Diastereo- and regioselectivity in Diels–Alder reaction of [1,4,2]diazaphospholo[4,5-a]pyridines

[1,4,2]Diazaphospholo[4,5-a]pyridines undergo diastereoselective Diels–Alder reaction at the >CP– functionality with 2,3-dimethylbutadiene and isoprene in the presence of sulfur or selenium. The reaction with isoprene occurs regioselectively. On carrying out the reaction with diene in presence of methyl iodide, the initially formed [2+4] cycloadduct is methylated regioselectively at the σ2,λ3-nitrogen. The results of the DFT calculations of the Diels–Alder reaction with isoprene are in accord with the observed regioselectivity. The relative stabilities of the two transition structures have been explained on the basis of NBO analysis.