sydnones are mesoionic heterocyclic chemical compounds possessing a 1,2,3-oxadiazole core with a keto group in the 5 position. like other mesoionic compounds they are di-polar, possessing both positive and negative charges which are delocalized across the ring. recent computational studies have indicated that sydnones and other similar mesoionic compounds are nonaromatic, "though well-stabilized in two separate regions by electron and charge delocalization." sydnones are a heterocyclic compound named after the city of sydney, australia.