Abstract5‐Methyl‐11H‐isoindolo[2,1‐α]benzirnidazolium halide (IV), obtained by direct quaternization of the precursor III, underwent facile condensation reactions at its 11‐methylene group similar to those of 11H‐isoindolo[1,2‐b]benzothiazolium cation II. Although the treatment of II with alkali regenerated its precursor I, treatment of IV with alkali caused ring opening to a phthalimidine. Attempted thermal cyclization of 2‐(alkylaminophenyl)phthalimidines,e.g., VII, resulted in dealkylatio