Investigation of azoles and azines. 76. Mass spectra of 5- and 6-substituted uracils

Abstract

Peaks of molecular ions that generally have the maximum intensity are observed in the mass spectra of most of the investigated 5- and 6-substituted uracils and 5-substituted orotic acids and their deutero analogs and methylated derivatives. The principal pathway of the fragmentation of the molecular ions is retrodiene fragmentation with the formation of [O=C(4)C(5)R5C(6)R(6)N(1)R1]+ (F1) ions. The stabilities of the latter depend on the nature and position of the substituents attached to the C(5) and C(6) atoms. The fragmentation of the F1 ions can be realized via four principal pathways (B-E) with the detachment of N-CR6 (B), O=C=CR5 (C), CO (D), and R6 (E) fragments. The most general pathway for the fragmentation of 5-substituted uracils is pathway C, whereas the most general pathway for 6-substituted uracils is pathway E. In the spectra of 5-substituted orotic acids the intensities of the molecular-ion peaks are high (∼100%) only in the case of electron-donor R5 and decrease sharply with an increase in the electron-acceptor strength of the substituent. The principal pathways of fragmentation of the molecular ions are decarboxylation (F) and retrodiene fragmentation (A), the contribution of which is appreciably smaller. The M-CO2 ions formed after decarboxylation undergo fragmentation via a scheme similar to that observed for 5-substituted uracils.