NEW ADVANCES IN THE SYNTHESIS OF CYCLOALKA[b]INDOLS

The review summarizes data on the synthesis of compounds whose backbone is a cycloalka[b]indole heterocycle. Approaches to the synthesis of homologous structures are considered, ranging from a carbocyclic fragment with three carbon atoms to eight-membered analogues. Examples of cyclizations are presented, including those with induction of chiral centers using appropriate catalytic systems, with different enantio- and diastereoselectivity. Attention is paid to carbocyclization reactions occurring in the presence of metal complexes, lithium alkylates, inter- or intramolecular cycloaddition of allenes, activated alkenes with 1,3-dipolarophiles or diene-like systems. In the preparation of cycloprop[b]indoles, [2+1]-cycloaddition reactions of indoles with various carbenes are most often used. In approaches to the synthesis of indoles annelated with cyclobutane, [2+2]-cycloaddition reactions are used. To obtain indoles, to which 2,3-annelated carbocycles of medium size, synthetic methods are effectively used, including the use of indole derivatives, 3-nitroindole derivatives as starting materials, intra- and intermolecular reactions of 3-ethynyl-, 3-allenyl -, 3-alkenyl- or functionalized 2-alkyl-substituted indoles in the presence of complexes of palladium, gold, rhodium, copper, silver, indium, organophosphorus acids and their amides, trifluoroethanol, hexafluoroisopropanol, organic sulfo- or trifluoroacetic acids. Methods of photochemical exposure are known, leading to cyclopenta[b]indoles. In the preparation of these heterocycles, cycloaddition reactions of 1,3-dipoles generated from (2-indolyl)diarylmethanols to dipolarophiles are successfully used. Equally effective are cycloaddition reactions of 2-vinylindole derivatives to activated alkenes or carbonyl compounds in the presence of various catalysts, leading to di- or tetrahydrocarbazoles or their seven-membered homologues. One of the frequently used approaches to obtain cycloalka[b]indoles are reactions based on catalyzed transformations of indole-substituted cyclopropanes or intermolecular interaction of cyclopropanes with indoles under various conditions.