Unexpected Discovery of an Inverse‐Electron‐Demand (5 + 4) Cycloaddition of Oxidopyridinium With Trifluoromethylated <i>o</i> ‐Quinone Methide Yielding a Bridged N, O‐Acetal Scaffold

We investigated the reaction of in situ generated 1‐(pyrimidin‐2‐yl)‐3‐oxidopyridiniums with a trifluoromethylated o ‐quinone methide and discovered that unexpected bridged N‐heterocycles bearing an N, O‐acetal moiety were predominantly formed via a (5 + 4) cycloaddition occurring at the 2‐ and 6‐positions of the oxidopyridinium ring. To rationalize the unusual regioselectivity, a non‐concerted mechanism was proposed based on density functional theory calculations.