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.