Structure of Six-membered Rings

1 Aromatic Nitrogen Systems without Exocyclic Conjugation 

Since N+ and C are isoelectronic, the simplest and most direct hetero-analogue of benzene (1) is the pyridinium ion (2). Further azonia substitution of this kind gives the disubstituted species (3)—(5). 

Synthetic derivatives (e.g. veronal) of barbituric acid are hypnotics. 

Azonia substitution at a naphthalene bridgehead position gives the quinolizinium ion (14). The two best known monoaza systems with three aromatic fused rings are acridine (15), derived structurally from anthracene, and phenanthridine (16), an azaphenanthrene. The better-known diaza systems include phenazine (17) and 1,10-phenanthroline (18), while systems with three linearly fused pyridine rings are called anthyridines, e.g. the 1,9, 10-isomer (19). 

Although most ring systems are numbered according to a fairly straightforward set of rules (see CHEC 1.02), there are several exceptions. Thus numbering in purine molecule follows historical tradition. Acridine now has its meso positions numbered 9 and 10 (see structure 15). (At least two other numbering systems have been widely used in the past.) Phenazine, however, is numbered systematically, as in structure (17). Phenanthridine (16) is now numbered in the systematic fashion as indicated. 

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Structure of Six-membered Rings Table 10 UV Absorption Bands of the Simple Monocyclic Azinesa... 

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