To 440 mg. of sodium hydride in oil dispersion is added a solution of 1.60 g. of thiobenzoic acid in 25 ml. of ethanol. The mixture is stirred for 30 minutes and then 4.33 g. of 1-[3-(4-bromobenzoyl)-3-bromopropionyl]-L-proline (Example 37) is added and the mixture is stirred at room temperature for 18 hours, the solvent is removed in vacuo and the residue is partitioned between dichloromethane and water. The organic layer is washed with water, then sodium chloride solution, dried over magnesium sulfate and evaporated to dryness in vacuo leaving a peach colored glass. A 2.8 g. portion of this glass is dissolved in ethyl acetate-hexane-acetic acid (75:25:2) and placed on a 1"×15" column of silica gel. The column is eluted with the same solvent system (100 ml. hold back volume) taking 50 ml. cuts. The desired product is recovered in cut number 2 as a glass.
