Triethylamine → N-[(4-Fluorophenyl)sulfonyl]-D-aspartic Acid 1,4-Dimethyl Ester

To a suspension of D-aspartic acid dimethyl ester hydrochloride (36.0 g, 182 mmol) in THF (360 mL) was added 4-fluorobenzenesulfonyl chloride (39.0 g, 200 mmol, 1.1 equiv.). To the previous mixture at 0° C. was added dropwise triethylamine (60.8 g, 601 mmol, 3.3 equiv.). After a period of 18 h at room temperature, the reaction mixture was filtered and the solid washed with methyl tert-butyl ether (30 mL). The filtrate was evaporated followed by the addition of methyl tert-butyl ether (300 mL). To the resulting mixture was added 100 mL of HCl 1 M. The organic phase was separated and sequentially washed with water, half-saturated sodium bicarbonate, HCl 0.5 M and half-saturated sodium chloride. The organic phase was dried over magnesium sulfate, filtered and evaporated under reduced pressure to give 50 g of the title compound as an oil.