A solution of (2S)-[4-(4-aminobutyl)benzenesulfonylamino]-3-methylbutyramide 15 (156 mg, 0.47 mmol), diisopropylethylamine (0.60 mL, 3.0 mmol), and 1-(3,5-diamino-6-chloropyrazine-2-carbonyl)-2-methylisothiourea hydriodide (230 mg, 0.61 mmol) in absolute ethanol (8 mL) was stirred at 70° C. for 5 h and then cooled to room temperature. The reaction mixture was concentrated by rotary evaporation. The crude residue was washed with water, filtered and the crude solid product was purified by flash silica gel column chromatography eluting with chloroform/methanol/concentrated ammonium hydroxide (5:1:0.1, v/v) to give the desired product as a yellow solid (137 mg, 54% yield). Part of the solid (86 mg) was further purified by semi-preparative HPLC (acetonitrile/water/0.1% TFA) to give the analytical pure sample which was then co-evaporated with 5% aqueous HCl to give the hydrochloride salt 16. mp 154-156° C. (decomposed). 1H NMR (300 MHz, CD3OD) δ 0.85 (d, 3H), 0.86 (d, 3H), 1.70 (m, 4H), 1.90 (m, 1H), 2.75 (m, 2H), 3.32 (m, 2H), 3.52 (d, 1H), 7.35 (d, 2H), 7.75 (d, 2H). m/z (ESI): 540 [C21H30ClN9O4S+H]+. [α]D25+5.2° (c 0.50, MeOH).
