Part IV. HETEROCYCLIC COMPOUNDS

Heterocyclic compounds constitute the larger group of organic compounds. They are constituents of many substances of natural origin and play an important role in metabolism, many of them show a high biological activity. Needless to say that most modern medicines contain heterocycles in their structure.

Heterocyclic compounds have a cyclic structure and contain atoms of other elements (heteroatoms) apart from carbon atoms in the cycle.

Some compounds with oxygen or nitrogen atoms in the cycle were discussed in previous sections of the course: cyclic anhydrides, cyclic esters (lactones, lactides), and cyclic amides (lactams, diketopiperazines). However, these compounds rather easily enter reactions accompanied by ring opening, e.g. hydrolysis. Due to this, they are usually not seen as heterocyclic compounds with a typically high strength of the ring.

This course discusses only nitrogen-, oxygen-, and sulfur-containing hetero-cycles predominantly of aromatic character.

Chapter 20

CLASSIFICATION AND NOMENCLATURE OF HETEROCYCLIC COMPOUNDS

20.1. Classification

Due to significant diversity of heterocyclic structures, it is difficult to select a single classification system for them. They are usually classified taking into account the features given below.

• According to the ring size, heterocyclic compounds are mostly three-, four-, five-, six-, and seven-membered. In examples given, the emphasis is on graphic representation of heterocycles, their names will be discussed later.

• According to the element type included into the ring, these are mainly compounds with nitrogen, oxygen, or sulfur atoms. Their combinations are also possible.

• According to the degree of saturation, heterocycles can be aromatic, unsaturated, and saturated.