F[C:2](F)(F)C(O)=O.[C:8]([C:10]1[C:15]2[N:16]=[C:17]([CH:19]3[CH2:21][CH2:20]3)[O:18][C:14]=2[C:13]([F:22])=[C:12]([NH:23][C:24](=O)OC(C)(C)C)[C:11]=1[CH3:31])#[N:9].C=O.C([BH3-])#N.[Na+]>C(OCC)(=O)C>[CH3:2][N:23]([CH3:24])[C:12]1[C:13]([F:22])=[C:14]2[O:18][C:17]([CH:19]3[CH2:20][CH2:21]3)=[N:16][C:15]2=[C:10]([C:8]#[N:9])[C:11]=1[CH3:31]
Trifluoroacetic acid (5 ml) was added to tert-butyl (4-cyano-2-cyclopropyl-7-fluoro-5-methyl-1,3-benzoxazol-6-yl)carbamate (I-318) (70 mg, 0.211 mmol), followed by stirring at room temperature for 1 hour. The reaction liquid was evaporated under reduced pressure, and the resulting residue was dissolved in acetic acid (2 ml). 38% formaldehyde (153 μl, 2.11 mmol) was added to the solution, then sodium cyanoborohydride (70 mg, 2.11 mmol) was added, followed by stirring at room temperature for 1 hour. Ethyl acetate was added to the reaction liquid, followed by washing with saturated brine. The organic layer was dried over anhydrous magnesium sulfate, then the solvent was evaporated away under reduced pressure. The resulting residue was purified by preparative TLC (eluent, ethyl acetate:n-hexane=3:1, v/v) to obtain the entitled compound (23 mg, 42%) as a white crystal.
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