1.2.4- Oxadiazoles forming imidazoles

A rearrangement that does not fall into the pattern described in this section has been found by Cusmano and Ruccia.191 This time a 1,2,4-oxadiazole is the final product formed by nitrosation of an imidazole. What intermediates lie between the nitroso compound (103) and the final product (104) have not been determined, but the reagent is alcoholic hydrochloric acid. Other examples of this type of rearrangement are known191,192 that also resemble the nitrosopyrrole to isoxazole rearrangement.192"... 

Nitrene-like intermediates can lead to CN bond formation and thus to imidazole derivatives. 1,5-Diphenyltetrazole 271 fragments to 272, which is trapped intramolecularly to form 273 (Scheme 152) (also see Section 3.4.1.2.1). Photolysis of the oxadiazole 274 also gives the intermediate 272 and thus 273 (Section 3.4.3.12.2). 

There is one example in which part of the amidine system is a C—N bond in a heterocyclic ring. The enamino ketone condensation products (42) of 3-amino-l,2,4-oxadiazoles and 1,3-dicarbonyl compounds cyclize in basic medium to form 60-80% yields of imidazoles. The driving force for this reaction is provided by the well-established, general attack of a nucleophilic centre in the side-chain at N-2 of the heterocyclic ring, but it is unusual in that a carbon nucleophile (rather than an oxygen or nitrogen species) is implicated (Scheme 23). 

Acylainino-4-acylimidazoles have been made from 3-amino-l,2,4-oxadiazoles and 1,3-dicarbonyl reagents (see Section 2.2.1 and Scheme 2.2.5). 4(5)-Acylimidazoles can be derived from 4-acylaininoisoxazoles (see Section 6.1.2 and Scheme 6.1.3). (See also the discussion in Section 2.2.1 on 4-acylimidazole synthesis.) 5-Acyl-l-arylimidazoles can be made from or-oxoketene-SJV-acetals and nitrosoaromatics (see Section 3.2 and Scheme 3.2.5), and 4-acyl-imidazoles by nitration of 1,3-dicarbonyl compounds in their enolic forms, reduction to iV-alkenylformamides and subsequent cyclization (see Section 3.2 and Scheme 3.2.4). Examples have also been isolated from reactions of 2-oximino-l,2,3-tricarbonyls and amines (see Section 4.1 and Scheme 4.1.7), from compounds such as 3-chloro-4,4-dimethoxy-2-butanone and 3,4-disubstituted 3-buten-2-ones (see Section 4.3 and Scheme 4.3.5), and by ultraviolet irradiation of 1-alkenyltetrazoles which bear an acyl group conjugated with the exocyclic double bond (see Section 6.1.2.3). 

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]... 

A variety of alditol-l-yl substituted heteiocyclic derivatives have been synthesized. The tetiazole derivative (94) was derived riom maltose, and convened into the corresponding 1,3,4-oxadiazoles (95) on acetylation or benzoylation. Reaction of l-arylamino- or 1-alkylamino-l-deoxy-D-arabino-hexuloses with cyanamide gave the imidazole derivatives (96). Fructosazine (97) and deoxyfiructosazine (98) were simultaneously formed by self-condensation of 2-amino-2-deoxy-D-glucose in neutral to basic aqueous solutions, and were shown to have DNA strand breaking activity. Condensation of 2-amino-2-deoxy-D-glucose with aryl isocyanates gave adducts such as (99) that exist in solution as (R,5)-mixtures in which the (R)-fonn predominates. ... 

---