Priorities heterocycles

TABLE 1.3 Specialist Nomenclature for Heterocyclic Systems Heterocyclic atoms are listed in decreasing order of priority. 

TABLE 1.5 Trivial Names of Heterocyclic Systems Suitable for Use in Fusion Names Listed in order of increasing priority as senior ring system. 

Multiple bonds in acyclic compounds in bicycloalkanes, tricycloalkanes, and polycycloalkanes, double bonds having priority over triple bonds and in heterocyclic systems whose names end in -etine, -oline, or -olene... 

When an exocycllc function has a higher priority, it may be necessary to name a cationic heterocyclic substituent group. The most important case is that in which the heterocyclic substituent is bonded through its cationic center. Such cases may be named in two ways, as in (182) and (183). The simplest is to use the suffix -io , as used for the H3N— substituent, ammonio terminal e is elided. More generally, however, an -yl suffix is appended after -I um , as shown in the second names given for the examples. This method applies equally well to situations with other sites of attachment, and also allows one to name divalent substituents, e.g. (184) and (185). 

Alternative names are shown in some cases this should emphasize that there is often no unique correct name. Sometimes, it can be advantageous to bend the rules a little so as to provide a neat name rather than a fully systematic one. Typically, this might mean adopting a lower priority functional group as the suffix name. It is important to view nomenclature as a means of conveying an acceptable unambiguous stmc-ture rather than a rather meaningless scholastic exercise. Other examples will occur in subsequent chapters, and specialized aspects, e.g. heterocyclic nomenclature, will be treated in more detail at the appropriate time (see Chapter 11). Stereochemical descriptors are omitted here, but will be discussed under stereochemistry (see Sections 3.4.2 and 3.4.3). 

The principal ring is selected using the criteria in Table 6.1. For instance, heterocycles have priority over carbocycles and unsaturated systems have priority over saturated systems. 

The rules for numbering heterocyclic skeletons have been mentioned already (see Section II, B, 2, b). However, these rules may not be sufficient to assign a unique numbering when substituents are present. Further rules are provided by IUPAC to deal with such cases (Rule C-15 see Appendix). These require no comment except to note (a) the high priority for lowest number given first to indicated hydrogen and then to the principal group, and (b) the fact that when the cyclic unit is named... 

Heterocyclic spiro compounds are named according to Rule A-42, the following criteria being applied where necessary (a) spiro atoms have numbers as low as consistent with the numbering of the individual component systems (b) heterocyclic components have priority over homocyclic components of the same size (c) priority of heterocyclic components is decided according to Rule B-3. Parentheses are used where necessary for clarity in complex expressions. 

The rules for potential nucleobases have become better defined by synthetic biologists.27 It is appropriate to couple design to experimental and theoretical considerations related to heterocycles that might be generated under prebiotic conditions. NASA should place high priority on Earth-based experiments that define what heterocycles might have been formed, especially in cometary bodies and on early Earth. 

The numbering of the skeleton in a ring system is generally a consequence of the way in which the skeleton is named. In order to avoid confusion as to the peripheral numbering sequence and locations of substituents in the fused systems, structural formulae are drawn as in Chemical Abstracts in accordance with the rules of heterocyclic nomenclature. Thereby the reader is not required to possess detailed knowledge of the elaborate series of priorities used in the structural presentation and numbering of ring systems. 

The priority in this chapter lies in the description of small organosubstituted silanes and of small heterocycles that might be precursors for the production of new materials for high tech applications (e. g. polycrystalline silicon for photovoltaic purposes, thin ceramic coatings from CVD processes). 

The actual observation of ester molecules in a polymerising system involving alkenyl monomers should be a priority among the tasks of future investigations in this field. Extremely interesting results have been obtained in similar studies vrith heterocyclic monomers (see and refs, therein), particularly with tetrahydrofliran. 

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