Stereoselective Synthesis of Chroman‐Fused Oxazinanes Via Lewis Acid‐Promoted Oximium Ion‐Initiated Intramolecular Formal [4 + 2] Cycloaddition Cascade

A TMSOTf‐promoted oximium ion‐driven intramolecular formal [4 + 2] cycloaddition of in situ generated ortho ‐quinone methides ( o ‐QMs) with cis / trans ‐ O ‐alkenyl hydroxylamines gave expedient access to chroman‐fused oxazinane derivatives with excellent diastereoselectivity. The observed stereochemical outcome is governed by the geometry of the olefinic partner. Furthermore, the chroman‐fused oxazinanes were successfully diversified via Suzuki, Stille, and Sonogashira cross‐coupling reactions, demonstrating the synthetic utility of this transformation.