Studies on Pyrimidine Derivatives and Related Compounds. LVII. Cyclization Mechanisms of Ethyl 2-(5-Pyrazolyl)aminoalkylidenecyanoacetates

It was found that the orientation of cyclization reaction of ethyl 2-(5-pyrazolyl)aminomethylenecyanoacetates (Va–c) and ethyl 1-cyano-2-(5-pyrazolyl)aminocrotonates (Xa–c) largely depends on the catalyst used: for example, the cyclization of Va by the acid catalyst results in only 7-amino derivative; on the other hand, the mixture of 7-amino derivative and 7-hydroxy derivative is obtained in the presence of the base catalyst. In order to clarify the mechanism of this reaction, we studied spectrophotometrically the behaviors of these compounds in basic solution, taking ethyl 2-(1,4-dimethylpyrazol-5-yl)aminomethylenecyanoacetate (XV), and ethyl 1-cyano-2-(1,4-dimethylpyrazol-5-yl)aminocrotonate (XVI) as model compounds. Consequently, it was found that the proton dissociation of the C2-amino group is greatly disturbed by the methyl substitution on that carbon atom. This finding is used for explaining the fact that the base catalyzed cyclization of X is insensitive to the amount of catalyst used while that of V shows some dependency on the concentration of alkali.