Oxatriazolium-5-olates

Some mesoionic heterocycles, structurally correlated to the sydnones and represented by a general 5-membered ring molecule that contains the NO-moiety, behave as NO-donors. They include sydnonimines 81 and mesoionic 1,2,3,4-oxatriazolium derivatives 82 and 83. The latter will be named 3-R-l,2,3,4-oxatriazolium-5-olates (82) and S-R-l S -oxatriazolium-S-Rj-amenates (83) [77]. 

Scheme 6.20 Synthetic routes to mesoionic 1,2,3,4-oxatriazolium-5-olates.

As for sydnonimines, the ability of mesoionic derivatives 82 and 83 to produce NO is strictly connected to their capacity to decompose to nitrosohydrazine intermediates. Mesoionic l,2,3,4-oxatriazolium-5-olates represent a thermally stable class of compounds, but they undergo a relatively facile nucleophilic ring-opening reac-... 

Oxatriazole derivatives are usually prepared by the cyclization of A-nitrosohydrazines as illustrated by the synthesis of 3-alkyl-1,2,3,4-oxatriazolium-5-olates 764 from semicarbazides via nitrosation (Scheme 330) . 

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