Copper-Mediated One-Pot Access to 2,3-Dihydrobenzo[b][1,4]oxathiines from o,o-Dihydroxydisulfides

o,o-Dihydroxydisulfides can be transformed to the corresponding o-thioquinones using a substoichiometric amount of Cu(II) salts to promote a thiolate anion 1,4-elimination at sulfur. o- Thioquinones are trapped in situ as dienes in an inverse electron-demand hetero Diels–Alder reaction with electron-rich alkenes acting as dienophiles to give, one-pot, 2,3-dihydrobenzo[b][1,4]oxathiine cycloadducts.