HETEROCYCLIC COMPOUNDS

**Definition:** This place covers:

Organic compounds containing at least one heterocyclic ring, and with no ring comprising a steroid, saccharide or peptide moiety. Said compounds of C07D contain ring heteroatoms selected from nitrogen, oxygen, sulphur, selenium, tellurium, halogen or a combination thereof.

The compounds may also contain metal atoms, but only as the cations of heterocyclic organic acid salts, alcoholates, phenolates or mercaptides, or as chelating atoms, e.g. in porphyrins.

Preparation of such compounds, including purification, separation, stabilisation or use of additives, unless a separate place is provided elsewhere in the classification scheme.

**Limiting references (this place does not cover):** - Macromolecular compounds -> C08

**Glossary:** - Acyclic: The absence of a ring structure. Acyclic chains may be linear or branched. - Bridged: Where two condensed rings share at least three adjacent ring members: - Carbocyclic: Where all ring members in a ring are carbon atoms. - Condensed: Where at least two rings share at least one ring member. - Condensed ring system: A ring system in which all rings are condensed among themselves, i.e. a ring system wherein the scission of a single connection between two ring atoms cannot result in the division of the ring system into separate entities. Two or more hetero rings are considered part of the same condensed ring system if they are condensed among themselves or to a common carbocycle or carbocyclic system. - Heterocyclic: Wherein at least one ring member in a molecule containing a ring of atoms is not a carbon atom. For the purposes of classification in this subclass, a narrower definition applies wherein heteroatoms may only be chosen from nitrogen, oxygen, sulphur, selenium, tellurium or halogen. - Number of relevant rings: In a condensed ring system, this equals the minimum number of scissions necessary to convert the ring system into an acyclic chain, a scission being the disconnection of two bonded atoms, without regard for the bond order. - Ortho-condensed: Where two condensed rings share two adjacent ring atoms in common. A ring system is deemed ortho-condensed if each ring shares only one face with any other ring, and no ring has two adjacent shared faces: - Peri-condensed: Where three rings in a condensed ring system share a single ring atom in common: - Relevant rings: These are the rings which account for all the bonds in a condensed system. In order to prevent ambiguity in classifying a condensed ring system, the rings which identify the ring system are determined according to the following hierarchy of criteria: the rings with the lowest number of members; the rings with the highest number of hetero atoms as ring members; the rings with the lowest number of members shared between rings; the rings with the last place in the classification scheme. - Rings: Rings are considered as heterocycles only if they contain at least one atom selected from halogen, N, O, S, Se or Te as a ring member. Heterocyclic rings may be present as distinct entities or condensed, either with carbocycles or among themselves. - Spiro-condensed: Where two condensed rings share only one atom in common:"free" "frozen" - [x,y]-condensed: The letters in the square brackets refer to the sides around the heterocyclic ring with "a" for the side 1,2; "b" for the side 2,3; etc. For example: