Lewis Acid‐Catalyzed Unusual (4+3) Annulation of <i>para</i> ‐Quinone Methides with Bicyclobutanes: Access to Oxabicyclo[4.1.1]Octanes

Abstract Over the past few years, there has been a surge of interest in the chemistry of bicyclobutanes (BCBs). Although BCBs have been used to synthesize bicyclo[2.1.1]hexanes and bicyclo[3.1.1]heptanes, the synthesis of bicyclo[4.1.1]octanes has remained elusive. Herein, we report the first Lewis acid‐catalyzed unexpected (4+3) annulation of para ‐quinonemethides ( p ‐QMs) with BCBs allowing the synthesis of oxabicyclo[4.1.1]octanes proceeding under mild conditions. With 5 mol % of Bi(OTf) 3 , the reaction afforded the (4+3) annulated product in high regioselectivity and good functional group compatibility via a simultaneous Lewis acid activation of BCBs and p ‐QMs. The reaction is likely initiated by the 1,6‐addition of Lewis acid activated BCBs to p ‐QMs followed by the C2‐selective intramolecular addition of the phenol moiety to the generated cyclobutyl cation intermediate. Moreover, detailed mechanistic studies provided insight into the mechanism of the reaction.