The fragmentation upon electron impact of the title compounds of general formula 1 has been investigated.

The primary fragmentation of pyrazolo[1,5-a]pyrimidine and its derivatives includes two important processes:

- expulsion of acrylonitrile or its derivative that originates from the decomposition of the pyrimidine ring,

- less characteristic loss of CH₂CN radical that occurs by the scission of the pyrazole ring.

The influence of substituents bonded to the pyrimidine and pyrazole rings of the pyrazolopyrimidines on the fragmentation pathways is discussed as well as the stability of these heterocyclic moieties toward ring cleavage.

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