Brønsted-Acid-Promoted Selective C2–N1 Ring-Expansion Reaction of Indoles toward Cyclopenta[b]quinolines

A novel Brønsted-acid-promoted selective C2–N1 ring-expansion reaction of indoles has been developed that provides a rapid and efficient protocol for the preparation of fused quinolines. A variety of corresponding quinolines were obtained in high yields. Controlled experiments revealed that C2-spiroindolenines might be intermediates of this C2–N1 ring-expansion reaction. The notable advantages of this process include excellent yields, good functional group tolerance, and operational simplicity.