Brønsted-acid-mediated dearomative cascade annulation of pyridines: one-pot synthesis of indolizin-5(3H)-one derivatives.

A direct dearomative cascade annulation of unactivated pyridines has been achieved using a Brønsted acid, enabling efficient access to indolizin-5(3H)-one frameworks. The transformation proceeds through a sequential [2+2] cycloaddition, followed by demethylation and intramolecular cyclization, furnishing fused bicyclic architectures in a one-pot two-step manner, using inexpensive hydrochloric acid. Deuterium incorporation studies further provide selectively deuterated indolizinone derivatives, offering opportunities for further synthetic diversification.