Fusion locants

One should be aware that, in the literature, some members of the quinazolino-quinazolines are described by either one of two fusion locants as a result of numbering either of the quinazoline moieties. 

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... 

Systematic names of the Hantzsch-Widman type, as well as fusion names, imply unsaturation to the extent of the maximum number of noncumulative double bonds. Suitable suffixes (Table 3) serve to identify the fully saturated derivatives. Partially unsaturated rings may be designated by hydro prefixes with locants and an even-numbered multiplier, such as 2,3-dihydro , applied to a fully unsaturated name, or by dehydro prefixes applied to the saturated name, e.g. (116). 

When a system does not have a retained name or a name that can be composed systematically as above, and when ortho- and ortho-peri- x% on are possible, it is named using fusion nomenclature, i.e. by combining the names of the two or more systems that are fused. One system is adjudged to be the senior according to criteria described elsewhere and is taken as a parent hydride, and the other is denoted in the name by a non-detachable prefix. The junction of the two systems is indicated in a specific manner. Instead of numerical locants, italic letters a , 6 , c , etc. are used to identify bonds in the parent hydride. The final letter o and normal locants are characteristic of the prefix. The examples below illustrate the fusion operation and the resulting fusion name. 

The locant indicates the position of the double bond other positions are possible. b Not considered suitable for use as components in fusion nomenclature (Section 1.02.3.3). c 4H-Pyran is also possible. 

A component fused to the base component (referred to as a primary component) is cited as a prefix terminating usually in o, with the locants of the bond engaged in fusion specified in square brackets in the order corresponding to the direction of lettering of the base component, as in example 13. A (secondary) component fused to a primary component is cited as a further prefix, with the use of primes to indicate the... 

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 ... 

This name, while still bearing some resemblance to (5) in that certain combinations of atom-bond sequences and of locant sequences are repeated, has a familial relation that is nowhere near as clear. By the time that one reaches the face fusion (part d of Figure 4), nearly all of the "local" similarity in the canonical name... 

When ring fusion is also present, the two ring systems that share the spiro atom are given in brackets with splicing locants as shown in the following example. 

The fusion in this ring places the benzene on the d side of the imidazole ring. Renumbering the new ring gives the locants as shown. 

According to a recent rule revision the numbering of each interior atom is now related to its lowest numbered peripheral neighbour atom by a superscript denoting the number of bonds between the two atoms. The following example stands among others for the intricacies involved in the application of orientation rules a) through e). Here the decision for the correct orientation is only possible after lowest locants for the fusion positions have been evaluated. 

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