3-Arylcyclohepta [d] isoxazolylium tetrafluoroborates were synthesized by the cycloaddition of tricarbonyl (2-5-η-2, 4, 6-cycloheptatrienone) iron with aromatic nitrile oxides followed by several reactions. The pK R+ values of these ions are less than+ 1.0 showing that the stability of the tropylium ions is much decreased by the fusion of the isoxazole ring.
![3-(4-methoxyphenyl)cyclohepta[d][1,2]oxazol-4-one Structure](https://image.chemsrc.com/caspic/086/109072-32-4.png)