Abstract A synthetic route to stable-isotope-substituted L-phenylalanine is presented, which allows the introduction of 13 C, 15 N, and deuterium labels at any position or combination of positions. For labelling of the aromatic ring, a synthetic route to ethyl benzoate (or benzonitrile) has been developed, based on the electrocyclic ring-closure of a 1, 6- disubstituted hexatriene system, with in situ aromatization by elimination of one (amino) ...