(R)-2-Amino-5-(benzyloxy)-5-oxovaleric acid (6.0 g, 25.3 mmol) was used as a starting material, which was dissolved in a mixed solvent of dioxane and water (1:1, 40 mL). To this solution was added di-tert-butyl dicarbonate (Boc2O, 6.62 g, 30.36 mmol) at 0 °C. The reaction mixture was stirred at room temperature for 16 hours. After completion of the reaction, the solvent was removed by distillation under reduced pressure. The residue was diluted with 30 mL of water and alkalized with sodium carbonate (Na2CO3). Subsequently, the aqueous layer was washed with ethyl acetate (EtOAc, 3 x 100 mL). The aqueous layer was adjusted to pH 2-3 with 5 M HCl aqueous solution and then extracted with ethyl acetate (EtOAc, 4 × 100 mL). The organic extracts were combined, washed with brine and dried with anhydrous sodium sulfate (Na2SO4). Finally, the solvent was removed by distillation under reduced pressure to afford the target product (R)-5-(benzyloxy)-2-((tert-butoxycarbonyl)amino)-5-oxopentanoic acid [Compound 3j, 8.19 g, 96% yield] as a viscous, colorless oil. The product was structurally confirmed by nuclear magnetic resonance hydrogen spectroscopy (1H NMR, CDCl3) and electrospray mass spectrometry (ESI MS).
