To a solution of 4-amino-3-hydroxy-N-(4-methoxybenzyl)-N-(thiazol-2-yl)benzenesulfonamide (INTERMEDIATE AD) (2.0 g, 5.12 mmol) in DMF (15 mL) was added K2CO3 (2.1 g, 15.3 mmol, Quaigens). To the above suspension was added 2-chloropropanoyl chloride (0.780 g, 6.15 mmol) at 0° C. and reaction mixture was stirred at 25° C. for 30 min, then heated 60° C. for 3 h. The reaction mixture was cooled to 25° C. The organic compound was extracted with EtOAc, washed with water, dried over sodium sulfate and concentrated. The resulting residue was purified by silica gel column chromatography using a gradient of 0-50% EtOAc in hexanes as the eluent to afford 1.45 g of N-(4-methoxybenzyl)-2-methyl-3-oxo-N-(thiazol-2-yl)-3,4-dihydro-2H-benzo[b][1,4]oxazine-7-sulfonamide as off white solid in 63.6% yield. 1H-NMR (400 MHz, DMSO): δ 7.45-7.46 (m, 2H), 7.40-7.44 (m, 1H), 7.23-7.26 (m, 3H), 7.02 (d, J=8.4 Hz, 1H), 6.85 (d, J=8.8 Hz, 2H), 5.19 (s, 2H), 3.70 (s, 3H), 1.44 (t, J=6.8 Hz, 3H); m/z (ESI, positive ion) 445.9 (M+H).
