Chemistry of Heterocyclic Ketene Aminals: Construction of Imidazo(pyrido)[1,2-a]pyridines and Imidazo(pyrido)[3,2,1-ij][1,8]naphthyridines via DABCO-Catalyzed Tandem Annulations

2-(2-Chloroaroyl)methyleneimidazolidines with four reactive sites show fascinating structural features and could be used as a new strategy for the synthesis of novel heterocycles. This paper presents our new findings in the reaction of 2-(2-chloroaroyl)methyleneimidazolidines with allenic esters affording functionalized imidazo(pyrido)[1,2-a]pyridines via DABCO-catalyzed tandem annulations. Of particular significance is the incorporation of an o-halo group into the aryl ring of 2-benzoylmethylene-imidazolidines to set up a convenient and general way for constructing unusual imidazo(pyrido)[3,2,1-ij][1,8]naphthyridines.