Metal- and Solvent-Free Cascade Reaction for the Synthesis of Amino Pyrazole Thioether Derivatives

Recent advances in the multicomponent synthesis of pyrazoles

Pyrazole moiety is considered as an important N-heterocycle in pharmaceuticals and many other functional molecules. The utilization of multicomponent reaction is a major tool in the current approaches of pyrazole synthesis. Considering the power and significance of multicomponent pyrazole synthesis, we review herein the latest developments in this field. According to the typical features, the contents are divided into reactions with different NN fragment sources, such as hydrazine, hydrazone, amidine, nitrile, and diazo compounds, in the pyrazole ring construction, covering the works published since 2019 to date.

Organocatalytic Dearomatization of 5-Aminopyrazoles: Synthesis of 4-Hydroxypyrazolines

Dearomatization is a fundamental chemical reaction that affords complex three-dimensional heterocyclic frameworks. We disclose the first organocatalytic dearomatization of 5-aminopyrazoles, which yields a range of structurally diversified C4-hydroxylated pyrazolines with yields of ≤95% in <1.5 h at room temperature. This catalytic process is achieved using in situ-generated hypervalent iodine. The method also yields a spirolactone via an intramolecular dearomatization process. Furthermore, we demonstrate that substrate-directed reduction of the resulting iminopyrazoline leads to 4,5-difunctionalized pyrazoline as a single diastereomer. Mechanistic studies suggest that the reaction proceeds through a dearomatized cationic intermediate.

Electrocatalytic C-H/S-H Coupling of Amino Pyrazoles and Thiophenols: Synthesis of Amino Pyrazole Thioether Derivatives

A mild method for the C-H/S-H coupling of pyrazol-5-amines and thiophenols was developed via electrochemistry, giving diverse amino pyrazole thioether derivatives in 37-98% yields. This electrochemical reaction is sustainable and an atom-efficient approach with good functional group tolerance and scalability by avoiding metal and external chemical oxidants.

Copper-Promoted Aerobic Oxidative [3+2] Cycloaddition Reactions of N,N-Disubstituted Hydrazines with Alkynoates: Access to Substituted Pyrazoles

A copper-promoted aerobic oxidative [3+2] cycloaddition reaction for the synthesis of various substituted pyrazoles from N,N-disubstituted hydrazines with alkynoates in the presence of bases is developed. This work involves a direct C(sp³)-H functionalization and the formation of new C-C/C-N bonds. In this strategy, inexpensive and easily available Cu₂O serves as the promoter and air acts as the green oxidant. The reaction exhibits the advantages of high atom and step economy, high regioselectivity, and easy operation.

Dimethyl Sulfoxide-Assisted, Iodine- and Ascorbic Acid-Catalyzed One-Pot Synthetic Approach for Constructing Highly Substituted Pyrazolo[1,5-a]quinoline Thioether Derivatives

A dimethyl sulfoxide-assisted and iodine/ascorbic acid-catalyzed simple approach to pyrazolo[1,5-a]quinoline thioether derivatives 22 is described. The compounds were identified using ¹H NMR, ¹³C NMR, high-resolution mass spectrometry, and single-crystal X-ray diffractometry. The pyrazolo[1,5-a]quinoline thioether was synthesized in a stepwise fashion through aryl sulfenylation and benzannulation strategies. The generated heteroaryl thioether compounds 23 were exposed to the benzannulation path to produce pyrazolo[1,5-a]quinoline thioether 22. The benzannulation reaction proceeds by way of diazotization of the pyrazole amine derivative 23, radical generation by the removal of nitrogen, and eventually trapping of the aryl radical with the support of phenylacetylene 19. A catalytic amount of ascorbic acid aided the benzannulation reaction. There were several other control studies conducted, including trapping reactions with isopropenyl acetate, tetramethylpiperidine N-oxyl reactions, and reactions without phenylacetylene. Since a change in the substitution has previously demonstrated substantial bioactivity, the core structure of pyrazole was evaluated for functional group tolerance. A reasonable mechanism is then proposed, accompanied by the support of control experiments and scope. A Suzuki reaction was used to create an aryl/heteroaryl compound 35 from one of the synthesized compounds 22b. In the controlled oxidation reaction paths, molecule 22a was selectively transformed into the corresponding sulfoxide 32 and sulfone 33.

Recent Advances in Synthesis and Properties of Pyrazoles

Pyrazole-containing compounds represent one of the most influential families of N-heterocycles due to their proven applicability and versatility as synthetic intermediates in preparing relevant chemicals in biological, physical-chemical, material science, and industrial fields. Therefore, synthesizing structurally diverse pyrazole derivatives is highly desirable, and various researchers continue to focus on preparing this functional scaffold and finding new and improved applications; this review highlights some of the most recent and strategic examples regarding the synthesis and properties of different pyrazole derivatives, mainly reported from 2017–present. The discussion involves strategically functionalized rings (i.e., amines, carbaldehydes, halides, etc.) and their use in forming various fused systems, predominantly bicyclic cores with 5:6 fusion taking advantage of our experience in this field and the more recent investigations of our research group.

Facile construction of C,N-disulfonated 5-amino pyrazoles through an iodine-catalyzed cascade reaction

A green and facile synthesis of previously unreported C,N-disulfonated 5-amino pyrazoles was established through an iodine-catalyzed cascade reaction.