Diazol-4-ones

Anhydro-acylation of JV-(JV-phenylbenzimidoyl)amino acids (90) and the corresponding nitriles yield the meso-ionic 5-acyl-l,3-diazol-4-ones (91, R4 = COR) derivatives (92) are analogously formed from N-2-pyridylglycines.28,5s The meso-ionic compounds 91 and 92 are remarkably stable they are unreactive toward alkalis, acids, and amines. 

Polycyclic meso-ionic l,3-diazol-4-ones (93) are also known. These include Besthorn s Red (93a) which was first described in 1904. The discovery of Besthorn s Reid and its subsequent formulation (93a) forms an interesting part of the history of meso-ionic compounds.2 9  

Recently, 8b an analogous meso-ionic l,3-diazol-4-one (93b) was unexpectedly encountered as the product of the reaction between 1-azaphenothiazine and chloroacetic anhydride. 

The triphenyl derivative (91, R = R3 = R4 = Ph, R2 = H) is formed390 in a mechanistically interesting reaction between benzoyl formic acid anil (Ph-N=CPh-COzH), trifluoroacetic anhydride, and pyridine. Its 1,3-dipolar cycloaddition reactions with alkynes and alkenes have been reported.59  

Two synthetic routes59 to meso-ionic l,3-diazol-4-ones (91) have recently been reported. 1,3-Cycloaddition39a of phenyl isocyanate to the meso-ionic l,3-oxazol-5-one (94) yields the intermediate (95 or 96) from which carbon dioxide was extruded in boiling xylene, giving the meso-ionic l,3-diazol-4-one (97). 

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Meso-ionic l,2-diazol-4-ones (373) were possibly first prepared in 1900 by Wolff and Fertig compounds of this ty (373) were again... 

A new method of preparing meso-ionic l,3-diazol-4-ones (91) in situ has been described. Condensation of a-haloacyl chlorides (R CHX.COCl) with benzamidines (R. C=NR NHR ) in benzene solution gives a-haloacylamidines (485) which in the presence of triethyl-amine cyclize to the meso-ionic l,3-diazol-4-ones (91). Using this procedure the l,3-diazol-4-ones (91) were not isolated but evidence for their formation was provided by (f) the development of a yellow coloration and (ii) the formation of pyrroles in good yield by 1,3-dipolar cycloaddition of alkynes." ... 

A general route39b to meso-ionic l,3-diazol-4-ones (91) involves the reaction of amino amides (R NHCOCH2NHR3) either with triethyl orthoformate or with JV-phenylbenzimidoyl chloride. 

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