5-Aminopentanoic acid (22.45 mmol, 1.0 equiv) was dissolved in 20-30 mL of anhydrous methanol in a two-necked flask equipped with a reflux condenser. Thionyl chloride (31.43 mmol, 1.4 equiv) was slowly added dropwise over 5-10 min. After the dropwise addition, the reaction mixture was heated and refluxed for 3 hours. Subsequently, the mixture was stirred at room temperature overnight. After completion of the reaction, the solvent was removed by rotary evaporator (RVE). The white or light yellow solid obtained was washed with hexane (3 x 50 mL) and evaporated with another portion of hexane. The product was dried under low pressure to give methyl 5-aminopentanoate hydrochloride (2a) as a translucent solid in a yield of 3.46 g (92% yield). Melting point: 141-143 °C (literature value 146.3 °C [42]); 1H NMR (300 MHz, D2O): δ = 3.57 (s, 3H, COOCH3), 2.88 (t, 2H, J = 7.0 Hz, C5-H2), 2.30-2.35 (m, 2H, C2-H2), 1.53-1.59 (m, 4H, C3-H2), 1.53-1.59 (m, 4H, C3-H2, C4-H2). C3-H2 and C4-H2) ppm; 13C NMR (75 MHz, D2O): δ = 179.2 (C1), 54.7 (COOCH3), 41.6 (C5), 35.5 (C2), 28.6 (C4), 23.6 (C3) ppm; IR (solid): ν = 3021(s), 2945(s), 1735(s) ), 1596 (m), 1576 (m), 1519 (s), 1474 (w), 1444 (w), 1414 (w), 1346 (w), 1271 (m), 1187 (s), 1152 (m), 1068 (w), 1055 (m), 984 (w), 952 (m), 899 (w), 882 (w), 865 (w ), 748(s), 651(w) cm-1.
