Introduction to Heterocycles
Nomenclature of Heterocyclic Compounds
There are several nomenclature systems of heterocycles that are in use. Aside from systematic nomenclature, trivial names are also frequently applied.
A heterocyclic compound is considered to be a carbocyclic compound in that one or more carbons are exchanged for heteroatoms. In replacement nomenclature, the heterocycle's name is composed of the carbocycle's name and a prefix that denotes the heteroatom. Thus, "aza", "oxa", and "thia" are prefixes for a nitrogen ring atom, an oxygen ring atom, and a sulfur ring atom, respectively. According to this nomenclature, tetrahydrofuran, for instance, is called oxacyclopentane.
The position number of the heteroatoms in heterocyclic rings must be as low as possible. If a ring contains different heteroatoms, the heteroatom with the highest CIP priority (highest atomic number) is assigned the lowest position number.
- Instruction : Numbering of heterocycles
The replacement nomenclature is virtually only used in connection with saturated heterocycles. However, trivial names are much more frequently used for saturated heterocycles, as well. The trivial names of saturated heterocycles are often derived from the names of the corresponding unsaturated heterocycles. The names of tetrahydrofuran and tetrahydropyran, for instance, are deduced from furan and pyran. Nitrogen-containing saturated heterocycles' names are usually not attributed to the names of the corresponding unsaturated heterocycles. Piperidine, for instance, is not referred to as pentahydropyridine.