Homoenolization of 1,3,3-trimethylbicyclo[2.2.2]octane-2,5-dione. Unusually rapid HD exchange of a bridgehead methyl

The title compound 2a undergoes unusually rapid H–D exchange at the bridgehead methyl at a rate 5300 times faster than exchange of the bridgehead methyl of 1,3,3-trimethylbicyclo[2.2.1]heptan-2-one (fenchone). From the isotopic composition of partially deuterated 2a, we deduce that exchange occurs via an isomer, in equilibrium with 2a, which contains an active methylene derived from the bridgehead methyl.