Inverse Electron-Demand Aza-Diels-Alder Reaction of α,β-Unsaturated Thioesters with In Situ-Generated 1,2-Diaza-1,3-dienes for the Synthesis of 1,3,4-Thiadiazines.

A highly regioselective inverse electron-demand aza-Diels-Alder reaction of α,β -unsaturated thioesters with 1,2-diaza-1,3-dienes generated in situ from α-halogeno hydrazones was developed. With α,β-unsaturated thioesters as C═S dienophiles, the developed protocol enables the formation of diverse 3,6-dihydro-2 H -1,3,4-thiadiazine derivatives in excellent yields. In the presence of lithium aluminum hydride, 3,6-dihydro-2 H -1,3,4-thiadiazine derivatives could be further transformed into 5,6-dihydro-4 H -1,3,4-thiadiazines in good yields.