[Cl:1][C:2]1[C:7]2[O:8][CH2:9][C:10](=[O:12])[NH:11][C:6]=2[N:5]=[C:4](/[CH:13]=C/C2C=CC=CC=2)[N:3]=1.[O:21]1CCOCC1>O.O=[Os](=O)(=O)=O>[Cl:1][C:2]1[N:3]=[C:4]([CH:13]=[O:21])[NH:5][C:6]2[C:7]=1[O:8][CH2:9][C:10](=[O:12])[N:11]=2To a solution of 4-chloro-2-[(E)-2-phenylethenyl]-6H-pyrimido[5,4-b][1,4]oxazin-7(8H)-one (0.45 g, 1.16 mmol) in 1,4-dioxane (25 mL) and water (10 mL) was added NaIO4 (1.26 g, 4.4 mmol) along with a catalytic amount of OsO4 (0.36 mL, 4 wt. % in water). The resulting mixture was stirred at room temperature for 5 h. The solvent was evaporated under reduced pressure and the residue was extracted with 10% methanol in DCM (3×50 ml). The organic extracts were combined, dried over anhydrous MgSO4, filtered and concentrated under reduced pressure to give a solid which was purified by column chromatography (silica gel) with a 0-10% methanol in DCM gradient to give the desired product as light yellow solid (0.28 g, 84%).
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