Tritium Labelling of Naltrindole, a δ‐Receptor‐Selective Opioid Antagonist via 1‐Bromonaltrexone

The synthesis of [1,5′‐ 3 H 2 ]naltrindole ( 9 ) with labels at both the morphine skeleton and the indole moiety was carried out by catalytic tritiodehalogenation of 1,5′‐dibromonaltrindole ( 8 ) resulting in a specific activity of 46.1 Ci/mmol (1705 GBq/mmol). The brominated precursor was prepared by the Fischer indole synthesis starting from 1‐bromonaltrexone ( 7 ) and (4‐bromophenyl)hydrazine.