From 2000 to mid-2010: a fruitful decade for the synthesis of pyrazoles

Iodolopyrazolium Salts: Synthesis, Derivatizations, and Applications

The synthesis of iodolopyrazolium triflates via an oxidative cyclization of 3-(2-iodophenyl)-1H-pyrazoles is described. The reaction is characterized by a broad substrate scope, and various applications of these novel cyclic iodolium salts acting as useful synthetic intermediates are demonstrated, in particular in site-selective ring openings. This was finally applied to generate derivatives of the anti-inflammatory drug celecoxib. Their application as highly active halogen-bond donors is shown as well.

Palladium-Catalyzed Synthesis of Polysubstituted Pyrazoles by Ring-Opening Reactions of 2H-Azirines with Hydrazones

A palladium–catalyzed ring-opening reaction of 2H-azirines with hydrazones has been developed. This protocol provides an alternative route for the construction of various polysubstituted pyrazoles with a wide substrate scope. Moreover, a plausible mechanism is proposed for this reaction, which should further enrich the chemical conversion of 2H-azirines.

Divergent synthesis of 1,3,5-tri and 1,3-disubstituted pyrazoles under transition metal-free conditions

Pyrazole cores are common structural motifs existing in various agrochemicals and pharmaceuticals. Herein, a transition metal-free, three-component reaction of arylaldehydes, ethyl acrylate and N-tosylhydrazones is described, which leads to the formation of 1,3,5-trisubstituted and 1,3-disubstituted pyrazoles divergently under slightly different conditions.

Further Exploration of Hydrazine-Mediated Bioconjugation Chemistries

The hydrazine group serves as a great anchor for bioconjugation; however, the application of hydrazone ligation has been limited by poor product stability. We aim to resolve such issues by optimizing the recently established pyrazolone ligation and investigating a new pyrazole ligation. We have identified a new, electron-deficient pyrazolone ligation and a regiospecific pyrazole ligation, both offering aqueous buffer stable and chemically inert products possessing triazole-like structures while not involving any heavy metal catalyst.

Denitrative imino-diaza-Nazarov cyclization: synthesis of pyrazoles

An iodine-catalyzed denitrative imino-diaza-Nazarov cyclization (DIDAN) methodology has been developed for the synthesis of pyrazoles with high to excellent yields by using α-nitroacetophenone derivatives and in situ generated hydrazones. The key transformation of this oxidative 4π-electrocyclization proceeds through an enamine-iminium ion intermediate. This rapid one-pot DIDAN protocol results in the selective generation of C-C and C-N bonds and cleavage of a C-N bond.

A One-Step Approach to N-(Hetero)aryl-3,5-dinitropyrazoles from (Hetero)aryl Amines

Potassium 1,1,3,3-tetranitropropane-1,3-diide (K₂TNP) was found to react readily with various (hetero)aryl amines (12 examples) to give corresponding N-(hetero)aryl-3,5-dinitropyrazoles in moderate to excellent yields. The reactions were performed at mild temperature, and most of the reactions completed in less than 4 h. Four potential energetic compounds show high enthalpy of formation, excellent thermal stability, and good sensitivity, with 3-(3,5-dinitropyrazol-1-yl)-1H-1,2,4-triazole (3j) being a potential 2,2',4,4',6,6'-hexanitrostibene (HNS) replacement.

Pyrazoline Hybrids as Promising Anticancer Agents: An Up-to-Date Overview

Pyrazolines are five-membered heterocycles possessing two adjacent nitrogens. They have attracted significant attention from organic and medicinal chemists due to their potent biological activities and the numerous possibilities for structural diversification. In the last decade, they have been intensively studied as targets for potential anticancer therapeutics, producing a steady yearly rise in the number of published research articles. Many pyrazoline derivatives have shown remarkable cytotoxic activities in the form of heterocyclic or non-heterocyclic based hybrids, such as with coumarins, triazoles, and steroids. The enormous amount of related literature in the last 5 years prompted us to collect all these published data from screening against cancer cell lines, or protein targets like EGFR and structure activity relationship studies. Therefore, in the present review, a comprehensive account of the compounds containing the pyrazoline nucleus will be provided. The chemical groups and the structural modifications responsible for the activity will be highlighted. Moreover, emphasis will be given on recent examples from the literature and on the work of research groups that have played a key role in the development of this field.

Synthesis and Antimicrobial Evaluation of Some New Organic Tellurium Compounds Based on Pyrazole Derivatives

A novel series of organic tellurium compounds based on pyrazole derivatives with a general formula of ArTeBr₃ and Ar₂TeBr₂ [Ar = 2-(3-(4-substituted phenyl) -5-(2-chlorophenyl)-1H-pyrazol-1-yl)-3,5-dinitrophenyl] were obtained by the refluxing of corresponding aryl mercuric chlorides with TeBr₄ in two different mole ratio of 1:1 and 2:1, respectively, in free-moisture dioxane solvent under an argon atmosphere. Compounds of ArTeBr₃ and Ar₂TeBr₂ were reduced by the action of ethanolic solution of hydrazine hydrate obtained Ar₂Te₂ and Ar₂Te, respectively. Reaction of Ar₂Te₂ with excess thionyl chloride or iodine gave the corresponding trihalides ArTeCl₃ and ArTeI₃, respectively while the reaction of Ar₂Te with thionyl chloride or iodine gave the corresponding Ar₂TeCl₃ and Ar₂TeI₃, respectively. The structures were elucidated according to their elemental analysis of carbon, hydrogen and nitrogen (CHN) and some of the spectroscopic techniques such as infrared IR and nuclear magnetic resonance for ¹H and ¹³C. The antimicrobial activity for all the synthetic compounds were assayed against both Gram-negative and Gram-positive bacteria by using the agar diffusion method. The tellurated pyrazole derivatives showed a good degree against bacteria growth. In some cases, the antimicrobial activities of the synthetic compounds were better than amoxicillin.

A Rh-catalyzed three-component reaction for the diastereoselective synthesis of pyrazolone derivatives with contiguous quaternary stereocenters

A diastereoselective three-component reaction of diazo compounds with alcohols and pyrazolinone ketimines by utilizing rhodium(ii) catalysis via interception of transient oxonium ylides is reported. The reaction provides an efficient approach for the facile construction of polyfunctionalized pyrazolone derivatives bearing two contiguous quaternary stereocenters in good yields with high regioselectivities and excellent diastereoselectivities.

Alkynes and Nitrogen Compounds: Useful Substrates for the Synthesis of Pyrazoles

The easy preparation and functionalization of pyrazoles associated with their innumerable biological properties have made this class of N-heterocycles very attractive for the development of new synthetic routes and applications. The cyclization reactions of alkynes and nitrogen compounds represent a powerful tool for the preparation of pyrazoles. This Review covers the recent advances in the preparation of pyrazoles by reacting alkynes and nitrogen compounds under transition-metal-catalyzed or metal-free conditions.

Molecular Hybrids Integrated with Benzimidazole and Pyrazole Structural Motifs: Design, Synthesis, Biological Evaluation, and Molecular Docking Studies

Synthesis of a series of benzimidazole-ornamented pyrazoles, 6a-6j has been obtained from arylhydrazine and aralkyl ketones via a multistep synthetic strategy. Among them, a hybrid-possessing para-nitrophenyl moiety connected to a pyrazole scaffold (6a) exerted the highest anti-inflammatory activity, which is superior to the standard, diclofenac sodium. While executing the 2,2-diphenyl-1-picrylhydrazyl radical-scavenging activity, a hybrid-possessing para-bromophenyl unit integrated at the pyrazole structural motif (6i) exhibited the highest activity among the hybrids examined. Besides, evaluation of anticancer potency of the synthesized hybrids revealed that the one containing a para-fluorophenyl unit tethered at the pyrazole nucleus (6h) showed the highest activity against both the pancreatic cancer cells (SW1990 and AsPCl) investigated. Considerable binding affinity between B-cell lymphoma and the hybrid, 6h has been reflected while performing molecular docking studies (-8.65 kcal/mol). The outcomes of the investigation expose that these hybrids could be used as effective intermediates to construct more potent biological agents.

Synthesis of fully substituted pyrazoles from pyridinium 1,4-zwitterionic thiolates and hydrazonoyl chlorides via a [[3 + 3] - 1] pathway

A novel and practical protocol for the synthesis of fully substituted pyrazoles from pyridinium 1,4-zwitterionic thiolates and hydrazonoyl chlorides in excellent yields under mild conditions is described. The transformation proceeds via an unusual [[3 + 3] - 1] pathway, which involves a formal [3 + 3] cascade cyclization followed by a spontaneous ring-contraction/sulfur extrusion reaction from 4H-1,3,4-thiadiazine intermediates.

Synthesis of 4-amino-5-fluoropyrimidines and 5-amino-4-fluoropyrazoles from a β-fluoroenolate salt

A synthesis of fluorinated pyrimidines under mild conditions from amidine hydrochlorides and the recently described potassium 2-cyano-2-fluoroethenolate was developed. A broad substrate scope was tested and mostly excellent yields were obtained. The synthesis of fluorinated aminopyrazoles from the same fluorinated precursor could be demonstrated but proceeded with lower efficiency.

Cascade and multicomponent synthesis of structurally diverse 2-(pyrazol-3-yl)pyridines and polysubstituted pyrazoles

The cascade reaction between N-tosylhydrazones and 2-alkynylpyridines leads to 2-(pyrazol-3-yl)pyridines, important structural motifs in ligands for transition metals and bioactive molecules. When the reaction is conducted with 2,6-diethynylpyridine, the important 2,6-bis(pyrazolyl)pyridines are obtained, featuring the arrangement of tridentate and also pentadentate ligands. A novel three-component version of the reaction has been designed, which involves the use of α-bromo-N-tosylhydrazones, alkynylpyridines and NH-azoles. The generality of the multicomponent reaction is further illustrated by the preparation of different polysubstituted pyrazoles by employing an array of terminal alkynes. In these multicomponent reactions, complex molecules featuring three different heterocycles are assembled in a single step from commercial materials, enabling the fast generation of molecular diversity.

Multicomponent Pyrazole Synthesis from Alkynes, Nitriles, and Titanium Imido Complexes via Oxidatively Induced N-N Bond Coupling

Pyrazoles are an important class of heterocycles found in a wide range of bioactive compounds and pharmaceuticals. Pyrazole synthesis often requires hydrazine or related reagents where an intact N-N bond is conservatively installed into a pyrazole precursor fragment. Herein, we report the multicomponent oxidative coupling of alkynes, nitriles, and Ti imido complexes for the synthesis of multisubstituted pyrazoles. This modular method avoids potentially hazardous reagents like hydrazine, instead forming the N-N bond in the final step via oxidation-induced coupling on Ti. The mechanism of this transformation has been studied in-depth through stoichiometric reactions of the key diazatitanacyclohexadiene intermediate, which can be accessed via multicomponent coupling of Ti imidos with nitriles and alkynes, ring opening of 2-imino-2H-azirines, or direct metalation of 4-azadiene-1-amine derivatives. The critical transformation in this reaction is the 2-electron oxidation-induced N-N coupling on Ti. This is a rare example of formal N-N coupling on a metal center, which likely occurs through an electrocyclic mechanism analogous to a Nazarov cyclization. Conveniently, these 2-electron-oxidized diazatitanacyclohexadiene intermediates can be accessed via disproportionation of the 1-electron-oxidized species, which allows utilization of weak oxidants such as TEMPO.

Regioselective Synthesis of 3-Trifluoromethylpyrazole by Coupling of Aldehydes, Sulfonyl Hydrazides, and 2-Bromo-3,3,3-trifluoropropene

A general and practical strategy for 3-trifluoromethylpyrazole synthesis is reported that occurs by the three-component coupling of environmentally friendly and large-tonnage industrial feedstock 2-bromo-3,3,3-trifluoropropene (BTP), aldehydes, and sulfonyl hydrazides. This highly regioselective three-component reaction is metal-free, catalyst-free, and operationally simple and features mild conditions, a broad substrate scope, high yields, and valuable functional group tolerance. Remarkably, the reactions could be performed on a 100 mmol scale and smoothly afforded the key intermediates for the synthesis of celecoxib, mavacoxib, SC-560, and AS-136A. Preliminary mechanism studies indicated that a 1,3-hydrogen atom transfer process was involved in this transformation.

Synthesis, crystal structure, and optical properties of fluorinated poly(pyrazole) ligands and in silico assessment of their affinity for volatile organic compounds

Increasing fluorination of an organic ligand forming porous coordination polymers results in enhanced affinity for volatile organic compounds (VOCs) without affecting the fluorescence properties, thus opening new perspectives for VOC sensing.

Copper-mediated tandem ring-opening/cyclization reactions of cyclopropanols with aryldiazonium salts: synthesis of N-arylpyrazoles

A general method for the synthesis of structurally diverse N-arylpyrazoles from readily available cyclopropanols and aryldiazonium salts is disclosed.

An expedient synthesis of novel spiro[indenoquinoxaline-pyrrolizidine]-pyrazole conjugates with anticancer activity from 1,5-diarylpent-4-ene-1,3-diones through the 1,3-dipolar cycloaddition/cyclocondensation sequence

A highly regio- and stereoselective two-stage route for the synthesis of spiro[indenoquinoxaline-pyrrolizidine]-pyrazole hybrids with anticancer activity has been developed.

Iodine-promoted synthesis of pyrazoles from 1,3-dicarbonyl compounds and oxamic acid thiohydrazides

A novel approach to 3,4-dicarbonyl-substituted pyrazoles from 1,3-dicarbonyl compounds and oxamic acid thiohydrazides was developed via iodine-promoted cascade imination/halogenation/cyclization/ring contraction reaction.

Access to highly enantioselective and diastereoselective spirooxindole dihydrofuran fused pyrazolones

Access to highly enantioselective and diastereoselective spirooxindole dihydrofuran fused pyrazolones from MBH-carbonates and pyrazole 4,5-diones.