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 highpressure 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