Cycloheptatriene Norcaradiene Equilibrium in Dimethyl trans-3,8-Dihydroheptalene-3,8-dicarboxylate

Abstract

Diels-Alder cycloadditions of dienophiles , such as 4-phenyl-1,2,4-triazoline-3,5-dione, dimethyl acetylenedicarboxylate, dimethyl fumarate , diethyl azodicarboxylate , maleic anhydride, benzyne and p-benzoquinone, to dimethyl trans-3,8-dihydrohexalene-3,8-dicarboxylate (4) have been investigated, and monoaddition products (9)-(16) have been isolated. It has been shown that a second addition to this system occurs only with singlet oxygen and with benzyne. Furthermore, it has been established that the second addition is retarded because of the strain in the cycloheptatriene molecule. The anti bicyclic endoperoxides (21)-(25) formed have been converted, either on heating or in a Co( tpp )- catalysed reaction, into the corresponding bisepoxides (27)-(31).