On the Diels−Alder Reaction of Allenes Bearing a Diphenylphosphoryl or (Trichloromethyl)sulfonyl Substituent

Diels−Alder reactions of two allenes, diphenyl(1,2-propadienyl)phosphane oxide (4) and 1,2-propadienyltrichloromethyl sulfone (7), with cyclic dienes were studied. While 4 only reacted with cyclopentadiene, to provide the endo adduct 5, the allene 7 proved to be much more reactive. Thus, the cycloaddition between 7 and N-Boc-pyrrole gave a mixture of the endo and exo adducts 9. The observed reactivities could be rationalized by semiempirical MO calculations.