Step 2: tert-butyl 4-(4-fluorophenyl)-3-oxopiperazine-1-carboxylate (2 g, 6.80 mmol) was dissolved in dichloromethane (5 ml) and cooled to 0°C. A solution of hydrogen chloride in 1,4-dioxane (16.99 ml, 68.0 mmol) was slowly added to the reaction system under stirring. Subsequently, the reaction mixture was warmed up to 25 °C and the reaction was continued with stirring for 3 hours. After completion of the reaction, the solvent was removed by evaporation under reduced pressure to give the crude product. The crude product was ground with ether (2 x 10 ml), decanted and dried to give 1-(4-fluorophenyl)piperazin-2-one hydrochloride (1.2 g, 5.20 mmol, 77% yield). The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 9.90 (broad single peak, 1H, D2O exchangeable), 7.38-7.34 (multiple peaks, 4H), 3.85-3.72 (multiple peaks, 4H), 3.55-3.50 (multiple peaks, 2H). Mass spectrometry analysis showed m/z 195 (M + 1).
