Sydnones are mesoionic heterocyclic chemical compounds possessing a C5-oxygenated 1,2,3-oxadiazole core,[1][2][3] named after the city of Sydney, Australia.

Like other mesoionic compounds they are dipolar, possessing both positive and negative charges which are delocalized across the ring.

N-phenylsydnone was first prepared in 1935 by John Campbell Earl [de] and Alan W. Mackney [Wikidata] by cyclodehydration of N-Nitroso-N-phenylglycine with acetic anhydride.

[4] Later work showed that this could be applied fairly generally to the nitrosamines of N-substituted amino acids.

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.