Tautomerism of Pyrazolidine-3,5-diones. I. N-Methylation and ultraviolet spectra

Abstract

Methylation of 3-phenyl-1H-1,2,4-triazol-5-ylpyrazolidine-3,5-diones attacks preferentially the pyrazolidine ring. In these cases and those of mono- and di-phenyl analogues methylation by methyl iodide gives C- and N-substitution only, while treatment with diazomethane affords O- methyl derivatives as well. In cyclohexane the keto-iminol tautomer is favoured or, if this tautomerization is blocked, the diketone form. In methanol again the keto-iminol form is favoured but with the keto-enol form as an alternative or main coexistent tautomer. Thus change from the more polar to the less polar solvent is associated with tautomeric shifts from enol to iminol or enol to ketone.