Coupling-Isomerization−N,S-Ketene Acetal-Addition SequencesA Three-Component Approach to Highly Fluorescent Pyrrolo[2,3-b]pyridines, [1,8]Naphthyridines, and Pyrido[2,3-b]azepines

Annelated 2-amino pyridines such as pyrrolo[2,3-b]pyridines, [1,8]naphthyridines, and pyrido[2,3-b]azepines can be synthesized in moderate to good yields in a consecutive one-pot three-component process by a coupling-isomerization−enamine-addition−cyclocondensation sequence of an electron-poor (hetero)aryl halide, a terminal propargyl N-tosylamine, and an N,S-ketene acetal. After the coupling-isomerization sequence, a Diels−Alder reaction with inverse electron demand of the intermediate enimine and the N,S-ketene acetal and subsequent aromatization furnish annelated 2-amino pyridines 4 that were unambiguously characterized by numerous X-ray structure analyses. These heterocycles are highly fluorescent and partially pH sensitive, and their electronic structure was studied with spectroscopic and computational methods.