Heterocycles form the largest group of organic compounds, and many have important biological properties. Most natural substances as well as synthetic and natural drugs contain heterocyclic fragments. More precisely, about two thirds of all organic compounds and over three quarters of drugs belong to heterocyclic compounds.
The main heteroatom that occurs in heterocyclic compounds is nitrogen, but there are also rings containing oxygen and sulfur.
In previous chapters we have encountered some compounds with the oxygen or nitrogen atom in a cycle. Formally, cyclic anhydrides, lactones, lactides, and lactams are heterocycles. But the mentioned compounds have the same chemistry as their open-chain counterparts: lactones and ordinary esters behave similarly, as well as lactams and ordinary amides, and so on. These cyclic compounds undergo many reactions with ring opening that is not typical of «genuine» heterocycles, therefore they do not relate to heterocyclic compounds.
16.1. GENERAL CHARACTERISTICS OF HETEROCYCLIC SYSTEMS 16.1.1. Classification
There is no unified classification of heterocyclic compounds because of their big variety (tens of thousands of types are known at present). They are usually classified in accordance with the following features of their skeleton.
According to the ring size. There are mainly three-, four-, five-, six-, and seven-membered heterocycles. Out of them, fiveand six-membered are the most widespread.
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According to the heteroatom incorporated in a ring. As has already been said, the most important are nitrogen-, oxygen-, and sulfur-containing heterocycles.
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According to the number of heteroatoms and their mutual arrangement in
a ring. The most common are compounds with one or two heteroatoms, but cycles