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.