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Updated: Aug 8, 2025

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
Published on: August 22, 2018
1,3-Migrative Ring Expansion of Spiroindolenines to Azepino[3,4-b]indoles
Hiroki Tanaka1, Toshihiro Yasui1, Muhammet Uyanik1
1Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan.
Abstract:
We serendipitously found an unprecedented 5-to-7-membered ring expansion of 2-alkylspiroindolenines to azepinoindoles mediated by n-tetrabutylammonium fluoride. The starting materials can be easily prepared by the hypoiodite-catalyzed oxidative dearomative spirocyclization of indole derivatives. Mildly basic conditions and electron-deficient protecting groups for the amines were found to be crucial to promoting chemoselective reactions. Moreover, the ring expansion of aniline-derived spiroindolenines proceeds smoothly under much milder conditions using only a catalytic amount of cesium carbonate.
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