3- Pyrrolidino-1,2,4-triazine 4-oxide, reaction

Another pathway for the aromatization of the cr -adducts was found in the reactions of 3-pyrrolidino-l,2,4-triazine 4-oxide 81 with amines. Thus the treatment of 1,2,4-triazine 4-oxide 81 with ammonia leads to 5-amino-1,2,4-triazine 4-oxides 54—products of the telesubstitution reaction. In this case the cr -adduct 82 formed by the addition of ammonia at position 5 of the heterocycle undergoes a [l,5]sigmatropic shift resulting in 3,4-dihydro-1,2,4-triazine 83, which loses a molecule of pyrrolidine to yield the product 54. This mechanism was supported by the isolation of the key intermediates for the first time in such reactions—the products of the sigmatropic shift in the open-chain tautomeric form of tiiazahexa-triene 84. The structure of the latter was established by NMR spectroscopy and X-ray analysis. In spite of its open-chain character, 84 can be easily aromatized by refluxing in ethanol to form the same product 54 (99TL6099). 

Interestingly, the reaction of 3-pyrrolidino-l, 2,4-triazine 4-oxide with ammonia has been established to give 5-amino-l,2,4-triazine 4-oxide, due to the te/e-substitution process with elimination of pyrrolidine (Scheme 25) [51 ]. As for the mechanism of this amino-dehydrogenation reaction, a sigmatropic shift of hydrogen has been postulated, substantiated by registration of the key intermediates. 

Scheme 25 te/e-substitution of hydrogen observed in the reaction of 3-pyrrolidino-1,2,4-triazine 4-oxide with NH3... 

---