Synthesis of 2‐Aminopyridine Derivatives via a Tandem CuAAC/Ring‐Cleavage/[4+2]‐Cycloaddition/Rearrangement Reaction Sequence

Abstract 2 H ‐Azirines are an important class of small‐ring heterocycle that have been employed as versatile synthetic building blocks. Herein, a one‐pot CuAAC/ring‐cleavage/[4+2]‐cycloaddition/rearrangement reaction sequence involving terminal ynones, sulfonyl azides and 2 H ‐azirines is reported that leads to 2‐aminopyridine derivatives. In the first step, highly reactive N ‐sulfonyl α ‐acylketenimines are generated, via a CuAAC/ring‐cleavage process, from sulfonyl azides and terminal ynones and these undergo inverse electron‐demand Diels–Alder cycloaddition reactions with 2 H ‐azirines to form (isolable) 1,3‐oxazines that themselves undergo base‐promoted ring‐cleavage and rearrangement reactions to afford the title compounds.