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 CO2 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.

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► [M - H]− of title compounds dissociate via sequential eliminations of CO2 and H2O.
► Products of dissociation are cyclized deprotonated benzimidazole-N-oxide derivatives.
► Structures determined by MS/MS, synthesis of reference compounds, and theory.
► Gas-phase cyclization is analog of the base-catalyzed solution reaction.