(4R,12aS)-7-Methoxy-4-Methyl-6,8-dioxo-3,4,6,8,12,12a-hexahydro-2H-[1,3]oxazino[3,2-d]pyrido[1,2-a]pyrazine-9-carboxylic acid

5-methoxy-6-(methoxycarbonyl)-4-oxo-1-(2-oxoethyl)-1,4-dihydropyridine-3-carboxylic acid (3.38 g, 107.24 mmol) was dissolved in 33 ml of acetonitrile. Add glacial acetic acid (3 ml) and methanesulfonic acid (0.21 ml, 3.24 mmol) in a 150 ml round bottom flask at room temperature. The reaction was heated to 60 ° C for 19 h. (R)-3-Amino-1-butanol (1.44 ml, 1.34 g, dissolved in 2.25 ml of CH3CN) was slowly added dropwise to the above reaction solution. The reaction was continued to stir at 60 ° C for 18 h to give a white suspension. The reaction solution was concentrated to dryness under reduced pressure to give a concentrate. Add 25 ml of CH2Cl2 and 1N HCl (25 ml). The organic layer was separated, and the aqueous layer was extracted with CH 2 Cl 2 (25 ml x 2). The organic layers were combined and concentrated to dryness under reduced pressure to give crude 6 2.2g. The crude product was recrystallized from 15 ml of MeOH/petroleum ether = 6/1 solvent. After filtration and drying, a pale yellow solid product of (4R,12aS)-7-Methoxy-4-Methyl-6,8-dioxo-3,4,6,8,12,12a-hexahydro-2H-[1,3]oxazino[3,2-d]pyrido[1,2-a]pyrazine-9-carboxylic acid was obtained, 1.72 g.