What is "sydnone"

n. (context organic chemistry English) Any of a class of 1,2,3-oxadiazoles having an oxygen atom attached to the 5 position; they are mesoionic

Sydnone is a mesoionic heterocyclic aromatic chemical compound. Recent computational studies have, however, shown Sydnone and other similar mesoionic compounds to be nonaromatic, "though well-stabilized in two separate regions by electron and charge delocalization." A sydnone imine in which the keto group of sydnone (=O) has been replaced with an imino (=NH) group can be found as a substructure in the stimulant drugs feprosidnine and mesocarb.