Rearrangement of 3,3-Disubstituted 1-Aryl-4,5-dihydro-5-oxo-3<i>H</i>-1,2,4-triazolium Tetrafluoroborates; Part 3. An Efficient Synthesis of 1,5-Heteroannulated 1,2-Dihydro-2-phenyl-3<i>H</i>-1,2,4-triazol-3-ones

Abstract

1-Isocyanato-1-(phenylazo)heterocycloalkanes 3a-f react with tetrafluoroboric acid to yield the respective 3-spiro-substituted 4,5-dihydro-5-oxo-1-phenyl-3H-1,2,4-triazolium tetrafluoroborates 4a-f. These compounds rearrange under ring enlargement in good yields to 1,5-heteroannulated 1,2-dihydro-2-phenyl-3H-1, 2,4-triazol-3-ones 5a-f. In two special cases, 4,5-heteroannulated 2,4-dihydro-2-phenyl-3H-1,2,4-triazol-3-ones 7g,h are obtained.