Gallium Bromide-Promoted Dearomative Indole Insertion in 3-Indolylmethanols: Chemoselective and (Z/E)-Selective Synthesis of 3,3'-Bisindole Derivatives.

Abstract

Gallium bromide (GaBr3)-promoted dearomative indole insertion in 3-indolylmethanols has been established, which chemoselectively constructs a biologically important 3,3'-bisindole framework bearing an all-carbon quaternary center in high yields and excellent (Z)-selectivities (up to 99% yield, all >95:5 Z/E). The reaction pathway was suggested to include a tandem sequence of Michael addition/C-C bond cleavage/nucleophilic addition, wherein the strong acidity of GaBr3 played a crucial role in the key step of C-C bond cleavage. This reaction not only provides a new strategy for dearomatization of indoles, but also represents a new reaction category for 3-indolylmethanols, which involves a rarely reported late-stage C-C bond cleavage of 3-indolylmethanol derivatives. In addition, this approach also offers an efficient method for the synthesis of biologically important 3,3'-bisindole derivatives.