ZnBr₂-Mediated Cascade Reaction of <i>o</i>-Alkoxy Alkynols: Synthesis of Indeno[1,2-<i>c</i>]chromenes

Abstract

A Lewis acid-mediated cascade annulation of o -alkoxy alkynols in the presence of ZnBr 2 has been developed. The cascade cyclization proceeds through a 5- exo -dig cyclization followed by a Friedel–Crafts reaction and ring-opening sequence to synthesize indeno[1,2- c ]chromenes. This protocol provides a broad substrate scope in moderate to good yields with high regioselectivity. The reaction with benzo-fused cycloalkyl ketones gave an unexpected alkyne C–C bond cleavage resulting in fused polycycles.