A solution (4S)-7-(3,5-dimethylisoxazol-4-yl)-2-oxo-4-pyridin-2-yl-1,2,4,5-tetrahydroimidazo[1,5,4-de][1,4]benzoxazine-9-carbaldehyde (250 mg, 0.664 mmol) in ethanol (12.5 mL) was treated with 1-(2,4-dimethoxyphenyl)methanamine [Aldrich #: 432725](150 μL, 0.996 mmol) and acetic acid (20.0 μL, 0.352 mmol) and heated at 60° C. for 1 h. The reaction mixture was cooled to room temperature, treated with sodium cyanoborohydride (210 mg, 3.3 mmol) and stirred at room temperature for 3 h. The reaction mixture was then quenched with acetic acid (1 mL) and diluted with ethyl acetate. The organic layer was washed with water and brine, dried over magnesium sulfate, filtered, and concentrated to give the desired product (0.40 g, 97%) as a single enantiomer which was used without further purification. LCMS calculated for C29H30N5O5 (M+H)+: m/z=528.2. found: 528.0.
