Hydrochloric Acid → 2-Tert-butoxycarbonylamino-3-(toluene-4-sulfonyloxy)-propionic acid methyl ester

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A solution of Boc-D-serine methyl ester (9.59 g, 43.8 mmol) in dichloromethane (100 ml) was stirred at −10° C. during the addition of 4-(dimethylamino)pyridine (270 mg, 2.19 mmol), triethylamine hydrochloride (4.19 g, 43.8 mmol) and tosyl chloride (8.35 g, 43.8 mmol). Triethylamine (6.10 ml, 43.8 mmol) was added dropwise at −10° C. over 35 minutes and the resultant slurry was allowed to stand at 6° C. overnight. The reaction mixture was poured into a mixture of ice (35 g), water (35 ml) and 2N hydrochloric acid (20 ml). The aqueous phase was extracted with dichloromethane (35 ml) and the combined organic solutions washed with brine (2*25 ml), dried (anhydrous magnesium sulfate) and concentrated in vacuo to a pale yellow oil. The oil was dissolved in diethyl ether (100 ml) and cooled to 6° C. Once crystallisation started, petrol ether (75 ml) was added in five portions over two hours, and crystallisation was allowed to proceed at 6° C. overnight to give 2-tert-butoxycarbonylamino-3-(toluene-4-sulfonyloxy)-propionic acid methyl ester (10.98 g, 67%).

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