To a solution of 0.7 g of 10H-pyrazino[2,3-b][1,4]benzothiazine-8-acetic acid in tetrahydrofuran (50 ml) were added 0.35 ml of triethylamine and 0.55 ml of diphenyl phosphate azide and the resulting mixture was stirred at room temperature for 3 hours. Next, it was heated under reflux for 1.5 hours. After adding 0.55 g of ethyl 4-(piperidin-4-yl)-2-methylbutanoate, the resulting mixture was reacted at room temperature for 1 hour. After adding ethyl acetate, the reaction mixture was washed with water and dried over anhydrous magnesium sulfate. After distilling off the solvent under reduced pressure, the residue was purified by silica gel column chromatography (eluted with dichloromethane/methanol) to thereby give 0.66 g of the title compound as a yellow oily substance.
