(2S,4S)-1-(p-Nitrobenzyloxycarbonyl)-2-hydroxymethyl-4-acetylthiopyrrolidine (177 mg), 76 mg of triethylamine and 61 mg of 4-N,N-dimethylaminopyridine were dissolved in 2 ml of dry dichloromethane. Ethyl chloroformate (81 mg) was added thereto under ice-cooling in a nitrogen stream. The mixture was stirred under ice-cooling for 3.5 hours, washed successively with water, dilute hydrochloric acid, water, a saturated sodium bicarbonate solution and water, dried over anhydrous sodium sulfate and distilled to obtain (2S,4S)-1-(p-nitrobenzyloxycarbonyl)-2-ethoxycarbonyloxymethyl-4-acetylthiopyrrolidine.
