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

This chapter discusses the five-membered heterocyclic compounds with four hetero-atoms in the ring. The compounds of tetrazolesmay be referred to one or other of the tautomeric 1H- or 2H- structures. Tetrazoles require the addition of azides to nitriles. 5-Alkoxy- and 5-aryloxy-tetrazoles are found from the interaction of cyanic esters and azide ion. Tetrazole-5-aldehyde is prepared by the action of hydrazoic acid upon diethoxyacetonitrile. Sulfur containing mesoionic tetrazoles are also known, of which the most commonly encountered is 2,3-diphenyl 2-H-tetrazolium-5-thiolate, often called dehydrodithizone. Nitrosation of the arylhydrazinium dithiocarbamates affords 1,2,3,4-oxatriazolium-5-thiolates, which in the presence of ammonia and ethanol rearrange to 1,2,3,4-thiatriazolium-5-olates. 1,2,3,5-Oxatriazolium salts are synthesized by the diazotization of 2-aminopyridine-l-oxide. 1,2,3,4-Thiatriazoles are compounds with a long and sometimes controversial existence. The usual synthesis of 5-substituted 1,2,3,4-thiatriazoles involves the reaction of thioacylhydrazines with nitrous acid. Thioacylazides are converted directly into 5-alkyl derivatives. In turn the azides are formed as intermediates in the reaction of 1-acetylthio-1-alkynes and ammonium azide. Such compounds are rare, but representative structures can be prepared by addition reactions of nitrile oxides and suitable reagents. 3H-1,2,3,5-oxathiadiazoles are formed and 1,3,2,4-dioxathiazole-2,2-dioxides are produced when sulfur trioxide is used.