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.