A solution of (S)-tert-butyl 1-(2-(7-(thiophene-2-carboximidamido)-2H-benzo[b][1,4]thiazin-4(3H)-yl)ethyl)pyrrolidine-2-carboxylate (200 mg, 0.423 mmol) in CH2Cl2 (5 mL) was treated with anisole (0.092 mL, 0.846 mmol). The reaction was cooled to 0° C., and trifluoroacetic acid (2.76 mL, 25.4 mmol) was then added. The mixture was stirred at 0° C. for 2 hours then concentrated to dryness. To this residue was added 4M HCl in dioxane (3.17 mL, 12.69 mmol), and the resulting suspension was stirred for 20 minutes. This mixture was diluted with Et2O (10 mL) and stirred for 1 hour. The solid was filtered, washed with Et2O (10 mL), and then dried under reduced pressure. The solid was dissolved in 3N HCl (4.0 mL, 12.00 mmol) and heated at 50° C. for 1 hour. The yellow solution was concentrated and dried under reduced pressure to give compound 18 as yellow solid (200 mg, 97%). 1H-NMR (CD3OD) δ 7.98-7.94 (m, 2H), 7.28 (dd, J=4.2, 4.8 Hz, 1H), 7.05-7.01 (m, 2H), 6.92 (d, J=8.7 Hz, 1H), 4.42 (dd, J=7.8, 9.0 Hz, 1H), 3.85-3.56 (m, 6H), 3.48-3.29 (m, 2H), 3.13-3.03 (m, 2H), 2.62-2.47 (m, 1H), 2.26-2.10 (m, 2H), 2.08-1.92 (m, 1H); ESI-MS (m/z, %): 417 (MH+, free base, 100); ESI-HRMS calculated for C20H25N4O2S2 (MH+, free base): 417.1419, observed: 417.1399; HPLC purity: 99.5% a/a; Optical Rotation: 25[α]589=−27.0°, c=0.52 in MeOH.
