Cl.[C:2]([N:6]([C@@H:10]1[CH2:15][C:14]2[C:16]([O:21]C)=[CH:17][CH:18]=[C:19]([F:20])[C:13]=2[O:12][CH2:11]1)[CH2:7][CH2:8][CH3:9])([CH3:5])([CH3:4])[CH3:3].B(Br)(Br)Br.C(=O)=O.C([O-])(O)=O.[Na+]>C(Cl)Cl>[C:2]([N:6]([C@@H:10]1[CH2:15][C:14]2[C:16]([OH:21])=[CH:17][CH:18]=[C:19]([F:20])[C:13]=2[O:12][CH2:11]1)[CH2:7][CH2:8][CH3:9])([CH3:3])([CH3:4])[CH3:5](R)-3-(N-tert-Butyl-N-n-propylamino)-8-fluoro-5-methoxy-3,4-dihydro-2H-1-benzopyran hydrochloride (1.3 g, 3.9 mmol) in dry methylene chloride (40 mL) under nitrogen was cooled on a dry ice-EtOH bath to -50° C. Boron tribromide (0.75 mL, 7.8 mmol) was added dropwise (in 1 min) to the stirred solution above. Five minutes after the addition of boron tribromide was complete, the dry-ice bath was changed to an ice bath (+4° C.). After stirring for 4 h at the same temperature the solution was poured on ice (100 g) and solid NaHCO3 was added to adjust pH to 8-9. When the ice had melted the mixture was extracted with ether (4×75 mL). The ether extracts were combined, washed with brine, dried (Na2SO4), filtered and concentrated in vacuo to afford 1.1 g (96% yield) of the title compound as a pale yellow oil. [α]21D =-91.7° (C=1.0; CHCl3). GC-MS (70 eV) M=281 (6%).
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