Synthesis
At 5°C, 25 mmol of D-leucine was dissolved in 10 ml of a 10% sodium hydroxide (NaOH) aqueous solution to obtain a solution. While stirring, 35 mmol (4.5 ml) of benzyl chloroformate (Cb2Cl) was added dropwise to the solution at a rate of 1 drop/4 seconds to carry out the reaction. A 10% sodium hydroxide (NaOH) aqueous solution was added dropwise to maintain the pH at 8. After the addition was complete, the solution was placed at 10°C and stirred for 15 hours. The reaction was then tested with ninhydrin as a colorimetric reagent; no purple color was observed. The solution was then washed three times with 16 ml of diethyl ether each time, and the ether was discarded, yielding an aqueous phase. The aqueous phase was adjusted to pH 3 with 30% concentrated hydrochloric acid, resulting in a milky white solid.
The liquid after the appearance of a milky white solid was extracted three times with ethyl acetate (16 ml/extract). The extracts were combined, and the combined liquids were washed twice with distilled water (16 ml/extract) and once with saturated brine (16 ml/extract). The mixture was then dried over anhydrous magnesium sulfate for 10 hours, filtered, and the filtrate was concentrated under reduced pressure to obtain a pale yellow oil. This oil was purified by column chromatography using a mixture of ethyl acetate and petroleum ether (1:3 v/v) as the mobile phase to give 4.0 g of N-Cbz-D-Leucine, in 61.5% yield, as an anhydrous, transparent oil.
