The cycloaddition reactions of o-benzoquinone and 4-chloro-o-benzoquinone with cyclopentadiene, and the reaction of o-benzoquinones with cyclohexa-1,3-diene, are described. The quinones behave as dienophiles yielding 1 : 1 quinoneâ diene adducts. The cyclopetadiene adducts undergo a novel thermal rearrangement to yield tricyclo[5,2,2,0^2,6]undeca-3,10-diene-8,9-diones.