Efficient Approaches to the Design of Six-Membered Polyazocyclic Compounds—Part 2: Nonaromatic Frameworks

This review summarizes and compares literature data on the main strategies for the synthesis of saturated and/or partially saturated analogs of six-membered polyazoaromatic heterocycles. With a few exceptions, this data was published within the last 15-20 years. These strategies include, first of all, hydrogenation, which can be carried out in a classical manner (i.e., in the presence of a catalyst and molecular hydrogen) or via hydrogen transfer techniques. Other approaches comprise saturation of aromatic frameworks, which can be achieved using compounds other than hydrogen, such as hydroboration and dearomatic binding, or the creation of saturated or partially saturated polyazo heterocycles through the coupling of two or more molecules. Each of the above strategies has certain advantages and serious shortcomings and limitations. Despite the apparent methodological difficulties, it is demonstrated that the described approaches can sometimes give impressive results, including the production of optically pure products under relatively mild conditions.