Trans-1-(p-Nitrobenzyloxycarbonyl)-4-hydroxy-L-proline p-methoxybenzyl ester (49 g) was dissolved in 300 ml of dry tetrahydrofuran, and 33 ml of triethylamine, 9.59 g of 4-dimethylaminopyridine and 33.2 g of benzoyl chloride were added thereto, followed by stirring under reflux for 5 hours. The reaction mixture was cooled down to room temperature and distilled to remove the solvent. The residue was diluted with ethyl acetate, washed successively with water, dilute hydrochloric acid, water, a saturated sodium bicarbonate solution and water, dried over anhydrous sodium sulfate and distilled to remove the solvent. Recrystallization of the residue from diethyl ether gave trans-1-(p-nitrobenzyloxycarbonyl)-4-benzoyloxy-L-proline p-methoxybenzyl ester.
