Discovering new plant growth regulators: 4,5-annelated bicyclic sydnone imines - a new type of mesoionic compounds

4,5-Annelated bicyclic derivatives of sydnone imines (6-cyano-1-substituted-[1,2,3]oxadiazolo[5,4-b]pyridin-1-ium-5-yl)amide hydrochlorides was prepared for the first time. In these bicyclic compounds mesomeric charge spreads out over the two fused aromatic fragments and they can be considered as a new type of mesoionic heterocyclic compounds. The molecular structures of the bicyclic derivatives was investigated in detail by X-ray and 1H, 13C, and 15N NMR studies. Biologycal studies showed promising activity of sydnone imines as phytoeffectors.