Step A: Ethyl cyanoacetate (4.7 mL, 44.3 mmol) was dissolved in a pyridine/triethylamine solvent mixture (500 mL, 1:1 v/v) under nitrogen protection. Hydrogen sulfide gas (H2S) was slowly passed into the solution until saturation. The reaction mixture was heated to 60 °C and stirred for 18 hours. After completion of the reaction, the solvent was evaporated under reduced pressure. The residue was separated by extraction with ethyl acetate (EtOAc) and dilute hydrochloric acid aqueous solution. The organic layer was dried with anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure. Removal of residual solid impurities by filtration afforded ethyl 3-amino-3-thiopropionate (6.25 g, 96% yield) as an orange oil. The product was characterized by 1H NMR (500 MHz, CDCl3): δ 8.92 (1H, broad single peak), 7.75 (1H, broad single peak), 4.21 (2H, quadruple peak, J = 7 Hz), 3.82 (2H, single peak), 1.29 (3H, triple peak, J = 7 Hz).
