Stereo- and regioselectivity in Diels–Alder reactions of 1,3-azaphospholo[5,1-a]isoquinoline and -[1,5-a]pyridine

1,3-Bis(ethoxycarbonyl)-1,3-azaphospholo[5,1-a]isoquinoline and -[1,5-a]pyridine undergo stereoselective Diels–Alder reactions at the CP– functionality with 2,3-dimethylbutadiene and isoprene in the presence of sulfur or methyl iodide. The reaction with isoprene occurs regioselectively as well, as confirmed by an X-ray crystal structure determination of one cycloadduct. Semiempirical PM3 calculations also support the regioselectivity.