Mechanism and Regioselectivity of the Cycloaddition of Thiones Derived from Meldrum’s Acid, Malonates, or Other Dicarbonyls

Several α,α-dioxothiones were generated in situ and reacted with 1,3-dienes of varying electronic and steric properties. It was found that thiones 10a and 11a reacted well with electron-rich or electron-poor dienes and are complementary in their regioselectivities when steric effects are at play. The calculated preferred mechanistic pathway implies a thiiranium zwitterion intermediate.