Synthesis of 4‐fluoro‐2, 3‐dimethyl‐1‐phenyl‐3‐pyrazoline‐5‐one (4‐fluoroantipyrine) and 18f‐labeled analog by direct fluorination of antipyrine with molecular fluorine

A facile procedure for preparing 4‐fluoroantipyrine is reported. Treatment of antipyrine with molecular fluorine gives 4‐fluoroantipyrine (3) and 4,4‐difluoro‐3‐hydroxy‐2,3‐dimethyl‐1‐phenylpyrazolidin‐5‐one (2). The product distribution depends on the ratio of antipyrine and molecular fluorine. The same procedure is used to prepare [18F]‐4‐fluoroantipyrine with high specific activity.