3-Oxo-3,4-dihydro-2H-1,4-benzoxazine-6-carboxylic acid (9.58 g, 48.0 mmol) was suspended with stirring in n-butyl acetate (48 mL). T3P in ethyl acetate (52.5 mL, 87.5 mmol, 50.0% w/w) was added followed by DIPEA (3.82 mL, 21.9 mmol). The resulting suspension was heated at 80° C. over night. 2-[(3S)-7-fluoro-3,4-dihydro-2H-1,4-benzoxazin-3-yl]-N-methylacetamide (Intermediate 13, 10.0 g, 43.7 mmol) was added portion wise over approximately 90 min and then held at 80° C. over night. Water (100 mL) was added and the mixture stirred at 40° C. The organic phase was collected and washed with aqueous sodium hydrogen carbonate (100 mL, 5.25% w/w) at 40° C. The organic phase was cooled to 10° C. and stirred over night. Isopropanol (100 mL) was added and the solid collected by filtration and dried under vacuum at 40° C. The title compound was obtained as an off-white solid (9.29 g, 22.3 mmol, 95.7% w/w, ee 98.4%, 50.9% yield).
