Lewis Base-Catalyzed Sequential [2 + 3]/[4 + 4] Annulations of α-Allylic Allenoates and Aldimine Esters: Construction of Functionalized Pyrrole-Fused Eight-Membered <i>O</i> -Heterocycles

The efficient construction of eight-membered rings has gained significant attention due to their widespread applications in natural products and pharmaceuticals. However, the synthesis of these molecules, especially by organocatalytic strategies, remains a formidable challenge. Herein, we disclose an efficient Lewis base-catalyzed sequential [2 + 3]/[4 + 4] annulation of α-allylic allenoates and aldimine esters, delivering facile access to the pyrrole-fused eight-membered <i>O</i>-heterocycles with good yields and excellent diastereoselectivity. Mechanistic studies suggest a mechanism involving the formation of two C-C bonds and one C-O bond, as well as hydrogen-bond-assisted activation. The practicality of this method has been demonstrated by the gram-scale synthesis and further synthetic transformations.