Heterocycles saturated, nomenclature

General. Continuous-chain alkanes are the sole parent hydrides for all compounds, the skeleton of which is composed of chains. Cyclic parent hydrides are more diverse. In nomenclature, they are classified according to their structure as carbo-cycles (composed of carbon atoms only) and heterocycles (composed of carbon atoms and other atoms, such as N, O and Si). They are also classified as saturated and unsaturated. Saturated cycles have the maximum number of hydrogen atoms attached to every skeletal atom, as judged by a prespecified valence unsaturated cycles have fewer hydrogen atoms and multiple bonds between pairs of atoms. 

A specialized system of nomenclature has been developed, principally by Smith,16 for naming skeletons consisting of aromatic residues linked in various ways by saturated bridges. Those skeletons containing benzene residues only are termed cyclophanes, and names for heteroaromatic analogues are based on the name of the heterocycle with the termination -ophane. The numbers of atoms in the bridges are indicated in square brackets, and the orientation of substitution on the aromatic residue(s) is shown in parentheses. A few simple examples are given (149-151), with alternative names. 

The nomenclature for fused 5 5 systems is generally related to the two fused component heterocyclic rings as indicated above for imidazo[4,5-rf]imidazole (1), although compounds are sometimes named as hetero derivatives of pentalene. Heterocycles which are not fully conjugated can be named as dihydro, tetrahydro, etc., derivatives of the fully conjugated parent system or as hetero derivatives of the saturated parent hydrocarbon. For example heterocycle (6) can be named as a tetraaza bicyclo[3.3.0]octane. 

Abstracts, only the Hantzsch-Widman nomenclature has been used. Occasionally, one encounters names not strictly in accordance with prescribed terminology, e.g., azocane or azacyclooctane to denote the saturated, eight-membered heterocycle containing one nitrogen. In this article, we are more concerned with clarity, than with the consistent use of IUPAC nomenclature. 

The nomenclature and numbering adopted are those used in Chemical Abstracts. The parent, 1-benzo, and dibenzo[6/]systems are numbered as shown in compounds (I) to (4). The name for the saturated monocyclic sulfide (4) is thiepane, not hexahydrothiepine. Since the lUPAC recommendation for the nomenclature for monocyclic heterocycles in 1983 <83PAC409>, thiepine is optionally used for the unsaturated system however, thiepin is favored by Chemical Abstracts, throughout this chapter thiepine is used. 

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