Studies on the oxidation of 1, 3-dithiane and 5, 5-disubstituted analogues including X-ray crystal structure, equilibration studies and pK a measurements on selected oxides

Oxidation of 1,3-dithiane has been carried out using eight different oxidants under several different reaction conditions. The most favourable conditions for obtaining the required trans-1,3-dithiane 1,3-dioxide were to use either MCPBA in Et2O or NaIO4 in MeOH–H2O. Other 5,5-disubstituted 1,3-dithianes were also prepared and subjected to similar oxidations. For these compounds the best oxidant for obtaining the trans-dioxides was O3. A rationalisation for the stereochemical outcome of the oxidations is presented and requires that trans-1,3-dithiane 1,3-dioxide is more stable than cis-1,3-dithiane 1,3-dioxide. This is borne out by equilibration studies of the two diastereoisomeric sulfoxides using N2O4. A rationalisation for the greater stability of the trans compared to the cis isomer is also presented and fits with the pKa values and melting points of the two compounds.