Approaches to the synthesis of compounds containing fused mesoionic rings

[1,2,3]Oxadiazolo[4,3-c][1,2,4]benzotriazinium-3-olate has been prepared by diazotization of 3-(2-aminophenyl)-1,2,3-oxadiazolium-5-olate [3-(2-aminophenyl)sydnone]. The tricyclic compound has been converted by hydrolysis, thermolysis, and reduction with sodium borohydride into 1,2,4-benzotriazine-3-carboxylic acid. It underwent alkylation with Meerwein's reagent but did not react with 1,3-dipolarophiles even under forcing conditions. Unsuccessful attempts were made to prepare azidoaryl derivatives of mesoionic triazoles, thiazoles, and tetrazoles. 3-(2-Azidocarbonylphenyl)-1,2,3- oxadiazolium-5-olate is converted photochemically into a mesoionic [1,2,3]oxadiazolo[3,4-a]quinoxaline derivative and 3-(2-carbamoylphenyl)-1,2,3- oxadiazolium-5-olate. The pyrolysis of mesoionic tetrazoles is described and aspects of the chemistry of 1,3-diphenyltetrazolium-5-thiolate are briefly discussed. A novel carbonylation reaction of 2,3-diphenyltetrazolium-5-thiolate (dehydrodithizone) is reported, as are complexes of this compound formed by treatment with dipotassium tetrachloropalladate.