A fast photoisomerization method for the preparation of tritium-labeled 9-cis-retinoic acid of high specific activity.

A method is described by which tritium-labeled all-trans retinoic acid of high specific activity (up to approximately 51 Ci/mmol corresponding to 85% of theoretical) is converted photolytically within a fraction of a second to a mixture of retinoic acid stereoisomers. One of these isomers, 9-cis-retinoic acid, was obtained in high radiochemical purity by reverse-phase HPLC of the stereoisomer mixture. This fast photolysis was obtained by using a high-pressure 100-W mercury lamp operated at 86 +/- 2 W. A copper sulfate solution was used as a light filter to eliminate short-wave ultraviolet radiation as well as much of the infrared radiation. The geometry of the experimental set-up allowed a maximal amount of the light output of the lamp to reach the retinoic acid solution. Reverse-phase HPLC of the photolytically generated retinoic acid stereoisomer mixture provided pure 9-cis-retinoic acid in 4.5% yield after irradiation for 0.6 s. A steady-state equilibrium of retinoic acid stereoisomers was reached when the irradiation time was extended to a total of 4-6 s (10-11% yield of 9-cis retinoic acid).