Concerted addition benzyl derivatives

A different mode of cycloaddition occurs with 7-azabicyclo[2.2.1]-heptadiene derivatives, in which the nucleophilicity of the nitrogen atom determines the point of attachment of the electrophilic dienophile. The addition depicted in 87, which may occur in two steps via a zwitterionic intermediate rather than by a concerted mechanism, accounts for the structures (88) of 1 2 adducts obtained with A-methyl- or A-benzyl-pyrrole and dimethyl acetylenedicarboxylate. At a higher temperature the reaction with A-methylpyrrole also afforded the indole tetraester... 

---