Ethylmorpholine → (S)-3-(2-benzyloxy-4-nitro-benzenesulfonylamino)-4,4-diethoxy-butyric acid tert-butyl ester

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A mixture of 7.69 g (31 mmol) 3-amino-4,4-diethoxy-butyric acid tert-butyl ester, 4.2 mL (33 mmol) N-ethylmorpholine, 0.37 g (3 mmol) N N-dimethylaminopyridine was stirred in 75 mL dichloromethane and cooled to 0° C. in an ice/water bath. 10.1 g (31 mmol) of % 2-benzyloxy-4-nitro-benzenesulfonyl chloride was added in one portion and the resulting mixture was stirred at room temperature 18 h. TLC analysis indicated complete reaction. The reaction mixture was transferred to a separatory funnel and washed with water and then brine. The organic layer was dried over anhydrous magnesium sulfate, filtered, and concentrated in vacuo to a pale yellow oil. The oil was purified over silica eluting with 1:1 ethyl acetate:hexanes to give 11.7 g (21.7 mmol, 70%) (S)-3-(2-benzyloxy-4-nitro-benzenesulfonylamino)-4,4-diethoxy-butyric acid tert-butyl ester as a pale yellow oil. MS (APCI) m/z 538.1 (M−1).

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