Synthesis of Tetraphenyl-furoindoles via Tandem Reactions

Abstract

We report a novel one-step synthesis of tetraphenyl substituted furoindoles involving acid-catalyzed cascade approach of commercially available m-aminophenols with readily accessible α-hydroxyaldehydes. The tandem reactions involved nucleophilic addition, aldimine condensation, pinacol rearrangement, α-iminol rearrangement, cyclization and dehydration aromatization process. The yield of furoindoles was up to 75%.Tetraphenyl substituted furoindoles are crucial structural motifs in various organic materials. 1 As a result, the synthesis of these target molecules by feasible methodologies have great significance, and indeed, various synthetic routes have been used for the construction of tetraphenyl substituted furoindoles and their derivatives. Some of synthetic strategies toward tetraphenyl substituted furoindoles are shown in Scheme 1. The classic method for the synthesis of tetraphenyl substituted furoindoles involves Pd catalyzed condensation of halogenated arenes with phenylacetylene (path a, Scheme 1). 1b Unfortunately, this method needs 4 steps to give access to tetraphenyl substituted furoindoles. Another method only needs two steps to give access to tetraphenyl substituted furoindoles, but this approcach needs special phosphorus ligand (path b and c Scheme 1). 1c,1f In 2018, Yan group introduced one approach for the p-toluenesulfonic acid (PTSA) catalyzed synthesis of tetraphenyl substituted furoindoles. The yield of this approcach can only reach to 35% showing only one example (path d, Scheme 1). 1e Sahoo developed a novel synthesis of tetraphenyl substituted furoindoles involving Pd and Rh catalyzed oxidative annulations of both commercially available phenols with 1,2-diphenylethyne. This method can synthesize two types of tetraphenyl substituted furoindoles. The yield can reach to 36% (over 2 steps) and 29% (over