1.2.4- Oxadiazoles, 3-benzoyl, formation

Base-catalyzed isomerization (called mononuclear heterocyclic rearrangement ) was described for the formation of Z-phenylhydrazone of 3-benzoyl-l,2,4-oxadiazole 156 into 4-benzoylamino-1,2,3-triazole 157242,243 (equation 61). 

The reaction of imidazole with benzonitrile oxide (in excess) gives little (Z)-l-benzoylimidazole oxime (70) besides (Z)-3-benzoyl-2,3-dihydro-l//-imidazol-2-one oxime (73) and benzonitrile (and in addition some side products). In the solid state only the chain form (70) is observed, but the formation of (72) indicates a chain-ring tautomerism in solution. The latter tautomer (71) is supposed to undergo nucleophilic addition to benzonitrile oxide. The resultant 7-benzoyl-3-phenyl-7, 7a-dihydroimidazo [4,5-a][l,2,4]oxadiazole (72) is considered the key intermediate suffering a [3 + 2]... 

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