An effective 1,3‐dipolar cycloaddition was developed for the synthesis of 5‐amino‐1,2,4‐triazoles by reacting hydrazonoyl hydrochlorides (nitrilimines) with carbodiimides in the presence of triethylamine as a base. Both symmetric and asymmetric carbodiimides are compatible with this newly developed reaction; diphenyl carbodiimide is the one exception. Mechanistic studies indicate that alkyl and cycloalkyl moieties of the quaternary N+–R on the 1,2,4‐triazolic ring serve as potential leaving groups and can efficiently depart from the intermediate 4,5‐dihydro‐4‐alkyl‐5‐imino‐1,2,4‐triazole.

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A novel one-flask synthetic method was developed in which 5-aminopyrazoles were reacted with N,N-substituted amides in the presence of PBr3. Hexamethyldisilazane was then added to perform heterocyclization to produce the corresponding pyrazolo[3,4-d]pyrimidines in suitable yields. These one-flask reactions thus involved Vilsmeier amidination, imination reactions, and the sequential intermolecular heterocyclization. To study the reaction mechanism, a series of 4-formyl-1,3-diphenyl-1H-pyrazol-5-yl-N,N-disubstituted formamidines, which were conceived as the chemical equivalent of 4-(iminomethyl)-1,3-diphenyl-1H-pyrazol-5-yl-formamidine, were prepared and successfully converted into pyrazolo[3,4-d]pyrimidines. The experiments demonstrated that the reaction intermediates were the chemical equivalents of 4-(iminomethyl)-1,3-diphenyl-1H-pyrazol-5-yl)formamidines. The rate of the reaction could be described as being proportional to the reactivity of amine reactants during intermolecular heterocyclization, especially when hexamethyldisilazane was used.