General procedure for the synthesis of Boc-D-alanine methyl ester from di-tert-butyl dicarbonate and D-alanine methyl ester hydrochloride: di-tert-butyl dicarbonate (11.7 g, 53.7 mmol) was added to an aqueous solution (100 mL) containing D-alanine methyl ester hydrochloride (5 g, 35.8 mmol) and sodium hydrogen carbonate (9.0 g, 107 mmol). The reaction mixture was stirred at room temperature. Upon completion of the reaction, the reaction mixture was extracted with dichloromethane (DCM, 3 x 100 mL), the organic phases were combined and dried over anhydrous magnesium sulfate (MgSO4). The dried organic phase was filtered and concentrated under reduced pressure. Nuclear magnetic resonance (NMR) analysis of the crude product showed the presence of a large amount of unreacted di-tert-butyl dicarbonate. Therefore, the crude product was dissolved in dichloromethane (DCM, 50 mL), treated with N,N-dimethylethylenediamine (5 mL) and stirred for 30 min. Subsequently, the solution was washed with 1 M hydrochloric acid (HCl, 50 mL), and the organic phase was again dried with anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure to give 5.35 g of Boc-D-alanine methyl ester in 73% yield as a colorless liquid.1H NMR (500 MHz, chloroform-d) δ 5.04 (s,1H), 4.31 (m,1H), 3.73 (s,3H), 1.44 (s,9H), 1.37 (d,J=7.2Hz,3H).
