Efficient Synthesis of New Phosphono-Substituted Dihydrothiopyrans via Hetero Diels–Alder Reaction, under Thermal and High Pressure Conditions

New α-phosphono-β-aryl- or β-heteroaryl-substituted α,β-unsaturated dithioesters 2 were easily prepared from diethyl phosphonodithioacetate 1 and used as thiadienes in thermal or high pressure hetero Diels–Alder cycloadditions with enol and thioenol ethers. The resulting new phosphono 3,4-dihydro 2H-thiopyrans 3 were isolated in excellent yields and with a cis- or trans-diastereoselectivity depending on the conditions of the reaction as well as the structure of the reagents. Some of the thiopyrans 3 were also favourably synthesized via a domino Knoevenagel–hetero Diels–Alder sequence.