Although the rate of proton abstraction (kinetic acidity) frequently plays an essential role in determination of reaction pathways and is of theoretical interest,1 it is still controversial whether an oxygen atom activates or deactivates the abstraction of an α-hydrogen atom of an ether. For example, it is well known that oxidative elimination of a seleno group gives an allyl ether as the major product, indicating the oxygen atom deactivates the kinetic acidity.2 Abstraction of the equatorial hydrogen atom at C-2 of 6-methyl-1,3-oxathiane-3,3-dioxide 1 is slower than that at C-4.3 On the other hand, the bridgehead hydrogen atom (Hb) adjacent to the oxygen atom of piperazinedione (2) is abstracted more readily than that of the alternative one (Ha).4