Structural reassignment of an ‘unusual’ derivative of 3-methyl-5-phenylpyrazol-5-one (Edaravone)

The structure of the major product from the reaction of 4-bromo-3-methyl-1-phenylpyrazol-5-one (1) with ethyl cyanoacetate is shown, by means of an X-ray crystal structure determination, to be the spirocyclic compound 4, rather than the previously proposed isomer 3.