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.