ABSTRACT
This chapter shows how just a few general principles allow one to understand the methods, at first sight apparently diverse, which are used in the construction of the heterocyclic ring of an aromatic heterocyclic compound from precursors which do not have that ring. It discusses the principles, and analyses the types of reaction frequently used in constructing an aromatic heterocycle, and also the way in which appropriate functional groups are placed, in the reactants, in order to achieve the desired ring synthesis. The preparation of benzenoid compounds nearly always begins with an appropriately substituted, and often readily available, benzene derivative only on very rare occasions is it necessary to start from compounds lacking the ring, and to form it during the synthesis. The preparation of heterocyclic compounds presents a very different picture, for it involves ring synthesis more often. A completely separate category is the increasing number of ring syntheses which involve electrocyclic processes.
