Advances in the synthesis of indolizines and their π-expanded analogues: update 2016–2024

Abstract

Indolizine (pyrrolo[1,2-a]pyridine) is an isomer of indole. The diversity of synthetic approaches leading to the indolizine skeleton is unrivalled, compared to many other heterocycles of this size. Increasing availability of densely functionalized pyrrole derivatives makes it possible to expand the scope of indolizine synthesis from these substrates. In this article, we describe progress in the development of new strategies that lead to indolizine and its π-expanded analogs during the period 2016–2024. Developments of the past decade combined the optimization of known strategies with the discovery of entirely new approaches. First, we discuss the synthetic pathways leading to the indolizine core from pyridine, followed by describing methods starting from pyrrole derivatives. Finally, we focus on π-expanded indolizines, also describing their optoelectronic properties. Although certain synthetic limitations exist, the newly developed methodologies provide impetus for numerous explorations that use indolizines. We anticipate that our review will help motivate further advances in indolizine synthesis.