A mass spectrometric investigation on some 4,7-dioxo-4,5,6,7-tetrahydroindole derivatives

The mass spectrometric behaviour of a series of 2-aryl substituted 4,7-dioxo-4,5,6,7-tetrahydroindoles has been studied in different ionization conditions (Electron Ionization and Fast Atom Bombardment), with the aid of the metastable ion studies. In electron ionization conditions all the compounds exhibit a highly favoured, primary H₂ loss giving rise to the corresponding indole-4,7-diones; in the usual spectra no evidence for the molecular ions in the enolic form was found, while the OH* loss observed in the MIKE (mass analyzed ion kinetic energy) spectra of molecular ions suggests that species at low internal energy content isomerize to the corresponding tautomeric enolic form. FAB mass spectra show easy formation of an unusual M + 2H]⁺ species, together with abundant M + H]⁺ and M⁺ cations.