Preparation of 3,5-disubstituted 1,2,4-oxadiazoles by reactions of nitrilium salts with amide oximes

The reaction of N-alkyl- and N-arylnitrilium salts 1a,d,g,j with the amide oximes 2a-c furnished the new iminium salts 3a-k (76-93\%). On heating, compounds 3a-c cyclized to salts, which were transformed with base to the 3,5-disubstituted 1,2,4-oxadiazoles 4a-c in good yields (ca. 80\%).