[CH:1]1([NH:5][C@@H:6]2[CH2:11][C:10]3[C:12]([O:17][CH3:18])=[CH:13][CH:14]=[C:15]([F:16])[C:9]=3[O:8][CH2:7]2)[CH2:4][CH2:3][CH2:2]1.[CH3:19][CH2:20][CH:21]=O.C([BH3-])#N.[Na+].C(O)(=O)C>CO>[CH:1]1([N:5]([C@@H:6]2[CH2:11][C:10]3[C:12]([O:17][CH3:18])=[CH:13][CH:14]=[C:15]([F:16])[C:9]=3[O:8][CH2:7]2)[CH2:19][CH2:20][CH3:21])[CH2:2][CH2:3][CH2:4]1(R)-3-(N-Cyclobutylamino)-8-fluoro-5-methoxy-3,4-dihydro-2H-1-benzopyran (1.01 g, 4.02 mmol) was dissolved in anhydrous methanol (20 mL) and to this was n-propionaldehyde (3.0 mL, 40.2 mmol) added. After 1 h the reaction was cooled (ice-bath) then sodium cyanoborohydride (0.46 g, 7.24 mmol) was added, the pH was adjusted to 4-5 with acetic acid and the reaction was allowed to stir at room temperature over the weekend. The solvent was removed in vacuo, the remains extracted thrice with diethyl ether. The combined ether portions were dried (MgSO4) filtered, and the solvent were taken into a 2M solution of NH3 and then removed in vacuo to give the crude residue. Chromatography on silica (eluent: 11% ethyl acetate/hexane) gave 0.95 g (80% yield) of the title compound as a clear oil. [α]21D =-95.4° (C=0.1; CHCl3) GC-MS (70 eV) M=293 (1%).
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