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.

Alkaloid quindoline, its structural analogues and δ-carbolines were obtained through the Cadogan reaction using DPPE under solvent-free conditions as a key stage of the synthesis.Various o-nitroarylpyridines prepared by one-pot synthesis from nitroacetophenones, methyl orthoformate and acetoacetic ester enamine were used as starting materials for the preparation of δ-carbolines. 2-Aryl-3-nitro-5,6,7,8-tetrahydroquinolines obtained by reacting 2-hydroxymethylenecyclohexanone with nitroacetophenones enamines were used for the synthesis of quindoline and its structural analogs. A relationship between the structure and optical properties of the synthesized compounds was found.