Synthesis
The general procedure for the synthesis of methyl (R)-2-amino-4-methylpentanoate hydrochloride from methanol and D-leucine was as follows: according to the method of SG Davies, HJ Sanganee, P. Szolcsányi in Tetrahedron 55 (1999) 3337, the procedure was as follows: D-leucine (60.24 mmol) was was suspended in anhydrous methanol (100 mL) and cooled to 0°C in an ice/salt bath. Thionyl chloride (SOCl2, 5.1 mL, 70.24 mmol) was added slowly and dropwise over 30 min. After removing the cooling bath, the reaction mixture was warmed to room temperature and stirred for 4 hours. Subsequently, the reaction mixture was concentrated under reduced pressure. The resulting white product was suspended in dichloromethane (DCM, 3 x 100 mL) and the solvent was removed by distillation under reduced pressure, a process repeated three times. The white crystalline compound was suspended in ethyl acetate (100 mL), filtered and the solid was washed with ethyl acetate (3 x 20 mL). The white product was dried to constant weight to give white powdered (R)-2-amino-4-methylpentanoic acid methyl ester hydrochloride in 92% yield. The product was characterized as follows: white solid; 92% yield; melting point 149.8-151.5 °C; [αD20] = -12.9° (c 1 , H2O).IR (ATR): 2957, 2921, 2842, 2632, 1731, 1588, 1506, 1451, 1248, 1225, 1040 cm-1.1H NMR ( 400 MHz, D2O): δ 4.10 (1H, t, J = 8 Hz, NH-CH-CH2- CH-(CH3)2), 3.79 (3H, s, O-CH3), 1.85-1.75 (1H, m, NH-CH-CH2- CH-(CH3)2), 1.75-1.62 (2H, m, NH-CH-CH2-CH -(CH3)2, NH-CH-CH2-CH-(CH3)2), 0.96-0.87 (6H, m, CH3-CH-CH3).13C NMR (100.79 MHz, D2O): δ 171.5, 53.6, 51.6, 38.9, 24.1, 21.6, 21.1.Elemental analysis: of C7H15N3O2 Calculated values (181.66): C, 46.28; H, 8.88; N, 7.71; Measured values: C, 45.99±0.03; H, 8.74±0.02; N, 7.69±0.02. HRMS: m/z Calculated values for C7H15N3O2: 146.11756 [M + H]+; Measured values: 146.11762 [M + H H]+.
References
[1] Angewandte Chemie - International Edition, 2016, vol. 55, # 34, p. 10113 - 10117
[2] Angew. Chem., 2016, vol. 128, p. 10267 - 10271,5
[3] Synlett, 2009, # 15, p. 2487 - 2491
[4] Liebigs Annalen der Chemie, 1985, vol. 1985, # 11, p. 2285 - 2300
[5] European Journal of Medicinal Chemistry, 2017, vol. 135, p. 142 - 158
