Hydrogen chloride-ethyl acetate → hexahydro-1,1-diphenyl-1H-oxazolo[3,4-a][1,4]diazepin-3-one

1,1-Dimethylethyl tetrahydro-3-oxo-1,1-diphenyl-1H-[1,3]oxazolo[3,4-a][1,4]diazepine-8(5H)-carboxylate (0.30 g, 0.73 mmol) was dissolved in ethyl acetate (5 mL). A 4 M hydrogen chloride/ethyl acetate solution (4 mL) was added thereto, and the mixture was stirred at room temperature for 1 hour. The reaction solution was concentrated, and then ethyl acetate was added to the residue. The resulting mixture was washed with a 0.5 M aqueous sodium hydrogen carbonate solution, dried over anhydrous magnesium sulfate and concentrated under reduced pressure to obtain the title compound (0.16 g, yield 70%).