Pyramidal Inversion in Cyclic Sulfonium Salts

5- and 6-membered cyclic sulfonium salts carrying methyl substituents on the ring have been synthetized and partially resolved. These salts undergo thermal stereomutation at rates which are 10³ times lower than that of non-cyclic sulfonium cations. Very stringent evidence is presented that racemisation or, where the case may be, epimerisation takes place by pyramidal inversion of the sulfur atom.