1,3-Migrative Ring Expansion of Spiroindolenines to Azepino[3,4-b]indoles

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.

Automated summary

We serendipitously discovered an unprecedented ring expansion reaction of 2-alkylspiroindolenines to azepinoindoles, mediated by n-tetrabutylammonium fluoride. The starting materials can be easily prepared by the oxidative dearomative spirocyclization of indole derivatives, catalyzed by hypoiodite. Mildly basic conditions and electron-deficient protecting groups for the amines were found to be crucial for promoting chemoselective reactions. Moreover, the ring expansion of aniline-derived spiroindolenines can proceed smoothly under much milder conditions using only a catalytic amount of cesium carbonate.