New synthesis of fluorinated pyrazoles.

A new synthesis of fluorinated pyrazoles, a class of compounds with potential in medicinal chemistry, is described. The treatment of benzoylfluoroacetonitrile with hydrazine yielded the expected new 3-amino-4-fluoropyrazole, while the analogous reaction of α-cyano-α,α-difluoroketones with hydrazine in refluxing isopropanol surprisingly gave rise to 3-unsubstituted 4-fluoropyrazoles via an unprecedented mechanism. The isolation of intermediate hydrazine adducts led to a mechanistic rationale for this transformation.