Two ethyl-1,2-dihydro-6,7-dimethoxyisoquinoline-3a-carboxylate derivatives were synthesized. These cyclic enamines in which the enaminic carbon atom was bonded with an ester group react regio- and stereospecifically with a diarylnitrilimine. The regiochemistry was specified by chemical and spectroscopic methods. The radiocrystallographic structure of one cycloadduct allows us to determine the stereochemistry of the reaction and to compare it with that of the reaction of a diarylnitrilimine with a 1,2-dihydro-1,2-dialkylisoquinoline without an ester group at the enaminic carbon atom. Keywords: 1,3-dipolar cycloaddition, diarylnitrilimine, cyclic enamine, regiochemistry, stereochemistry.