GENERAL STEPS: To a stirred solution of 4-amino-butyl 4-hydroxy-3,5-dimethoxybenzoate (5.6 g, 20 mmol) in N,N-dimethylformamide (DMF, 15 mL) was slowly added S-methylisothiourea sulfate (15% aqueous solution, 23.1 g, 25 mmol). The reaction mixture was heated to 120 °C and kept at reflux with continuous stirring for 6 hours. The progress of the reaction was monitored by high performance liquid chromatography (HPLC) and the heating was stopped after confirming the complete consumption of raw materials. The solvent was evaporated under reduced pressure and the residue was dissolved in an appropriate amount of water. Sodium bicarbonate (NaHCO3, 2.31 g, 27.5 mmol) was added to the aqueous solution in batches, and a large amount of solid precipitated during stirring. The solid product was collected by filtration and washed with an appropriate amount of water, and dried to give 4-guanidinobutyl 4-hydroxy-3,5-dimethoxybenzoate (Leonurine, 4.0 g, 65% yield) as a white solid. The structure of the product was confirmed by nuclear magnetic resonance hydrogen spectroscopy (1H NMR, 400 MHz, with trifluoroacetic acid as solvent): δ 7.38 (s, 2H), 4.42 (t, 2H, J=6.2 Hz), 3.91 (s, 6H), 3.30 (t, 2H, J=6.7 Hz), 1.89 (dt, 2H, J=13.3,6.5 Hz), 1.83- 1.71 (m, 2H).
