Dearomative [4 + 2] annulations between 3-nitroindoles and enals through oxidative N-heterocyclic carbene catalysis

Hydrocarbazolone and its derivatives are attractive organic molecules, which have been widely applied in the synthesis of natural products and pharmaceuticals. Herein, we describe a novel intermolecular dearomative [4 + 2] annulation of 3-nitroindoles and enals under oxidative N-heterocyclic carbene catalysis. With this organocatalytic strategy, a variety of dearomatised hydrocarbazolone products were obtained in high yields. This protocol was also suitable for the oxidative cyclisation of 2-nitrobenzothiophenes with enals, thereby affording a collection of 4-hydroxyldibenothiophenes.