Stabilization of even‐electron ions by cyclization of substituents on 3 N ‐ and 4 N ‐nitrogens in 4 N ‐substituted cytosines

Electron impact mass spectra of 4 N ‐substituted derivatives of cytosine are reported. The strong influence of 4 N ‐substituents on the mode of mass fragmentation occurring upon electron impact ionization was proved. The presence of α‐carbon in the 4 N ‐substituent extends the possibilities of fragmentation via rearrangement leading to formation of even‐electron bicyclic ions containing quaternary 3 N ‐nitrogen. When β‐carbon was present in the 4 N ‐substituent, the decomposition based on elimination of alkenes was detected as well as the rearrangement leading to formation of bicyclic even‐electron ions containing quaternary 3 N ‐ or 4 N ‐nitrogen.