Potassium carbonate (9.1 g, 65.8 mmol) was added to a solution of compound D (23 g, 65.8 mmol) in methanol (100 mL) at 0 °C and the reaction mixture was stirred at room temperature for 1 hour. The progress of the reaction was monitored by thin layer chromatography (TLC) and after confirming the completion of the reaction, the mixture was filtered. The filtrate was concentrated under vacuum to remove methanol, and the residue was dissolved in ethyl acetate, washed sequentially with water and brine, dried over anhydrous magnesium sulfate, filtered and concentrated to give the crude product. The crude product was purified by silica gel column chromatography (petroleum ether/ethyl acetate = 4/1 to 1/1 gradient elution) to afford the target product (2S,4S)-N-CBZ-4-hydroxyproline methyl ester (12.6 g, 70% yield) as a light yellow oil.
