Electrophilic halogenation of hydrazones of CF3-ynones. Regioselective synthesis of 4-halo-substituted 3-CF3-pyrazoles

Trifluoromethylated Pyrazoles via Sequential (3 + 2)-Cycloaddition of Fluorinated Nitrile Imines with Chalcones and Solvent-Dependent Deacylative Oxidation Reactions

A general approach for preparation of two types of polyfunctionalized 3-trifluoromethylpyrazoles is reported. The protocol comprises (3 + 2)-cycloaddition of the in situ generated trifluoroacetonitrile imines with enones leading to trans-configured 5-acyl-pyrazolines in a fully regio- and diastereoselective manner. Initially formed cycloadducts were aromatized by treatment with manganese dioxide. Depending on the solvent used, the oxidation step either led to fully substituted pyrazoles (DMSO) or proceeded via a deacylative pathway to afford 1,3,4-trisubstituted derivatives (hexane) with excellent selectivity.

A convenient approach for the electrochemical bromination and iodination of pyrazoles

Electrochemical bromination and iodination of some pyrazoles were investigated under constant-current (CC) electrolysis in an undivided electrochemical cell. Anodic oxidation of KX salt produces X2 in-situ which can be consumed as an expedient electrophile in pyrazoles aromatic electrophilic substitution reactions or may participate in an X–N coupling reaction with electrochemically catalyzed pyrazolesox to form the halogenated pyrazoles. All reactions proceeded without the need to use any hazardous reagents or catalysts. The reaction conditions are mild and environmentally compatible.

Polyfluoroalkylated antipyrines in Pd-catalyzed transformations

In the direct C–H arylation with arylhalogenides in the presence of Pd(OAc)₂, trifluoromethyl-containing antipyrine reacts very slowly and incompletely owing to the low nucleophilicity of its C4 center. However, it was effective in modifying polyfluoroalkyl-substituted 4-bromo- and 4-iodo antipyrines by the Suzuki and Sonogashira reactions. It was established that using Pd₂(dba)₃ as catalyst and XPhos as phosphine ligand was the optimal catalytic system for the synthesis of 4-aryl- and 4-phenylethynyl-3-polyfluoroalkyl-antipyrines. Moreover, iodo-derivatives as the initial reagents were found to be more advantageous compared to bromo-containing analogs. It was found that 4-phenylethynyl-5-CF₃-antipyrine has a moderate activity against the influenza virus A/Puerto Rico/8/34 (H1N1) and 4-iodo-5-CF₃-antipyrine reveals a weak activity against the vaccine virus (strain Copenhagen) and bovine diarrhea virus (strain VC-1).

Peculiarities of heterocyclic systems with electron-withdrawing groups (polyfluoroalkyl-containing antipyrines) in Pd-catalyzed C–H arylation and cross-coupling reactions.

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.

Synthesis of 4-chalcogenyl pyrazoles via electrophilic chalcogenation/cyclization of α,β-alkynic hydrazones

A facile method for the synthesis of 4-chalcogenylated pyrazoles has been developed via electrophilic chalcogenation/cyclization of α,β-alkynic hydrazones. The cyclization of α,β-alkynic aldehyde hydrazones could be induced by using either sulfenyl chloride or the S-electrophiles generated in situ from the reaction of NCS and arythiol. The developed method was successfully applied to the synthesis of the sulfenyl analogue of celecoxib.

Regioselective Synthesis of Functionalized 3- or 5-Fluoroalkyl Isoxazoles and Pyrazoles from Fluoroalkyl Ynones and Binucleophiles

A facile synthetic route toward either 3- or 5-fluoroalkyl-substituted isoxazoles or pyrazoles containing an additional functionalization site was developed and applied on a multigram scale. The elaborated approach extends the scope of fluoroalkyl substituents for introduction into the heterocyclic moiety, and uses convenient transformations of the side chain for incorporation of fluoroalkyl-substituted azoles into the structures of biologically active molecules. The utility of the obtained building blocks for isosteric replacement of alkyl-substituted isoxazole and pyrazole was shown by the synthesis of fluorinated Isocarboxazid and Mepiprazole analogues.