3-Amino-1,2,4-triazol-5-ones, tautomerism

Iminoimidazolin-4-ones 227b are the only tautomers detected for 4-hydroxy-2-aminoimidazoles (R = H, Ph R = H) (91KGS62). Similar tautomeric forms are also characteristic of l-methyJ-2-imino-5-benzylideneim-idazole-4-one 228 (91KGS62) and 3-amino-l,2,4-triazol-5-ones 229 (R = R = H, COMe) (Scheme 78) [76AHC(S1), p. 482]. 

Similarly, cyclization of 3-amino-l, 2,4-triazoles (65) with methyl propio-late or methyl phenylpropiolate gave a mixture of the l,2,4-triazolo[4,3-a]pyrimidin-7-ones 97 and the l,2,4-triazolo[l,5-a]pyrimidin-7-ones 98 (70CB3266 71CB2702). In addition, methyl tram-3-(3-amino-l,2,4-triazol-l-yl)acrylates (99) were also obtained. Production of the 1,2,4-triazolopy-rimidines 97 and 98 started by condensation of the ester function with the amino group of 65, followed by cycloaddition of the triazole N4 or N1 of the two tautomeric intermediates 96a and 96b, respectively, onto the carbon-carbon triple bond of the side chain. In contrast, formation of the triazolyl acrylates 99 took place through addition only of the triazole N1 onto the propiolate carbon-carbon triple bond. The relative amounts of products were found to depend on the reaction conditions (temperature, solvent, and time) (70CB3266) (Scheme 42). 

Several papers have dealt with 1,2,4-triazolo[3,4-fc][ 1,3,4]thiadiazines including synthesis of new derivatives and tautomeric studies <02SC3455 02ZN(B)552 02PS487> and a one-pot synthesis starting from 4-amino-5-substituted-l,2,4-triazole-3-thiones involving a solid acid-induced cyclocondensation <02PS2403>. 

Hydrazidoyl halides react with 2-amino[l,2,4]triazole to give 5-substituted 6-arylhydrazono-6//-imidazo[l,2-i][l,2,4]triazoles (350) and 6-arylhydrazono-5,6-dihydro-4//-imidazo[l,2-/>][ 1,2,4]-triazol-5-ones (351), respectively the latter products are not determined with respect to the tautomeric structures of the H-, 3H-, or 4/7-form (Scheme 32) <83JHC639>. 

Within the huge field of heterocyclic rearrangements [53], the Dimroth rearrangement gives 5-mercapto-l,2,3-triazoles from 5-amino-l,2,3-thiadiazoles and allows interconversion of 5-amino-l,2,3-triazoles into a mixture of isomers under basic condition [54]. Thus, this reaction, discovered in 1909, involves a linear intermediate that can cyclize into two isomeric compounds (Scheme 15) [55]. This ring-chain tautomerization is reversible, but the equilibrium can be pushed toward an end if one of the two isomers is stabilized. 

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