Base component in fusion names

Rings are cited in increasing order of preference for selection as base components in fusion names, i.e. the ring which comes later in the list takes precedence. 

In Table 4 (drawn from Sections B and D of the lUPAC Blue Book (B-79MH0200)) are listed most of the heteropolycyclic systems having trivial names still in common use. This Table also includes monocycles described in the preceding section, so as to make clear the order of preference for selection as base component in a fusion name (Section 1.02.3.3). 

For derivation of a fusion name, the molecule is considered as a combination of two or more cyclic units by a fusion (or fusions) such that each component has at least one bond in common with another. All the components should normally have maximum unsaturation (i.e., contain the maximum number of noncumulative double bonds) names based on... 

Orf/io-fused and ortho- and pen-fused ring compounds containing hetero atoms are named according to the fusion principle described in Rule A-21 for hydrocarbons. The components are named according to Rules A-21, B-l and B-2. When the name of a component in a fusion name contains locants (numerals or letters) that do not apply also to the numbering of the fused system, these locants are placed in square brackets (as are also the locants for fusion positions required by Rule A-21.5). The base component should be a heterocyclic system. If there is a choice, the base component should be, by order of preference ... 

In a compound name for a fusion prefix when more than one pair of square brackets is required), the points of fusion in the compound prefix are indicated by the use of unprimed and primed numbers, the unprimed numbers being assigned to the ring attached directly to the base component, thus ... 

Generally, many isomers are possible for fused systems and these have to be differentiated by appropriate descriptors. To obtain these, the sides of the parent component are labelled consecutively by italic letters a, b, c... tracing the locant path 1,2,3,4. .. and ignoring potential non-standard numbering. For the attached secondary components the inherent numbering is maintained. Combination of the partial names is then effected by intercalating the fusion locants in square brackets between the component names. Clearly, the principle of lowest locants is to be respected here too first, lowest letters for the base component second, lowest numbers for the higher order components the sequence of numbers follows... 

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