Synthetic Strategies for the Construction of δ-Carbolines: A Chemical Ladder in Search of Novel Drugs

δ-Carboline and its naturally obtained analogs are of importance due to their wide range of biological properties. In the present review, we intend to discuss the main trends in the synthesis of δ-carboline analogs based on pyridine derivatives (including the Fischer synthesis, Graebe-Ullmann reaction, photochemical cyclization, arylation of substituted pyridines, quinolines, etc.) and indole derivatives in order to compare the potential of finding novel drugs in this series. In other words the development, current status and innovative new options for the design and synthesis of δ-carboline analogs are reviewed here. Published data about the various pharmacological functions and the spectroscopy of compounds belonging to this series are summarized. It also discusses the scope, future perspectives and the problems yet to be explored with respect to the synthesis of functionalised bioactive heteroannulated δ-carbolines in search of novel drugs.