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0 °C

(2S,4R)-4-(Tert-butyl-dimethyl-silanyloxy)-2-hydroxymethyl-pyrrolidine-1-carboxylic acid tert-butyl ester (intermediate 53, 51.4 g, 155 mmol) dissolved in 500 ml of dichloromethane was cooled to 0° C., then p-nitrobenzoic acid (28.5 g, 171 mmol), N,N′-dicyclohexylcarbodiimide (35.2 g, 171 mmol) and 4-dimethylaminopyridine (1.90 g, 1.60 mmol) were added. The mixture was maintained under stirring at room temperature for 18 hours, then filtrated; the filtrate was concentrated to obtain crude that was purified by silica gel chromatography (petroleum ether/ethyl acetate from 15/1 to 10/1). (2S,4R)-4-(tert-Butyl-dimethyl-silanyloxy)-2-(4-nitro-benzoyloxymethyl)-pyrrolidine-1-carboxylic acid tert-butyl ester was obtained as white solid (70 g, yield: 94%).

1. temperatureThe mixture was maintained
2. filtrationfiltrated
3. concentrationthe filtrate was concentrated
4. customto obtain crude that
5. customwas purified by silica gel chromatography (petroleum ether/ethyl acetate from 15/1 to 10/1)