In this topic we will study reactions in which heterocyles are generated by cyclization reactions. The formation of a cycle can take place from a molecule that contains a nucleophilic center and another electrophilic one, which unite forming a bond. But cycles can also be obtained from two molecules, in this case two nucleophilic centers are necessary in one of them and two electrophilic centers in the other, or each one contains an electrolyle and nucleophilic center. When we speak of a nucleophilic center, we usually refer to an atom with lone pairs and capable of attacking a center of positive polarity (electrophile) that can be found in the same molecule or in another.