Synthesis of 3,6-Dihydro-2<i>H</i>-thiopyrans from α-Diazo-β-diketones and Vinylthiiranes via [5 + 1] Annulation

Functionalized 3,6-dihydro-2<i>H</i>-thiopyrans are synthesized in moderate to good yields from α-diazo-β-diketones and vinylthiiranes under Cu(hfacac)₂ catalysis and microwave irradiation via [5 + 1] annulation. The reaction mechanism includes a tandem sequence of the Cu-carbenoid generation from α-diazo-β-diketones, nucleophilic attack of vinylthiiranes to the carbenoids, and subsequent [2,3]-sigmatropic rearrangement. The process is a carbene-induced ring expansion of vinylthiiranes.