Synthesis of δ-Carbolines and the Alkaloid Quindoline through a Molybdenum-Catalyzed Cadogan Cyclization and their Photoluminescent Properties

Cadogan reductive cyclization of substituted 2-aryl-3-nitropyridines to give δ-carbolines was performed under MoO2Cl2(DMF)2 catalysis with triphenylphosphine as a ligand. A new approach for the synthesis of the alkaloid quindoline based on a Mo(VI)-catalyzed Cadogan reductive cyclization of 2-phenyl-3-nitro-5,6,7,8-tetrahydroquinoline followed by aromatization of the resulting 2,3,4,10-tetrahydro-1H-indolo[3,2-b]quinoline is proposed. Various о-nitroarylpyridines, obtained by reacting acylpyruvates and cyclic hydroxymethylene ketones with nitroacetophenone enamines, were used as starting compounds for the preparation of δ-carbolines. The synthesized δ-carbolines were found to act as phosphors; their photophysical properties were studied and a structure–property relationship was revealed.