Macrocyclization Reactions: The Importance of Conformational, Configurational, and Template-Induced Preorganization

The success of a given macrocyclization reaction involves a very delicate balance of many different factors. First, a proper understanding of the basic thermodynamic and kinetic concepts underlying these processes is essential in defining the strategies to obtain the targeted cyclic structures and the experimental elements to be optimized. essential element for achieving high yields in a macrocyclization process is the appropriate selection of the disconnection site. This defines the reaction used for the key step, which will clearly affect the overall process, but also delineates the nature and structure of the immediate linear precursor. The presence of structural elements, including configurational elements, able to induce a favorable folding of this linear precursor in such a way that both reactive ends approach with the proper orientation provides significant enhancements in macrocyclizations. The use of templates of very different natures is a versatile strategy to overcome the limitations of the other macrocyclization strategies. The use of templates has allowed the preparation of a variety of macrocyclic structures in better yields and usually in shorter reaction times, and often allows easier purification protocols.