidines to form pyrimidines, with β-ketocarbonyl compounds and related nitrile derivatives to form polysubstituted pyridines and with 3/5-aminopyrazoles to form pyrazolo[1,5-a]pyrimidines, in high yield. These practical reactions are rapid at room temperature, are base catalyzed, and offer a diversity of structural modifications.

Dynamic and Stimuli-Responsive Hydrogel Systems for Controlled Release of Proteins

Background and HypothesisHydrogels have been extensively used as biomaterials for controlled release applications due to their biocompatibility and tunable networks. Stimuli-responsive, or smart, hydrogels are of interest due to their potential for localized release based upon local stimuli such as pH. In addition to smart materials, dynamic hydrogels have been utilized to obtain unique release profiles such as biphasic release curves. We hypothesized that by leveraging dynamic and stimuli-responsive click chemistry we can fabricate hydrogels with unique and controlled release profiles.
 Project MethodsA new reversible addition fragmentation chain transfer (RAFT) polymer known as PADO, was synthesized bearing a ketone functional group allowing for crosslinking with hydrazide bearing motifs to form a pH labile acylhydrazone bond. In addition, polyethylene glycol (PEG) based hydrogels have been extensively used in controlled release applications due to their biocompatibility and antifouling properties. Previously, we have developed a novel conjugation technique for functionalizing norbornene onto hydroxyl-terminated PEG through a cyclic desymmetrization reaction with carbic anhydride. The new polymer, PEGNBCA, was further conjugated with dopamine via amide conjugation to form PEGNBD. Mass loss and protein release analysis were conducted for PADO and PEGNBD hydrogels, respectively.
 ResultsMass loss studies confirmed that PADO hydrogel degraded faster at mildly acidic pH of 6 compared to physiological pH of 7.4. In addition, the rapid hydrolysis of PEGNBD crosslinked via inverse electron demand Diels-Alder reaction demonstrated higher protein release compared to conventional PEGNB hydrogels, which are hydrolytically resistant.
 Conclusion/ImpactThe sensitivity to degradation PADO hydrogels in an acidic environment, and the hydrolytically sensitive PEGNBD hydrogels, can be utilized for targeted therapeutic release. For example, the pH-sensitive hydrogel system can be utilized for therapeutic release into an acidic neoplasm while having minimal release in neutral pH tissues.

Recent Advances in Hydrogels via Diels–Alder Crosslinking: Design and Applications

The Diels-Alder (DA) reaction is a promising tool for obtaining covalently crosslinked hydrogels due to its reaction bioorthogonality, the absence of by-products, and the application of mild conditions without a catalyst. The resulting hydrogels are in demand for use in various fields of materials science and biomedicine. While the dynamic nature of the cycloaddition of diene and dienophile has previously been used extensively for the fabrication of self-healing materials, it has only recently spread to the expansion of the functional properties of polymer gels for bioapplications. This review describes strategies and recent examples of obtaining hydrogels based on the DA reaction, demonstrating that the emerging functional properties go beyond self-healing. The types of classifications of hydrogels are listed, depending on the type of reaction and the nature of the components. Examples of obtaining hydrogels based on the normal and inverse electron-demand DA reaction, as well as the application of hydrogels for cell culture, drug delivery, injectable gels, and wound dressings, are considered. In conclusion, possible developmental directions are discussed, including the use of diene-dienophile pairs with a low temperature for the reversal of DA reaction, the modification of nanoparticles by diene and/or dienophile fragments, and new applications such as ink for 3D printing, sensing hydrogels, etc.

Facile de Novo Sequencing of Tetrazine-Cyclized Peptides through UV-Induced Ring-Opening and Cleavage from the Solid Phase.

While most FDA-approved peptide drugs are cyclic, the robust cyclization chemistry of peptides and the deconvolution of cyclic peptide sequences by using tandem mass spectrometry render cyclic peptide drug discovery difficult. Here we present the successful design of cyclic peptides on solid phase that addresses both of these problems. We demonstrate that this peptide cyclization method using dichloro-s-tetrazine on solid phase allows successful cyclization of a panel of random peptide sequences with various charges and hydrophobicities. The cyclic peptides can be linearized and cleaved from the solid phase by simple UV light irradiation, and we demonstrate that accurate sequence information can be obtained for the UV-cleaved linearized peptides by using tandem mass spectrometry. The tetrazine linker used in the cyclic peptides can further be explored for inverse electron-demand Diels-Alder (IEDDA) reactions for screening or bioconjugation applications in the future.

Post-polymerisation diversification of conjugated polymers by an inverse electron demand Diels–Alder reaction

Post-polymerization diversification to tune the optical properties of conjugated polymers was achieved via an effective tetrazine–alkyne inverse electron demand Diels–Alder reaction.

Catching up with tetrazines: coordination of Re(I) to 1,2,4-triazine facilitates an inverse electron demand Diels-Alder reaction with strained alkynes to a greater extent than in corresponding 1,2,4,5-tetrazines.

Inverse electron demand Diels Alder (IEDDA) reactions of 1,2,4-triazines are of interest to biorthogonal chemistry but suffer from slow kinetics. It is shown here that coordination of Re(I) to a 1,2,4-triazine ring speeds up the IEDDA reaction with bicyclooctyne (BCN) by a factor of 55. Comparative analysis with corresponding 1,2,4,5-tetrazine analogues reveals that the origin of the increased reactivity is markedly different and more profound than in tetrazine analogues. DFT calculations and subsequent analysis indicated the greater increase for the triazine than the tetrazines on coordination could be attributed to the triazine's lower distortion energy and more favourable interaction energy for the triazine, the latter attributable to lower Pauli repulsion than the tetrazines rather than to favourable frontier orbital energies.

Tetrazine-induced activation of a trimethyl lock as a click-to-release system for protected doxorubicin.

We herein report a novel chemically triggered click-to-release system, that combines the trimethyl lock (TML) lactonization with the bioorthogonal inverse electron demand Diels-Alder (IEDDA) reaction of a vinyl ether and a tetrazine. Kinetic studies were carried out on a vinyl phenol model system with six tetrazines using NMR and UV/Vis spectroscopy, revealing that within the three step sequence the IEDDA reaction was rate-limiting. The reaction rates were enhanced by increasing the electrophilicity of the tetrazine, while balancing reactivity and stability of the tetrazines. The anticancer drug doxorubicin was conjugated to a vinyl-modified TML. Its subsequent liberation from vinyl-TML was triggered by dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate and followed quantitatively by NMR, thereby providing a proof-of-concept for the tetrazine/TML click-to-release system. In addition the applicability of the reaction under physiolgoical conditions could be shown.

Radiolabelling of peptides with tetrazine ligation based on the inverse electron-demand Diels-Alder reaction: rapid, catalyst-free and mild conversion of 1,4-dihydropyridazines to pyridazines.

Click chemistry reactions, such as the tetrazine ligation, based on the inverse-electron demand Diels-Alder (IEDDA), are chemoselective cycloaddition reactions widely used for chemical modifications and synthesis of biomolecule-based radiopharmaceuticals for positron emission tomography (PET). The reactions have potential also for pretargeted PET imaging. When used as a bioconjugation method in production of biomolecule-based radiopharmaceuticals, IEDDA-based tetrazine ligation has one significant drawback, namely the formation of a mixture comprising reduced metastable dihydropyridazines (DHPs) and oxidized cycloadducts. Conversion of the reduced DHPs to stable pyridazines requires oxidation, which is typically achieved by using oxidants or by photo-irradiated air-oxidation, both methods requiring added reagents or reaction times of several hours, not compatible with short-lived radionuclides. Here we report a mild, rapid, and catalyst-free conversion of the DHPs to pyridazines. In this study, a model peptide Tyr3-octreotide (TOC) was modified with polyethylene glycol (PEG) linkers and with trans-cyclooctenes (TCOs) for rapid IEDDA-mediated radiolabeling. Fluorine-18-labelled alkylammoniomethyltrifluoroborate ([18F]AmBF3) tetrazines were conjugated to the TCO-TOC analogs at room temperature for rapid synthesis of PET imaging agent candidates. The formed DHPs were successfully converted to the oxidized form, after heating the radiolabelled bioconjugates in aqueous solution (≥95% water) at 60 °C for a minimum of 10 minutes in the presence of air, resulting in one-pot back-to-back IEDDA reaction and DHP conversion. The water content of the reaction mixture was to be found critical for the coversion. Our finding offers a straightforward method for conversion of the metastable DHPs from the IEDDA-based tetrazine ligation to stable, oxidized pyridazines. The method is especially suitable for applications requiring rapid conversion.

Click'n lock: rapid exchange between unsymmetric tetrazines and thiols for reversible, chemoselective functionalisation of biomolecules with on-demand bioorthogonal locking.

The late-stage functionalisation and diversification of complex structures including biomolecules is often achieved with the help of click chemistry. Besides employing irreversible click-like reactions, many synthetic applications benefit from reversible click reaction strategies, so called de-/trans-click approaches. Yet, the combination of both, reversible and irreversible click chemistry - while still respecting the stringent criteria of click transformations - remains so far elusive for modifications of biomolecular structures. Here, we report click'n lock as a concept that enables reversible click reactions and on-demand locking of chemical entities, thus switching from reversible to irreversible modifications of complex biomolecules. For this purpose, we employ the tetrazine-thiol exchange (TeTEx) reaction as a fully traceless click reaction with second order rate constants k2 higher than 2 M-1 s-1 within aqueous environments. Employing TeTEx as a reversible click reaction for the chemoselective modification of biomolecules is made possible by the use of 3,6-disubstituted 1,2,4,5-tetrazines bearing a single sulfide residue. The inherent reactivity of tetrazines towards inverse electron demand Diels-Alder (IEDDA) reactions allows to stabilize the clicked structure, switching from reversible to irreversible systems (click'n lock).

Toward Precise Antitumoral Photodynamic Therapy Using a Dual Receptor-Mediated Bioorthogonal Activation Approach.

Targeted delivery and specific activation of photosensitizers can greatly improve the treatment outcome of photodynamic therapy. To this end, we report herein a novel dual receptor-mediated bioorthogonal activation approach to enhance the tumor specificity of the photodynamic action. It involves the targeted delivery of a biotinylated boron dipyrromethene (BODIPY)-based photosensitizer, which is quenched in the native form by the attached 1,2,4,5-tetrazine unit, and an epidermal growth factor receptor (EGFR)-targeting cyclic peptide conjugated with a bicycle[6.1.0]non-4-yne moiety. Only for cancer cells that overexpress both the biotin receptor and EGFR, the two components can be internalized preferentially where they undergo an inverse electron-demand Diels-Alder reaction, leading to restoration of the photodynamic activity of the BODIPY core. By using a range of cell lines with different expression levels of these two receptors, we have demonstrated that this stepwise "deliver-and-click" approach can confine the photodynamic action on a specific type of cancer cells.

Making functional connections: A summary of the Nobel Prize in Chemistry 2022

<p indent="0mm">The Nobel Prize in Chemistry 2022 was awarded to three scientists for their work in the development of click chemistry and bioorthogonal chemistry: Carolyn R. Bertozzi from Stanford University, Morten Meldal from the University of Copenhagen, and K. Barry Sharpless from Scripps Research. Notably, this is the second time Sharpless was awarded this prize. In 2001, he received his first Nobel Prize for the development of catalytic asymmetric oxidation reactions. This year’s prize, however, is in a completely different area, as will be summarized below. For both click chemistry and bioothogonal chemistry, their focus is on making easy connections between molecules and snapping them together. Although modern chemistry has experienced substantial development since its birth in the eighteenth century, dealing with complex molecules and systems remains a challenging and laborious process that requires skill and knowledge. Synthesis has become the bottleneck for the discovery of molecular functions. In 2001, Sharpless and co-workers coined the concept of click chemistry, by defining a set of stringent criteria so that few good reactions could be sorted out for making easy and modular connections: Molecules could be linked together simply like buckling up through a “click”. Later in 2002, Sharpless group published the first click-type reaction, namely the copper-catalyzed azide-alkyne cycloaddition (CuAAC). The reaction is silent without catalyst due to the high kinetic barrier, but become as fast as a “click” to generate triazoles in the presence of copper(I) with a strong thermodynamic driving force. Independently, Meldal made a similar discovery during the peptide synthesis. Over the years, CuAAC has proved itself to be a reliable tool for connecting chemical units of various forms together, nearly under any circumstances with high fidelity. In the late 1990s, in Bertozzi’s search for chemical tools for sugar labeling on the surface of living cell, she realized that the key for successful utilization of a chemical handle was the non-interfering nature of the involved reactions with the surrounding biological environments. She termed this kind of non-interfering reactivity as bioothogonal chemistry. By metabolically incorporating unnatural sialic acids to the cell surface, including the azide derivatives, Bertozzi successfully used Staudinger reaction to attach imaging probes on the cell with amide connections. The iconic breakthrough took place in 2004 when click chemistry was merged with bioothogonal chemistry for the first time. Bertozzi developed a stain-promoted azide-alkyne cycloaddition (SPAAC), which gave improved connection capability for biorthogonal cell imaging without the need of a catalyst, nor any compromise on bioorthogonality. The impact of click chemistry and bioorthogonal chemistry is tremendous and ever-growing. They have inspired the innovation of many connection tools, such as the inverse electron-demand Diels-Alder reaction, photoclick chemistry, and the sulfur(VI)-fluoride exchange (SuFEx) reaction. Apart from serving as tools for tracking and manipulating biomolecules and related events <italic>in vivo</italic>, they have also been used for material sciences and medicinal chemistry. For instance, sacituzumab govitecan is a Trop-2-directed antibody and topoisomerase inhibitor drug conjugate approved by the FDA for the treatment of metastatic triple-negative breast cancer and metastatic urothelial cancer. The key linkage is a CuAAC-derived 1,2,3-triazole. In short, there remain a lot of challenges in chemistry and the related fields to be addressed through diverse strategies and chemical approaches nowadays. The mission of chemistry is to bring the greatest benefit to mankind through continuous innovation and integration.

Pyridazine Synthesis from 1,2,4,5-Tetrazines and Alkynes in 1,1,1,3,3,3-Hexafluoro-2-propanol through the Inverse Electron Demand Diels–Alder Reaction

Abstract A facile method to synthesize pyridazines from 1,2,4,5-tetrazines and alkynes is disclosed. The reaction was significantly facilitated by 1,1,1,3,3,3-hexafluoro-2-propanol. The wide scope and the predictable regioselectivity by theoretical calculations will serve to synthesize a wide range of multisubstituted pyridazines.