Synthesis of tritiated clenbuterol

Tritiated clenbuterol was prepared starting from 4‐aminoacetophenone (I) which was selectively brominated to 4‐amino‐3,5‐dibromoacetopheno ne (II), then to 4‐amino‐α,3,5‐tribromoacetophenone (III) and reacted with tert.butylamine to 4‐amino‐3,5‐dibromo‐α‐tert.butylaminoacetoph‐none(IV). (IV) was dehalogenated and reduced with tritium gas to give 2‐(tert.butylamino)‐1‐(4‐amino‐[3,5‐³H]‐phenyl)‐[1‐³H]‐ethanol (V). This tritiated compound underwent selective aromatic chlorination to give the desired 2‐(tert.butylamino)‐1‐(4‐amino‐3,5‐dichlorophenyl)‐[1‐³H]‐ethanol, [ethanol‐1‐³H]clenbuterol, with specific activity of 13.4 Ci/mmol (496 GBq/mmol).