Metal-Free Dehydrogenative Diels-Alder Reactions of 2-Methyl-3-Alkylindoles with Dienophiles: Rapid Access to Tetrahydrocarbazoles, Carbazoles, and Heteroacenes.

An unprecedented strategy for in situ generation of indole-based ortho-quinodimethanes (oQDMs) from 2-methyl-3-alkylmethylindoles by either a metal-free DDQ- or BQ-mediated dehydrogenative process was developed. These oQDMs were trapped by electron-deficient dienophiles to provide a facile approach to synthetically valuable tetrahydrocarbazoles, carbazoles, and hetereoacenes. The salient features of this transformation include direct C(sp(3))-H bond functionalizations, readily available starting materials, metal-free conditions, high efficiency, operational simplicity, and ease of scale-up.