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

Abstract

1-Isocyanato-1-(phenylazo)cycloalkanes 3 react with tetrafluoroboric acid to yield 3-spiro substituted 4,5-dihydro-5-oxo-1-phenyl-3H-1,2,4-triazolium tetrafluoroborates 4. Compounds 4 rearrange with ring expansion in good yield to give, after basic workup, 1,5-annulated 1,2-dihydro-2-phenyl-3H-1,2, 4-triazol-3-ones 5.