Fragmentation and isomerization reactions of phenyl substituted 2-oxo-1,3,2-dioxathianes (trimethylene sulfites)

The preparation and Grob-like fragmentation of several phenyl substituted 2-oxo-1,3,2-dioxathianes (trimethylene sulfites) have been examined. Preparation of the 4,4,6-triphenyl derivatives gives a chair S = O equatorial isomer which fragments readily in polar solvents to produce benzaldehyde and 1,1-diphenylethylene. Isomerization to an unreactive twist boat isomer occurs as a minor reaction in solution and in the solid state on prolonged storage. Activation parameters and substituent effects on the fragmentation reaction are consistent with an ionic mechanism. The diphenyl derivatives react only in the presence of boron trifluoride etherate giving mainly isomerization to the stable chair, S = O axial form; fragmentation is a minor pathway.