(2S,4S)-1-(p-Nitrobenzyloxycarbonyl)-2-hydroxymethyl-4-acetylthiopyrrolidine (78 mg) was dissolved in 1 ml of dry pyridine, and 135 mg of 4-N,N-dimethylaminopyridine and 119 mg of dimethylaminocarbonyl chloride were added successively thereto, followed by stirring at 90° to 100° C. for 20 hours. The reaction mixture was diluted with water and extracted with ethyl acetate. The extract was 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-dimethylaminocarbonyloxymethyl-4-acetylthiopyrrolidine.
