Heterocyclic peripheral numbering

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 confusion that can arise when the formula of a relatively simple fused heterocycle is wrongly drawn may be illustrated with a tricyclic compound which may be drawn in several ways, some of which are shown in (1,8) to (1.12). None of these is in accord with lUPAC recommendations (1.13) is the correct formula and is the one that should be used. Peripheral numbering of this compound is shown and its name is pyrazinol2, 3 4.5]thieno[3,2-[Pg.21]

Conclusive peripheral numbering of fused heterocyclic systems must first of all pay attention to the generalized orientation rules detailed for corresponding carbocyclics (p. 22). If these are not sufficient, that orientation is chosen which leads to lowest locants by evaluating the following criteria one after another ... 

As structural criteria of aromaticity, indexes N= + b (where R are bond lengths, and a and b are parameters characteristic for a given pair of atoms) and Julg s parameter J = 1 — 225/ S,s(l —d Jdf (where n is the number of peripheral bonds rs, are their lengths, and d is their mean length) have been applied. The less the value SaA and closer to 1 the value J, the greater is the aromaticity of a heterocycle. Polarizability exaltations F = M — M , where M and M arc the mean dipole polarizability and the mean atomic or group polarizability, respectively. 

Calculated F s in several cases are in accord with the experimental trends. Quinodimethanes are more reactive than benzo[c]heterocycles and the reactivity of the linear polycyclic hydrocarbons increases with the number of rings. The changes in the NICS values for the rings is consistent with the changing aromaticity. In the case of polycyclic hydrocarbons, the aromaticity in the peripheral rings increases. The aromaticity of the center ring is transformed to the aromaticity of the TS and then diminishes as the reaction is completed. ... 

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