cyclopentadiene) undergo Diels−Alderreactions with the 1,3,5-triphosphabenzenes 1 under mild conditions to furnish the dihydrotriphosphabarrelenes 9, 11a−c, 13a−e, 15a−d, 17a,b, and 24. The monoadduct 26 as well as the diadducts 27 and 28 are isolated following the reaction with norbornadiene. Cyclopropene is the only alkene to undergo a Diels−Alder/homo Diels−Alderreaction sequence to afford the hexacyclic