To a solution of meso-2,3-bis-(4-chlorophenyl)-2,3-butanediamine (3 g, example 1) in toluene cooled to 0° C. was added dropwise 2 M solution of trimethylaluminum in toluene (5.33 mL, 10.66 mmol). After the addition, the mixture was stirred at 0° C. for 15 min then the ice bath was removed. After 30 min, the mixture was warmed up to 80-100° C. for about 30 min before being slowly cooled down to room temperature. At room temperature, a solution of 4-bromo-2-ethoxy-benzoic acid ethyl ester (3.18 g, 11.64 mmol) in toluene (10 mL) was added, and the yellow reaction mixture was heated at reflux for 48 h. Upon cooling to room temperature, 1 M Rochelle solution (˜20 mL) and ethyl acetate (˜50 mL) were added. The biphasic mixture was stirred rigorously at room temperature for 3 h. The layers were separated and the aqueous layer was extracted with ethyl acetate (1×100 mL). The combined organic layers were washed with saturated solution of sodium bicarbonate (1×20 mL), brine (1×20 mL), dried over anhydrous sodium sulfate. The solids were filtered off and the filtrate was concentrated to dryness. Purification of the crude residue by flash chromatography (120 g of silica gel, eluting with 5-10%, 20%, 40% then 80% ethyl acetate in hexane) gave the title compound as light yellow foam (3.25 g, 65%). HR-MS (ES, m/z) calculated for C25H24N2OCl2Br [(M+H)+] 517.0444, observed 517.0443.
