4-Pyrones mesomeric structures

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

The ozonolysis of 2,6-dimethylthio-y-pyrone yields mesoxal dialdehyde (0.12 mole), glyoxylic acid (0.04 mole), methylglyoxal (0.02 mole), acetic acid (0.53 mole), and formic acid (0.82 mole). These scission products have been formed from mesomeric structures ... 

In contrast, 2- and 4-pyrones are considered to have relatively little aromatic character. Whereas in an analogous nitrogen series 4-pyridone 5.23 has significant aromatic character (mesomeric representation 5.23a making a considerable contribution to the overall electronic distribution), aromatic mesomeric representation 9.3a makes less of a contribution to the overall electronic structure of 4-pyrone. As with furan, the higher electronegativity of oxygen leads to heterocycles of little aromaticity in cases where delocalisation of electron density from the heteroatom is a prerequisite for that aromaticity. 

In the ozonolysis of 3-methyl-y-pyrone and of tetramethyl-y-pyrone, scission products are formed which are characteristic for polar structures. In the ozonolysis of 2,3,6-dimethyl-y-pyrone particularly interesting results were obtained. The following characteristic scission products have been isolated pyruvic acid (0.11 mole), dimethylglyoxal (0.07 mole), methylglyoxal (0.08 mole), glyoxylic acid (0.11 mole), and acetic acid (1.85 moles). This means that the trimethyl-y-pyrone reacts according to mesomeric polar structures. 

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