Synthesis of high specific activity R‐and S‐ warfarin‐³H

4‐Hydroxy‐3‐[3‐oxo‐1‐(3‐bromophenyl)butyl]‐2H‐1‐benzopyran‐2‐one (3′ ‐bromowarfarin) was synthesized and resolved into its enantiomers by formation of the d‐10‐camphorsulfonate diastereoisomers, which were separated by liquid chromatography and hydrolyzed. The pure enantiomers were reduced with tritium gas to yield R‐ and S‐4‐hydroxy‐3‐[3‐oxo‐1‐(phenyl‐3‐³H)butyl]‐2H‐l‐ benzopyran‐2‐one (R‐ and S‐warfarin‐3′‐³H) of specific activity 31.2 and 24.9 Ci/mmole, respectively. The compounds have been used to develop a stereoselective radioimmunoassay for warfarin enantiomers.