The goal of my thesis was to synthesize indole-based polycyclic structures through intramolecular domino cyclization reactions, focusing on forming condensed heterocycles. I started with N-protected indole derivatives and performed transformations, including reduction, oxidation, reductive amination, and aromatic nucleophilic substitution, to prepare the substrates for domino cyclization. Various active methylene compounds, such as cyclic diketones, pyrones, and acids, were used in the cyclization, yielding seven derivatives of novel condensed O,N-heterocycles via intramolecular HDA reactions. Additionally, acyclic esters, nitriles, and nitro-substituted active methylenes were reacted with the indole substrate, forming three derivatives of polycyclic O,N-heterocycles. The cyclizations were diastereo- and regioselective, with only the ketone carbonyl involved in the IMHDA reaction. Contrary to our expectations, the nitro-substituted active methylene reagents underwent DK-IMHDA cyclization instead of the nitro-HDA pathway. These synthesized compounds are proposed for further biological evaluation, especially for their potential anti-cancer activity, to compare their effectiveness with known benzene-fused analogues.