(3S,4S)-4-(1-Hydroxy-1-methylethyl)-3-(1-(R)-hydroxyethyl)-1-di(p-anisyl)methyl-2-azetidinone (26 g) and 4-dimethylaminopyridine (16 g) were dissolved in dry dichloromethane (200 ml). Benzyl chloroformate (20 g) was added dropwise thereto over a period of 1 hour with ice-cooling, and the resultant mixture was stirred for 2 hours and warmed to room temperature, followed by stirring at the same temperature as above for 10 hours. 5% Hydrochloric acid (100 ml) was poured into the reaction mixture with ice-cooling, and the resulting mixture was stirred for 0.5 hour and allowed to stand. The organic layer was washed successively with water, a saturated sodium bicarbonate solution and brine and dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated in vacuo to give (3S,4S)-4-(1-hydroxyl-methylethyl)-3-(1-(R)-benzyloxycarbonyloxyethyl)-1-di(p-anisyl)methyl-2-azetidinone.
