Recent Advances in the Synthesis of Indoles with Partially Hydrogenated Benzene Ring (Tetrahydroindoles)

Abstract In this review, a fragmentary analysis of 4,5,6,7-tetrahydroindoles was performed and, on its basis, the existing methods of their synthesis published over the past 15 years have been summarized, and analyzed, as well as, if necessary, an analysis of earlier works is carried out. The proposed reaction mechanism is considered, as well as factors that significantly influence the course of the process. Among such factors: temperature, type and content of the catalyst, promoting additives, method of the process, etc. Particular attention is paid to fundamentally new methods that make it possible to synthesize various derivatives of the mentioned heterocyclic systems from available and cheap reagents, and in some cases under environmentally benign conditions. 1 Introduction 2 Intramolecular Reactions Leading To Tetrahydroindoles 2.1 Ring Closure Reactions with N–C2 Bond Formation (a) 2.2 Ring Closure Reactions with C2–C3 Bond Formation (b) 2.3 Ring Closure Reactions with C3–C3a Bond Formation (c) 2.4 Ring Closure Reactions with C7a–N Bond Formation (e) 2.5 Ring Closure Reactions with C4–C5 Bond Formation (g) 2.6 Ring Closure Reactions with C3a–C4 Bond Formation (h) 3 Intermolecular Reactions Leading To Tetrahydroindoles 3.1 Ring Closure Reactions with N–C2 + C3–C3a Bond Formation (ac) 3.2 Ring Closure reactions with N–C2 + C7a–N Bond Formation (ae) 3.3 Ring Closure reactions with C3–C3a + C7a–N Bond formation (ce) 3.4 Ring Closure Reactions with N–C2, C3–C3a, and C7a–N Bond Formation (ace) 4 Recovery Processes (Hydrogenation) of Indoles and Oxidation of Perhydroindoles in the Synthesis of 4,5,6,7-Tetrahydroindoles 5 Conclusion