A 2 ml dichloromethane solution of methyl 3-({[(2,5-dioxopyrrolidin-1-yl)oxy]carbonyl}oxy)-2,2-dimethylpropionate prepared from 136 mg of methyl 3-hydroxy-2,2-dimethylpropionate in the same manner as in Reference Example 46 was added to 5 ml dichloromethane solution of 235 mg of tert-butyl [(1R)-1-(1-naphthyl)ethyl]{[(3R,4S)-4-phenylpyrrolidin-3-yl]methyl}carbamate and 0.21 ml of triethylamine. After 12Hours of stirring at room temperature, the reaction solution was concentrated under a reduced pressure, and the thus obtained residue was purified by a silica gel column chromatography (hexane-ethyl acetate) to obtain 227 mg of 3-methoxy-2,2-dimethyl-3-oxopropyl (3R,4S)-3-({(tert-butoxycarbonyl)[(1R)-1-(1-naphthyl)ethyl]amino}methyl)-4-phenylpyrrolidine-1-carboxylate as a colorless amorphous substance.
