New Insight into the Reaction of Singlet Oxygen with Sulfur-Containing Cyclic Alkenes: Dye-Sensitized Photooxygenation of 5,6-Dihydro-1,4-dithiins

The reaction of 3-methyl-5,6-dihydro-1,4-dithiins with singlet oxygen affords dicarbonyl compounds
and/or ring-contracted ketosulfoxides, the latter regio- and stereoselectively, depending on the nature of
the substituent at C-2 and on the reaction conditions. In competition with normal fragmentation, the
intermediate dioxetanes, derived from [2 + 2] cycloaddition of singlet oxygen to the double bond, undergo
an intramolecular oxygen transfer to the sulfur-1 atom, leading to labile epoxide intermediates. The latter
convert to cis- and trans-ketosulfoxides through a non-concerted S-4 migration. This pathway is promoted
by the electron-withdrawing group at C-2 and, for monosubstituted amide, by the solvent basicity.
S-Oxidation of dithiins is insignificant, except for the monosubstituted amide derivative or in the presence
of protic species, and occurs selectively at the S-1 atom.