Formal [3 + 1 + 2] Cyclization/Cyclopropanation Cascade: A Modular Approach to Cyclopropane-Fused Dihydro-α-carbolines

A cascade strategy has been established for the efficient assembly of cyclopropane-fused dihydro-α-carbolines through the formation of four chemical bonds and two fused ring systems. Mechanistically, this transformation proceeds via sequential C−N coupling, double Michael addition, intramolecular cyclopropanation, and sulfonyl group elimination. This protocol showcases broad substrate scope and excellent synthetic modularity. Interestingly, cyclopropane-fused dihydro-α-carboline products could be further converted into indole-fused seven-membered azacycles via acid-promoted ring-opening reaction.