2, 2′-Biindolyl 9 reacts with electron-deficient dienophiles at 100–110° C to give low to moderate yields of Michael addition and formal [42] cycloaddition products, with the former predominant; the products derived from 9 and 2-(phenylsulphinyl) maleimides 14 and 15 undergo in situ elimination of benzenesulphenic acid, leading to 2, 2′-biindolylsubstituted maleimides which can be efficiently photocyclised into indolo [2, 3-a] carbazoles.
[Benitez, Aida; Herrera, F. Ruth; Romero, Margarita; Talamas, Francisco X.; Muchowski, Joseph M. Journal of Organic Chemistry, 1996 , vol. 61, # 4 p. 1487 - 1489]