The perchlorates of 7-tropylionorbornane, 2-tropyliobicyclo[2.2.2]octan be, and 2-tropylioadamantane were found to undergo stereomutation in acetonitrile at 25-85°C, as evidenced by following the change in the 13 C NMR spectra of their deuterium-labeled compounds is acetonitrile-d 3 . Addition of tetracyanoethylene to their solutions in acetonitrile gave the [8+2] cycloaddition products of the corresponding heptafulvenes, indicating the intermediacy of the heptafulvenes. This stereomutation was too slow to detect in dichloromethane at the same temperature. Therefore, it is concluded that the stereomutation proceeds through the abstraction of the α-hydrogen by acetonitrile as a base followed by the protonation of the heptafulvene intermediate from the rear side