Lewis Acid-Promoted Intermolecular Acetal-Initiated Cationic Polyene Cyclizations

摘要

The intramolecular acetal-initiated cationic polyene cyclization reaction was first introduced by W. S. Johnson. Since then, this method has been extensively developed to afford bicyclic, tricyclic, tetracyclic, and even pentacyclic products in respectable to good yields. Asymmetric induction has also been achieved using chiral acetal templates, providing enantiomeric excess of up to 90%. However, there exist some disadvantages in using acetals for intramolecular polyene cyclizations. The need to incorporate the required acetal into the acyclic precursor introduces added synthetic complexity. In addition, the accommodation of the acetal moiety also diminishes the structural flexibility in the acyclic precursor. These two problems, though minor, could reduce the scope and applicability of the method substantially. In order to overcome these issues and yet retain the advantages of using acetals as initiators, we devise the intermolecular acetal-initiated polyene cyclization reaction.