Five-membered heterocyclic compounds with two nitrogen atoms in the ring

Five-membered aromatic systems with two ring nitrogen atoms are known as diazoles, and the isomer in which these nitrogens are adjacent is pyrazole. Thus, pyrazole can be regarded as a composite of the pyrrole and pyridine structures. In this chapter, methods of preparation, general properties of the ring system, and typical chemical reactions of the pyrazoles are discussed. The various homologs and derivatives of pyrazole are analyzed, such as the pyrazole-2-oxides, N-alkyl and N-aryl derivatives, N-acylpyrazoles, C-alkyl and C-Aryl derivatives, halogenopyrazoles, nitroso- and nitro-pyrazoles, amino and diazo compounds, cyanopyrazoles, aldehydes and ketones, carboxylic acids and derivatives and reduced systems. Indazole, or benzopyrazole, bears the same relation to pyrazole as indole does to pyrrole. Isomeric with pyrazole, the imidazole system carries its two nitrogen atoms in the alternative relationship, with an intervening methine group. The imidazole ring system occurs widely in nature, from the simple amino-acid histidine, to the imidazole alkaloids, such as pilocarpine, the purines and the imidazole nucleosides. The discovery of the 5,6-dimethylbenzimidazole unit in the vitamin B12 structure stimulated interest in the synthetic methods of benzimidazoles; they include systemic fungicides, such as benomyl, several veterinary anthelmintics, and PBI, a highly thermostable polybenzimidazole with specialist textile uses.