Tautomerism of heterocycles: five-membered rings with one

Friedrichsen, W., Traulsen, T., Elguero, J., Katritzky, A. R., Tautomerism of Heterocycles Five-Membered Rings with One Heteroatom, 76, 85. 

Heterocyclic compounds are ubiquitous in biology, medicine, and biochemistry, and the azoles form an important class of nitrogen heterocyclic compounds with five-membered aromatic rings that can contain one to five nitrogen atoms (other heteroatoms can also be present). Many drugs and pharmaceuticals are derived from azole structures. The structures of representative azoles are shown in Figure 6.1. Note that some of these compounds exist in tautomeric equilibrium in solution. 

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