Recent developments in the synthesis of 4-, 5-, 6- and 7-azaindoles

Azaindole structures, also named as pyrrolopyridines, have gained significant relevance in the last three decades. Many different synthetic approaches have been proposed due to their structural relation with the indole moiety. The outstanding development in the synthesis of heterocycles and also, in cross-coupling reactions, has provided a wide range of synthetic approaches to azaindoles. Azaindoles are extremely relevant in different scientific domains: drug research, natural products, photochemistry, etc. The purpose of this review is to structure all these methods with a retrosynthetic approach inspired by indole (pyrrole) or quinoline (pyridine) synthesis in order to provide an overall perspective of how to attempt the preparation of these heterocycles.