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

Abstract

Catalyst‐controlled switchable (4 + 3) and (4 + 2) annulation reactions of Morita–Baylis–Hillman carbonates with benzofuran‐derived azadienes have been established. Employing PCy3 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 PCy3 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.