The trace impurities discovered after extended storage of a 5-amino- benzimidazole library were determined as imidazo[4,5-g]quinazoline derivatives by extensive spectroscopic data analysis. The formation of this highly aromatic heterocyclic ring involved a novel multi-component reaction, using which several novel compounds were prepared. Its mechanism was deduced as a cascade of chemical transformations, including the formation of a Schiff's base, intramolecular hetero-Diels-Alder reaction, defluorination and dehydrogenation.