General procedure for the synthesis of 1-(4-fluorophenyl)-2-piperazinone from tert-butyl 4-(4-fluorophenyl)-3-oxopiperazine-1-carboxylate: To a solution of 4-(4-fluorophenyl)-3-oxopiperazine-1-carboxylic acid tert-butyl 4-(4-fluorophenyl)-3-oxopiperazine-1-carboxylate (2 g, 6.80 mmol) in methylene chloride (5 ml), was slowly added a solution of 1,4-dioxane with hydrogen chloride (16.99 ml. 68.0 mmol). The reaction mixture was stirred at 0°C and subsequently warmed up to 25°C and continued stirring for 3 hours. After completion of the reaction, the solvent was removed by evaporation under reduced pressure to give the crude product salt. 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 9.9 (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).
