Deprotonated N-(2,4-dinitrophenyl)amino acids undergo cyclization in solution and the gas phase

The collisionally activated mass spectral fragmentations of N-(2,4-dinitrophenyl)alanine and phenylalanine [M - H]^- may be gas-phase analogs of the base-catalyzed cyclization of N-(2,4-dinitrophenyl) amino acids in aqueous dioxane. This latter reaction is one source of the 2-substituted 5-nitro-1H-benzimidazole-3-oxides, which are antibacterial agents. The fragmentation of both compounds, established by tandem mass spectrometric experiments and supported by molecular modeling using DFT methods, indicate that the [M - H]^- ions dissociate via sequential eliminations of CO ₂ and H₂O to produce deprotonated benzimidazole-N-oxide derivatives. The gas-phase cyclization reactions are analogous to the base-catalyzed cyclization in solution, except that in the latter case, the reactant must be a dianion for the reaction to occur on a reasonable time scale. The cyclization of N-(2-nitrophenyl)phenylalanine, which has one less nitro group, requires a stronger base for the cyclization than the compound with a second nitro group at the 4-position. Following losses of CO₂ and H₂O are expulsions of both neutral molecules and free radicals, the latter being examples of violations of the even-electron ion rule.