Mononuclear Rearrangement of the Z-Phenylhydrazones of Some 3-Acyl-1,2,4-oxadiazoles: Effect of Substituents on the Nucleophilic Character of the >c=N-NH-C 6 H 5 Chain and on the Charge Density of N-2 of the 1,2,4-Oxadiazole Ring (Electrophilic Counterpart)