Synthesis of 3-(4-oxo-2-thioxothiazolidin-3-yl)propionic acid (33): β-alanine 32 (3.4 g, 38.1 mmol) was dissolved in 17 mL of 22% potassium hydroxide solution, and the temperature of the reaction was controlled not to exceed 25 °C. Carbon disulfide (2.5 mL, 42 mmol) was added slowly and dropwise. The reaction mixture was stirred at room temperature. Subsequently, batchwise addition of bromoacetic acid (5.3 g, 38.1 mmol) was made over about 20 minutes, and the whole process was completed in about 3 hours. After addition, stirring was continued at room temperature for 3 hours, during which precipitation was observed. The pH of the reaction mixture was adjusted with concentrated hydrochloric acid to 3-4. After addition of sulfuric acid, the solution was stirred at room temperature overnight. The precipitate was collected by filtration and washed with water to give pure target compound 33 (3.52 g, 45% yield) as a yellow solid. Melting point 155-156 °C. 1H NMR (400 MHz, MeOD-d4) δ (ppm): 5.00 (brs, 1H), 4.22 (t, J = 7.5 Hz, 2H), 4.11 (s, 1H), 2.64 (t, J = 7.6 Hz, 2H). 13C NMR (100 MHz, MeOD-d4) δ (ppm): 202.58, 174.37, 172.87, 39.64, 35.08, 30.28. mass spectrum (ESI) m/z: 204.1 [M-H]+. Elemental analysis (C6H7NO3S2) Calculated values: C, 35.11; H, 3.44; N, 6.82. Measured values: C, 35.10; H, 3.48; N, 6.88.
