Imidazole canonical structures

Scheme 12.20 Formation of 1-acetylimidazole from imidazole and its canonical structures.

In the anionic form of xanthine (as a sodium salt) no hydrogen appears at N-3 or N-7 and the data has been interpreted by assignment of the canonical structures (Scheme 1) (69BCJ3099). It is interesting to note that in this case all the forms are assigned as 9H tautomers, the imidazole proton in solution usually being assigned to N-7. 

Oxazolones are simply cyclic anhydrides of A-acyl-a-amino acids, and are constructed in the way that this implies. If the nitrogen also carries an aUcyl group, cyclisation " can only lead to an overall neutral product by adopting a zwitterionic stracture, for which no neutral canonical form can be written - a mesoionic structure. Mesoionic oxazolones (named miinchnones by Huisgen after their discovery at the University of Munchen, Germany) undergo ready dipolar cycloadditions,with loss of carbon dioxide from the initial adduct the examples" show the conversion of a miinchnone into a mesoionic thiazolone and into an imidazole. 

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