Aromatic Systems with Exocyclic Conjugation

Of the mesoionic systems, (40) and its aza derivatives (43), (44) and (49) have been designated as Class B by Ollis 76AHC(19)1), including compounds with X = CRR there 

Triazolium, Oxadiazolium, Thiadiazolium, Dioxazolium, Oxathiazolium, Dithiazolium  

Tetrazolium, Oxatriazolium, Thiatriazolium, Dioxadiazolium, Oxathiadiazolium, Dithiadiazolium  

For unsubstituted aromatic hydrocarbons all the carbon atoms are assigned the same Coulomb integral (a) and all C—C bonds are assigned the same resonance integral fi). 

The resonance integral of the vr-bond between the heteroatom and carbon is another possible parameter in the treatment of heteroatomic molecules. However, for nitrogen compounds more detailed calculations have suggested that this resonance integral is similar to that for a C—C bond and moreover the relative values of the reactivity indices at different positions are not very sensitive to change in this parameter. 

Structure of Five-membered Rings with Two or More Heteroatoms 

Important compounds of three classes result when a positively charged ring carries a negatively charged substituent. 

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Li Aromatic systems withoUt exocyclic conjugation Aromatic systems with exocyclic conjugation... 

Applying the same analysis to pyrimidines (3 and 6) leads to pyrimidones, examples of which are the pyrimidine bases in DNA and RNA. Thus deoxycytidine 46 and deoxythymidine 47 are two of the four 2 -deoxyribonucleosides that are the building blocks of DNA and uridine 48 is one of the four nucleoside building blocks of RNA. As for pyridones, the contribution of dipolar resonance hybrids to pyrimidones and other systems with exocyclic conjugation often helps to understand their properties, including their aromatic character. 

Lithiation with subsequent alkylation results in alkylation of the exocycUc phosphorus atom, and formation of an aromatic system in the central heterocycle analogous to a cyclopentadienide. Spectroscopically, the effect is a severing of the exocyclic phosphorus atom from the conjugated ring system, accompanied by a significant upheld shift of the phosphorus resonance by over AS= -300 ppm, dependent on the alkyl substituent. 

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