Electrocyclic Ring-Opening of 1,2,4-Oxadiazole[4,5-a]piridinium Chloride: a New Route to 1,2,4-Oxadiazole Dienamino Compounds.

1,2,4-Oxadiazole[4,5-a]piridinium chloride adds nucleophiles to undergo electrocyclic ring opening affording 1,2,4-oxadiazole dienamino derivatives. These pyridinium salts represent a special class of Zincke salts that are prone to rearrange when treated with primary amines or in the presence of bicarbonate to give the pyridones. The pivotal tuning of the experimental conditions leads to a straightforward synthesis of valuable 1,2,4-oxadiazole dienamine derivatives. The mechanism is also discussed in the light of NMR experiments and theoretical calculations.