Diastereo- and regioselective Diels–Alder reactions of 2-phosphaindolizines

1,3-Bis(alkoxycarbonyl)-2-phosphaindolizines undergo Diels–Alder reactions at the 2 bonds on the lefthand sideCdouble bond; length as m-dashP– functionality with 2,3-dimethylbutadiene and with isoprene in the presence of sulfur with complete diastereoselectivity. The reaction with isoprene occurs with 100% regioselectivity as well. 3-Ethoxycarbonyl-1-methyl-2-phosphaindolizine, however, fails to undergo Diels–Alder reaction under these conditions. Difference in the dienophilic reactivities of mono- and bis(alkoxycarbonyl) substituted 2-phosphaindolizines and the observed regioselectivity in the Diels–Alder reaction has been rationalized on the basis of DFT calculations. The relative stabilities of the transition structures have been explained on the basis of the NBO analysis.