Ring-chain tautomerism 1.2.3- oxadiazole

The 1,2,5-oxadiazole ring is a stable system and annular-group tautomerism is not favored. Although three tautomeric forms can be drawn for 3-hydroxyfurazans (Scheme 3) IR and NMR data for chloroform solutions show only the presence of the hydroxy compound. Ring-chain tautomerism is an important feature of furoxan chemistry and the equilibration between the isomeric furoxans is discussed in detail later in this chapter (Section 4.05.5.2.1). 

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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