In this paper we present a new approach for the realization of tetrahydroisoquinoline scaffolds via stereoselective proto-activation of suitable allenamide precursors. The elusive and rather unstable iminium ion derived from acrylaldehyde is generated in situ and this electrophilic intermediate can be engaged in stereoselective intramolecular Friedel–Crafts-type allylic alkylation with electron-rich aromatic rings. The highest enantioselectivity for tetrahydroisoquinoline intermediates, obtained by organocatalytic transformation, is reported.