De Novo Synthesis of Benzannelated Heterocycles.

Benzannelated heterocycles such as indoles and indazoles are prominent structural motifs found in natural products, pharmaceuticals and agrochemicals. For their synthesis, chemists traditionally either functionalize commercially available heterocycles or resort to transformations that make use of benzene-derived building blocks. Here, we report a powerful cascade reaction that enables the de novo construction of variously substituted indoles, indazoles, benzofurans and benzothiophenes from readily available bicyclo[3.1.0]hexan-2-ones. The transformation can be conducted under mild, non-anhydrous conditions. For the synthesis of indoles, mechanistic studies revealed that the electrocyclic ring-opening of the bicyclic ring-system and aromatization precedes the 3,3-sigmatropic rearrangement.