Brønsted acid-catalyzed cascade cyclization: an efficient strategy for divergent synthesis of cyclohepta[b]indole derivatives

Abstract

An efficient acid-catalyzed three-component cascade strategy was established on the basis of a crucial conversion of BIMs to vinylindoles to synthesize a privileged scaffold, phenyl-substituted dihydrocyclohepta[b]indole carboxylate. The vinylindole intermediates worked as 1,4-bisnucleophiles and could be trapped by atropaldehyde acetals to construct diphenyl-substituted dihydrocyclohepta[b]indoles. A [5 + 2] cyclization of 2-aryl-3,4-dihydropyrans with 2,3-unsubstituted indoles was also established, which provided another route to access tetrahydrocyclohepta[b]indoles. Readily available materials, inexpensive metal-free catalysts, eco-friendly solvents and divergent synthesis ensured these methods are practically attractive.