Azafluorenes containing two bridgehead nitrogen atoms

Abstract

Four new tricyclic cationic ring systems containing two bridgehead nitrogen atoms have been synthesised: pyrido[1′,2′ : 3,4]imidazo[1,2-a]pyrimidinium, and pyrido[1′,2′ : 3,4]imidazo[1,2-c]pyrimidinium, pyrido[1′,2′ : 3,4]imidazo[1,2-a]pyrazinium, and pyrido[1′,2′ : 3,4]imidazo[2,1-b]thiazolium. In the case of the first named system the parent heterocycle has been prepared. A new, efficient synthesis of the previously described imidazo[a,c]dipyridinium cation is also noted. The synthetic route to these compounds involved N-alkylation of an azinol with 2-chloromethylpyridine and ring closure of the resulting lactam with phosphoryl chloride. In the case of uracil and 6-methyluracil Nalkylation was proved to have occurred at N-1 by unambiguous synthesis. Nucleophilic attack by hydroxide ion upon the 1,3-dimethylpyrido[1′,2′ : 3,4]imidazo[1,2-c]pyrimidinium cation led to 3-(2-acetylaminoprop-1-enyl)imidazo[1,5-a]pyridine the structure of which was confirmed by unambiguous synthesis of its hydrolysis product.