Oxazoles, conversion into imidazoles

It has been known for many years that treatment of oxazoles with ammonia (or amines) results in their conversion into imidazoles.133-134 Similarly, 4-hydroxymethylene-5-oxazolones (28) yield (decarboxy-lated) imidazoles under the same conditions.135... 

The oxazole (232) heated with formamide gave the imidazole (233) oxazolium cations undergo similar conversions. Primary amines convert oxazole-4-carboxylic acids (234) at 150°C into imidazoles (235) with accompanying decarboxylation (53CB88) (see CHEC 4.07 and 4.18). 

It has long been known that the conversion of oxazoles into imidazoles may be accomplished by treatment at high temperatures with ammonia or amines.132 160 379 Oxazole-4-carboxylic acids are converted similarly into the corresponding decarboxylated imidazoles (228).302... 

C==0, C=N, C=S, C=C, and N=N containing substrates. Thus oxazoles imidazoles, thiazoles, pyrroles, and 1,2,4-triazoles have been prepared, respectively. Furthermore, jj-tolylsulfonylmethyl isocyanide has found use in a one-step conversion of ketones into cyanides and in a two-step synthesis of a-hydroxyaldehydes from ketones. ... 

Such synthetic approaches can only be valid if the other heterocycles are readily available, or if their transformations lead to imidazoles difficult to make by other means. It is certainly important to be able to aromatize imidazolines since a number of ring-synthetic procedures lead to reduced imidazoles. 4-Aminoisoxazoles are sources of a-acylaminoenaminones which cyciize with bases to give 4-acylimidazoles. Oxazole-imidazole conversion has largely historical importance, but it is also implicated in some ring-synthetic procedures (e.g. the Bredereck method, see Chapter 5). Transformations of benzofuroxans into 2-substituted benzimidazole iV-oxides have some synthetic importance. Few, if any, ring contractions appear to have major application. 

There are general reviews on heterocyclic syntheses by cycloaddition reactions of isocyanates and on the use of heterocyclic cations in preparative organic chemistry. More specific topics are 5-hydroxymethylfuran-2-carb-aldehyde, isobenzofurans and related ort/io-quinonoid systems, the conversion of 2//-cyclohepta[Zj] furan-2-one (1) into derivatives of azulene, the synthesis of indoles from o-alkylphenyl isocyanides, and abnormal Fischer indolization reactions of o-methoxyphenylhydrazones. Two reviews on isoindoles have appeared and a lecture on highly conducting charge-transfer complexes that are based on heterocyclic selenium and tellurium donors has been reprinted.Recent advances in the chemistry of imidazole and in the use of nitro-imidazoles as radiosensitizers have been summarized. There have been reviews on benzimidazole A -oxides and on dihydrobenzimidazoles, benzimidazolones, benzimidazolethiones, and related compounds. Other topics are synthetic applications of 1,3-dithiolium and 1,3-oxathiolium salts and of isoxazoles, the chemistry of benzisoxazoles, 2-amino-oxazoles, 5-oxazolones (2), furoxans, benzofuroxans, and related systems, the synthesis of five-membered meso-ionic compounds, and tetrazoles. ... 

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