Five-Membered Heterocyclic Compounds with Three Hetero-Atoms in the Ring

This chapter discusses the five- membered heterocyclic compounds with three hetero-atoms in the ring and their preparation. Triazoles are one of the heterocyclic compounds. It has two classes: 1,2,3-triazoles and 1,2,4-triazoles. 1,2,3-triazoles are important compounds, which are applicable in the textile industry as optical brighteners and as polymer additives, and in medicine as drug substances. 1,2,3-triazoles have been prepared by a number of distinct routes. 1,2,4-triazoles are alkylated in basic media with alkyl halides to give predominantly 1-alkyl derivatives. Benzotriazoles exist as 1H- or 2H- structures. When unsubstituted at nitrogen the 1H-aromatic isomers are preferred. Four ring systems of oxadiazoles are namely 1,2,3-, 1,2,4-, 1,2,5- and 1,3,4-oxidiazoles. 1, 2, 5-oxadiazoles are known furazans and their benzo derivatives are known as 2,1,3-benzoxadiazoles. Five-membered rings containing one sulfur and two nitrogen atoms are called thiadiazoles. There are four classes of thiadiazoles and in addition there are two types of benzothiadiazoles. The chemistry and preparative methods of selenadiazoles are usually very similar. These compounds are available through the action of selenium dioxide in acetic acid upon semicarbazones.There are only a few references in the chemical literature to dioxazoles. It has six possible representations including tautomers.