Fused-Ring Five-Membered Heterocycles Indoles and Purines

These ring systems occur in nature. For example, the imidazole skeleton is present in the amino acid histidine, where it plays an important role in the reactions of many enzymes. Decarboxylation of histidine gives histamine, a toxic substance present in combination with proteins in body tissues. It is released as a consequence of allergic hypersensitivity or inflammation (for example, in hay fever sufferers). Many antihistamines, compounds that counteract the effects of histamine, have been developed. One of the better known of these is the drug benadryl (diphenylhydramine). 

Antihistamines are compounds that counteract the effects of the toxin histamine, which contains an imidazoie ring. 

The thiazole ring occurs in thiamin (vitamin Bi), a coenzyme required for certain metabolic processes and hence essential to life (thiamin also contains a pyrimidine ring). In its reduced form, the tetrahydrothiazole ring appears in penicillins, which are important antibiotics. 

Fused-Ring Five-Membered Heterocycles Indoles and Purines 

Another aromatic or heteroaromatic ring can be fused to the double bonds of five-membered heterocycles. For example, indole has a benzene ring fused to the C-2-C-3 bond of pyrrole. 

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