1.2- Disubstituted 4-acylimidazoles

Catalytic reduction of iminoethers derived from 4-aminoisoxazoles (85TL3423, 87JOC2714) or 4-amino-5(477)-isoxazolones (91S127) leads to a-(acylamino)enaminones (231) which cyclize in the presence of bases to form 4-acylimidazoles (Scheme 34). Treatment of the enaminone with a primary amine incorporates a substituted amino function in the (3-position with concomitant expulsion of ammonia and allows access to 1-substituted and 1,2-disubstituted 4-acylimidazoles in 75-92% yields (87JOC2714). 

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). 

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