To a suspension of 8-[(2R)-2-hydroxy-2-(4-methyl-1-oxo-1,3-dihydro-2-benzofuran-5-yl)ethyl]-2-(4-methyl-5-oxo-2,5-dihydrofuran-3-yl)-2,8-diazaspiro[4.5]decan-1-one (Example 7, 300 mg, 0.68 mmol) and diphenylphosphoryl azide (578 mg, 1.36 mmol) in a mixed solvent of toluene and dichloromethane (v:v, 10:1, 11 mL) was added DBU (310 mg, 1.36 mmol). The reaction mixture was stirred at 35° C. for 20 h. The solvent was removed by evaporation and the residue was purified by flash column chromatography (0-100% ethyl acetate in petroleum ether gradient) to afford the title compound. 1H NMR (400 MHz, CDCl3): δ 7.32 (d, J=8.0 Hz, 1H), 7.50 (d, J=8.0 Hz, 1H), 5.20 (s, 2H), 5.18 (s, 2H), 4.94 (dd, J=4.0 Hz, 9.2 Hz, 1H), 3.94 (t, J=6.8 Hz, 2H), 2.88-2.85 (m, 2H), 2.72-2.67 (m, 1H), 2.54-2.50 (m, 1H), 2.42-2.39 (m, 1H), 2.33-2.27 (m, 4H), 2.04 (t, J=6.8 Hz, 2H), 1.95 (s, 3H), 1.94-1.87 (m, 2H), 1.51-1.47 (m, 2H); MS-ESI (m/z): 466 (M+1)+.
