π-Extension of Indoles Using Acrolein Linker: Synthesis of Indolo[3,2-a]carbazole-6-carbaldehydes and Racemosin B

A simple method for the synthesis of indolo[3,2-a]carbazole-6-carbaldehydes by the π-extension of indoles with acrolein is reported. The scope of the method is demonstrated with 33 examples. The products exhibit high activities toward advanced synthesis and are proved to be able to produce valuable chemicals, such as natural products, dyes, and organic fluorescent materials. In addition, the alkaloid racemosin B can be prepared by this method in two steps in ∼50% overall yield.