Syntheses of Hydrocarbazole Derivatives via Brønsted acid-Initiated Diels-Alder Cycloaddition/retro-Michael Addition Cascade Reaction of Azepino[4,5-b]indoles and Acrolein

A unique approach to hydrocarbazoles bearing an all-carbon quaternary center at C4a position was developed via a Brønsted acid-initiated Diels-Alder cycloaddition/retro-Michael addition cascade process from facilely prepared azepino[4,5-b]indoles and commercially available acrolein. The method provided a range of hydrocarbazoles in good to excellent yields. The practi-cality of this transformation was demonstrated by scale-up experiment and various transformations to several hydrocarbazole derivatives.