Lewis Base Controlled (4 + 3) and (4 + 2) Annulations of MBH Carbonates with Benzofuran‐derived Azadienes: Access to Highly Substituted Benzofuro[3,2‐b]azepines and Spirotetrahydroquinolines

Catalyst‐controlled switchable (4+3) and (4+2) annulation reactions of Morita–Baylis–Hillman carbonates with benzofuran‐derived azadienes have been established. Employing PCy 3 as the catalyst, the reaction could provide a variety of synthetically useful benzofuro[3,2‐ b ]azepines in good yields (80–92%) with excellent chemo‐ and regioselectivities via (4+3) cycloaddition reactions. Whereas changing the catalyst from PCy 3 to DMAP, the chemo‐ and regioselectivities were switched to construct highly substituted spirotetrahydroquinoline scaffolds with three sequential stereocenters containing all‐carbon spiro‐quaternary stereocenters in excellent efficiency and diastereoselectivities (92–96% yields with all cases>25:1 dr ) via (4+2) annulations. In addition, the synthetic utility of this method was further showcased by gram‐scale reactions and synthetic transformations of the product.