Cobalt(III)‐Catalyzed [4+2] Annulation of Heterobicyclic Alkenes by sp2C−H Activation

An efficient and convenient cobalt(III)‐catalyzed highly diastereoselective strategy was demonstrated for the synthesis of epoxybenzophenanthridinone derivatives. The sp2C−H activation of N‐methoxybenzamides and their annulation reaction with 7‐oxa/aza benzonorbornadienes proceeds under mild conditions and exhibits excellent functional group tolerance. In addition, the products were transformed to biologically relevant phenanthridinones by an acid‐catalyzed ring opening/aromatization sequence. A bicycle made for 4+2: A highly diastereoselective redox‐neutral [4+2] annulation of N‐methoxybenzamides and strained 7‐oxa/azabenzonorbornadienes for the synthesis of epoxybenzo[b]phenanthridinones using earth‐abundant and cost‐effective Co‐salts was demonstrated.