Hetero-Diels–Alder reactions of hetaryl thiochalcones with acetylenic dienophiles

Hetaryl-substituted thiochalcones react with acetylenic mono- and diesters in the THF solution in the presence of LiClO4 at 65°C to give, after 24 h, 4H-thiopyran carboxylates and dicarboxylates, respectively, in moderate to good yields. The same reactions were performed also in the THF solution without a catalyst under microwave irradiation. In that case, the reaction time was reduced to three minutes and, in most cases, an improvement in the yield of the [4+2]-cycloadduct was observed. The reactions with methyl propiolate occurred regioselectively and the 3-carboxylates were formed exclusively.