[Synthesis]
The general procedure for the synthesis of 2,4-difluoro-6-methoxybenzaldehyde (Intermediate 26B) and 2,6-difluoro-4-methoxybenzaldehyde (Intermediate 26A) from 1,1-dichlorodimethyl ether and 3,5-difluoroanisole is as follows (Intermediates 26A and 26B were prepared similarly to the method described in WO 2004046133): to a stirred 3 ,5-difluoroanisole (1.0 g, 7.0 mmol) to a stirred solution of dichloromethane (6 mL) was slowly added titanium tetrachloride (1.23 mL, 11.2 mmol) and dichloromethyl methyl ether (0.63 mL, 7.0 mmol) dropwise. After the dropwise addition was completed, the reaction mixture was continued to be stirred for 1 hour and subsequently poured into ice water (50 mL). The reaction mixture was extracted with dichloromethane (3 x 50 mL). The organic phases were combined, washed sequentially with H2O (50 mL) and brine (50 mL), dried with Na2SO4 and filtered. The solvent was removed by concentration under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography with 0 to 20% EtOAc/hexane as eluent to isolate 2,6-difluoro-4-methoxybenzaldehyde (Intermediate 26A, less polar component, 230 mg, 19% yield) and 2,4-difluoro-6-methoxybenzaldehyde (Intermediate 26B, more polar component, 740 mg, 62% yield), both as a white solid. 1H NMR (400 MHz, CDCl3) data of intermediate 26A: δ 3.85 (s, 3H), 6.47 (d, J=10.55Hz, 2H), 10.17 (s, 1H). 1H NMR (400MHz, CDCl3) data for intermediate 26B: δ 3.91 (s, 3H), 6.43-6.51 (m, 2H), 10.30 (s, 1H). |