Reaction of 2-aminobenzimidazole with bifunetional carboxylie acid derivatives. Formation of pyrimido[1,2-a] benzimidazolones

2-Aminobenzimidazole reacts with α,β-unsaturated carboxylic acid chlorides or esters to give only pyrimido 1,2-a] benzimidazol-2-(1H) ones. β-Ethoxymethylenemalonie acid derivatives or β-ketocarboxylic acid derivatives give only pyrimido 1,2-a] benzimidazol-4-(1H)ones. Structural assignments based on nmr and chemical manipulations are discussed.