A solution of (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 (1.30 g, 3.45 mmol) (single enantiomer from Example 137, step 3) in tetrahydrofuran (30.0 mL) was treated with 3.0 M methylmagnesium iodide in diethyl ether (4.03 mL, 12.1 mmol) and stirred at room temperature for 30 min. The reaction mixture was diluted with ethyl acetate, washed with water and brine, dried over magnesium sulfate, filtered, and concentrated to give the crude product. Purification by preparative LCMS (XBridge C18 column, eluting with a gradient of acetonitrile/water containing 0.1% ammonium hydroxide, at flow rate of 60 mL/min) gave the desired product as a mixture of diastereoisomers. The diastereoisomers were separated by chiral HPLC (Chiracel AD-H column, 20×250 mm, 5 micron particle size, eluting with 50% ethanol in hexanes at 12 mL/min, 45 mg per injection) to give Peak 1 (Diastereoisomer 1, RT=10.2 min) and Peak 2 (Diastereoisomer 2, RT=12.6 min).
