Direct Assembly of [6–5–6–7–5] Heteropentacyclic Skeletons Via a Base-Mediated (4 + 3) Cascade Annulation

Herein, we report a base-mediated (4 + 3) annulation of enone-tethered β-carbolines and α-alkylidene succinimides, which enables highly functionalized assembly of [6–5–6–7–5] heteropentacyclic skeletons (3,7a-diazacyclohepta[jk]fluorene) via a domino strategy. The method displays a broad substrate scope of products with good to excellent yield with high diastereoselectivity. Additionally, the potential utility of this approach was proved by late-stage functionalization, scale-up synthesis, and product derivatizations. Furthermore, a plausible reaction pathway has been proposed with a few mechanistic studies.