N → N acyl group migration in N-acylpyrazoles: Isomerization of 1,4-diacyl-5-methyl-1H-pyrazoles to 1,4-diacyl-3-methyl-1H-pyrazoles

1,4-Diacyl-5-methyl-1H-pyrazoles on heating in toluene undergo isomerization to 1,4-diacyl-3-methyl-1H-pyrazoles via intermolecular N → N acyl group migration. 1,4,5-Trisubstituted pyrazoles obtained by reaction of 2-ethoxymethylidene derivatives of 1,3-diketones with 1,3-benzothiazol-2-ylhydrazine or phenylhydrazine failed to isomerize to 1,3,4-trisubstituted pyrazoles.