Water → 5-Chloro-3-fluoro-2-methoxybenzaldehyde

To 5 g (34 mmol) 4-Chloro-2-fluorophenol and 416 mg (3.41 mmol) 4-dimethylaminopyridine in 18 ml THF are added 3.7 ml (37.5 mmol) isopropyl isocyanate and the mixture is heated for 20 hours at 60° C. After cooling down to room temperature 2 M HCl is added and the aqueous phase is extracted with diethyl ether. The combined organic phases are washed with brine, dried over sodium sulfate and evaporated to yield 7.2 g isopropylcarbamic acid 4-chloro-2-fluorophenyl ester as the crude product. To 7.2 g (31 mmol) isopropylcarbamic acid 4-chloro-2-fluorophenyl ester and 5.1 ml tetramethylene diamine in 300 ml diethyl ether are added 5.9 ml (32.5 mmol) (trimethylsilyl)(trifluoromethan)sulfonate at room temperature. After 30 minutes the mixture is cooled to −70° C., 10.2 ml tetramethylene diamine and 27 ml of a 2.5 M n-BuLi solution are added successively. After one hour at −70°24 ml DMF in are added at −70° C. and the mixture is stirred another hour at −70° C. 130 ml ethanol and 36 ml of a 2 M aqueous NaOH are added and the reaction is warmed to ambient temperature over 18 hours. The reaction mixture is set acidic by addition of 100 ml 2 M aqueous HCl and partitioned between diethyl ether and water. The aqueous phase is extracted with diethyl ether, the combined organic phases are washed with brine, dried over sodium sulphate and evaporated. The crude product is purified by chromatography on silica gel to yield 1.1 g 5-chloro-3-fluoro-2-hydroxybenzaldehyde. 1.1 g (6.1 mmol) 5-chloro-3-fluoro-2-hydroxybenzaldehyde and 1.56 g (11.3 mmol) potassium carbonate are stirred vigorously in 11 ml DMF while 0.7 ml methyliodide are added. Stirring is continued for 18 hours and water is added. The aqueous phase is extracted with diethyl ether, the combined organic phases are dried over sodium sulfate and evaporated. The crude product is purified by chromatography on silica gel (ethyl acetate in hexane 0 to 10%) to yield 570 mg 5-chloro-3-fluoro-2-methoxybenzaldehyde. 1H-NMR (CDCl3); δ=4.10 (d, 3H), 7.35 (dd, 1H), 7.59 (m, 1H), 10.35 (s, 1H).