A Convenient Synthesis of Tetrasubstituted Pyrazoles from Nitrile Imines and 2-(Thioxothiazolidin-5-ylidene)acetates

Pyrazoles are prominent structural motifs utilized extensively as pharmaceuticals and agrochemicals,as well as coloring agents [1]. For these reasons, the development of new methodologiesfor the synthesis of polysubstituted pyrazoles continues to be an active area of researchin fine chemistry. Among the conventional approaches developed over the past decadesfor the construction of the pyrazole skeleton [2,3], the most commonly used the cyclocondensationof hydrazine derivatives with 1,3-disubstituted three-carbon units including1,3-diketones and α,β-unsaturated ketones, and 1,3-dipolar cycloaddition reaction of nitrogenbased1,3-dipoles with alkynes or alkenes. We now report a 1,3-dipolar cycloaddition/elimination process for the synthesis of pyrazole derivatives in the presence of Et3N inacetonitrile (Fig. 1).