Reactions of 2-Mono- and 2,6-Disubstituted 4-Pyrones with Phenylhydrazine as General Method for the Synthesis of 3-(N-Phenylpyrazolyl)Indoles
Phenylhydrazine reacted regioselectively with 6-substituted 4-pyrone-2-carboxylic acids (or esters) in protic and aprotic solvents, leading to phenylhydrazones of 3-(3-R-1-phenylpyrazol-5-yl)- or 3-(5-R-1-phenylpyrazol-3-yl)pyruvic acids (or esters), respectively. 6-Di(tri)fluoromethylcomanic acids (or esters) reacted analogously, forming the corresponding phenylhydrazones with RF group in the side chain. The obtained phenylhydrazones underwent Fischer reaction under acidic conditions, forming the respective 3-(N-phenylpyrazolyl)indoles. In contrast to comanic acid and its ester, the reactions of 2-substituted 4-pyrones occurred non-selectively and gave mixtures of regioisomeric pyrazoles with phenylhydrazone group in the side chain or 3-(N-phenylpyrazolyl)indoles. A mechanism was proposed to explain the effect of solvent on the course of reaction.