Reaction of 3-ethyl-5-aryl-1, 3, 4-oxadiazolium salts (X) with dialkyl acylphosphonates (V) in the presence of triethylamine afforded 2-aryl-4-ethyl-6-alkyl (or aryl)-5, 6-dihydro-4H-1, 3, 4-oxadiazin-5-one derivatives (XII) via an acyclic intermediate (XI) which is a 1 : 1 adduct of X and V. The mechanism of this novel reaction involving ring expansion is discussed briefly.
