Oxazoles benzils

One of the first recognized preparations of an oxazole was that of 2,4,5-triphenyloxazole by the action of ammonia on benzil.141 The reaction is very complex and leads to a variety of products (including 2,4,5-tri-phenylimidazole) depending on the temperature, time, and solvent. A number of mechanisms to explain the mode of formation of these products have already been critically discussed in previous reviews.1-4 In the latest study by Wenkert and Mekler,142 the reaction between benzil and ammonia is interpreted as involving the ionic cleavage of a benzil-ammonia complex containing two benzil units. 

Diarylquinoxalines are similarly prepared by reaction of o-phenylenediamine with benzils. The required benzils have been prepared traditionally by oxidation of the corresponding benzoins, which in turn have been prepared by treatment of the appropriate aldehydes with potassium cyanide. In a more recent procedure, benzils have been obtained in good yield by oxidation of para-substituted diphenyloxazoles with bromine in acetic acid. The oxazoles themselves are readily prepared by cyclization of the benzoin acetate with urea (Scheme 3). ... 

A soln. of benzil monoxime, benzaldehyde, and NH4-acetate in glacial acetic acid refluxed 1 hr. 2,4,5-triphenylimidazole 3-oxide. Y 85.5%. F. e. s. H. Lettau, Z. Chem. 10, 338 (1970) 1-subst. analogs with amines instead of NH4-acetate s. ibid. 10, 431 isolation of 2-imidazolones from formaldehyde s. ibid. 10, 462 oxazole N-oxides in the absence of NH4-acetate or amines cf. Y. Goto, M. Yama-zaki, and M. Hamana, Chem. Pharm. Bull. 19, 2050 (1971). 

Scheme 8.70 Synthesis of polyarylated oxazoles from benzylamine and benzil derivatives.

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