Synthesis of [³H]DIPPA: A potent irreversible antagonist selective for the κ opioid receptor

2‐(3,4‐Dichlorophenyl)‐N‐methyl‐N‐[(1S)‐1‐(3‐isothiocyanatophenyl)‐2‐(1‐pyrrolidinyl)ethyl]acetamide (1, DIPPA) has been previously reported to be an opioid receptor affinity label that produces selective and long‐lasting κ opioid receptor antagonism in mice, High specific activity [³H]DIPPA (39.7 Ci/mmol) was prepared by bromination and catalytic tritiation of the amino precursor of DIPPA followed by conversion to the isothiocyanate with thiophosgene.