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A solution of tert-butyl 2-(7-(furan-2-carboximidamido)-2H-benzo[b][1,4]thiazin-4(3H)-yl)ethyl(methyl)carbamate (0.823 g, 1.978 mmol) in CH2Cl2 (20 mL) was cooled to 0° C. and was treated with trifluoroacetic acid (5 mL). The mixture was stirred at 0° C. under argon atmosphere for 2 hours. The mixture was quenched with 1N NaOH solution (70 mL) and transferred to a separatory funnel. The solution was diluted with water (50 mL) and extracted with CH2Cl2 (2×50 mL). The organic phase was washed with brine (50 mL) and dried (Na2SO4). The crude material was concentrated and subjected to flash chromatography on silica gel (5-15% (2M NH3MeOH):CH2Cl2). The collected fractions were concentrated to give compound 20 as a yellow viscous oil (0.47 g, 76%). 1H NMR (CDCl3) δ 7.85 (s, 1H), 7.44-7.43 (m, 1H), 6.77-6.74 (m, 2H), 6.70 (d, J=2.4 Hz, 1H), 6.62 (dd, J=2.4, 8.7 Hz, 1H), 3.59-3.56 (m, 2H), 3.47 (s, 1H), 3.40 (t, J=6.6 Hz, 2H), 3.05-3.01 (m, 2H), 2.81 (t, J=6.3, 2H), 2.48 (s, 3H); ESI-MS (m/z, %): 319, 317 (MH+, 96), 260 (100).

1. customThe mixture was quenched with 1N NaOH solution (70 mL)
2. customtransferred to a separatory funnel
3. additionThe solution was diluted with water (50 mL)
4. extractionextracted with CH2Cl2 (2×50 mL)
5. washThe organic phase was washed with brine (50 mL)
6. dry with materialdried (Na2SO4)
7. concentrationThe crude material was concentrated
8. concentrationThe collected fractions were concentrated