Unexpected products via singlet oxygen oxygenation of functionalized 5,6-dihydro-1,4-oxathiins

[formula: see text] Single oxygen oxygenation of 5,6-dihydro-1,4-oxathiins substituted at C-3 with an electron-withdrawing group leads stereoselectively to ketosulfoxides 5 and 6, instead of the expected dicarbonyl compounds 3. A mechanism involving an unprecedented intramolecular rearrangement of the corresponding dioxetanes 2 is proposed.