GENERAL STEPS: DL-3-aminoisobutyric acid (CAS No. [10569-72-9], 5 g, 47.52 mmol) was dissolved in 2N NaOH solution (24.7 mL), and a solution of tetrahydrofuran (THF, 75 mL) of di-tert-butyl dicarbonate (Boc2O, 11.73 g, 53.22 mmol) was added slowly dropwise with the cooling of a cold water bath, controlling the The reaction temperature was below 30 °C. The reaction mixture was stirred at room temperature for 18 hours and then concentrated under reduced pressure. The concentrate was diluted with water (75 mL) and washed with methyl tert-butyl ether (MTBE, 3 x 150 mL). The aqueous phase was acidified to pH 3 with 100 g/L citric acid solution (150 mL) and subsequently extracted with ethyl acetate (EtOAc, 3 × 150 mL). The organic phases were combined, washed with water (3×45 mL), dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure to afford 9.4 g of the target product 3-((tert-butoxycarbonyl)amino)-2-methylpropanoic acid (Compound 11) as a colorless oil in 97% yield. Nuclear magnetic resonance hydrogen spectrum (1H NMR, 400 MHz, DMSO-d6, δ, ppm): 1.01 (d, J = 7.1 Hz, 3H); 1.38 (s, 9H); 2.48 (m, 1H); 2.91 (dt, J = 6.1, 6.5 and 13.5 Hz, 1H); 3.15 (ddd, J = 6.1, 7.4 and 13.5 Hz , 1H); 6.80 (t, J = 6.1 Hz, 1H); 12.15 (broad s, 1H).
