To a mixture of 4-(6-acetyl-3-oxo-2H-benzo[b][1,4]oxazin-4(3H)-yl)butanoate (1.3 g, 0.003 mol), urea hydrogen peroxide (0.85 g, 0.009 mol) and sodium bicarbonate (0.76 g, 0.009 mol) in anhydrous dichloromethane (10 mL) under nitrogen atmosphere at ice-bath temperature was added trifluoroacetic anhydride (0.8 mL, 0.006 mol) drop-wise over a period of 10 minutes. The reaction mixture was gradually warmed to room [...]