To a solution of (4S)-1-(tert-butyloxycarbonyl)-4-hydroxymethyl-L-proline tert-butyl ester (364 mg, 1.21 mmol) in pyridine (2 mL) cooled to 0° C. was added methanesulfonyl chloride (0.19 mL, 2.42 mmol). The reaction mixture was stirred for 2 hours at ice temperature, diluted with ethyl acetate (100 mL), washed with 1M sodium hydrogencarbonate solution, 1M citric acid, water, saturated brine solution, dried (Na2 SO4), and evaporated. The crude product was purified by flash silica gel chromatography eluting with 20% acetone/hexane; yield 437 mg (95%).
