Formal Carbene Insertion into C−O or C−N Bond: An Efficient Strategy for the Synthesis of 2‐Substituted 2H‐Chromene Derivatives from Chromene Acetals or Hemiaminal Ethers

Abstract We report the palladium/Brønsted acid co‐catalyzed formal insertion of carbene into the C−O or C−N bond of 2 H ‐chromene acetals or hemiaminal ethers. This transformation was initiated by the Brønsted acid‐promoted cleavage of the C−O or C−N bond, followed by modification of the leaving alcohol or amino fragments with palladium carbenes, and reassembly of the modified fragments. A variety of C‐2 functionalized 2 H ‐chromene derivatives were obtained in moderate yield (43∼75%) with good to excellent diastereoselectivities (up to >95:5 dr) under mild conditions. magnified image