Ring transformations in reactions of heterocyclic compounds with nucleophiles (III): Conversion of pyrimidine and some of its methyl derivatives by hydrazine and by methylhydrazine sulfate into pyrazoles and methylpyrazoles

Investigations on ring contraction of pyrimidine and some of its homologues into pyrazoles have been extended. When pyrimidine, 4‐methyl‐ and 4,6‐dimethylpyrimidine are allowed to react with hydrazine, pyrazole, 3(=5)‐methylpyrazole and 3,5‐dimethyl‐pyrazole, respectively are formed in excellent yields. The same compounds, when treated with methylhydrazine sulfate yield 1 ‐methylpyrazole, an isomeric mixture of 1,3‐ and 1,5‐dimethylpyrazole and 1,3,5‐trimethylpyrazole, respectively. The quaternary salts 1‐methylpyrimidinium iodide and 1,4,6‐trimethylpyrimidinium methyl sulfate are converted by hydrazine into pyrazole and 3,5‐dimethylpyrazole. A mechanism of this ring transformation, involving an open‐chain intermediate, is proposed.