Tetra-n-butylammonium tribromide (64 mg, 0.13 mmol) was added to a solution of the 6-acetyl-8-(tert-butyl)-3,4-dihydro-2H-1,4-benzoxazin-3-one (30 mg, 0.12 mmol) in a methylene chloride (0.4 ml)-methanol (0.1 ml) mixed solvent under a nitrogen atmosphere, and the mixture was stirred at room temperature for 3 hours. After distilling off the solvent under reduced pressure, the residue was subjected to silica gel chromatography (Wakogel-C200,1 g) to yield the title compound (36 mg, 91% yield) as light yellow needle-like crystals from the chloroform fraction.
