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A mixture of 0.98 g (0.0078 moi) of 2-methoxy-pyrazine 4-oxide (Example P6) in 4 ml of phosphoryl chloride is stirred for 2 hours whilst heating at reflux, with 1.25 ml (0.0078 mol) of N, N-diethylaniline being metered in with the aid of a syringe before the start of the reaction. The resulting solution is cooled to 20°C and poured onto ice-water. After adjusting the mixture to pH 9 with 30 percent aqueous sodium hydroxide solution, extraction is carried out four times, using 10 ml of chloroform each time. The combined extracts are washed with 3N hydrochloric acid and saturated sodium chloride solution and are dried over sodium sulfate. A mixture of the desired target compound and of the isomeric 2-chloro-3-methoxy-pyrazine is obtained, which can be separated on a silica gel column (eluant : ethyl acetate/isohexane 1/8). The desired title compound is obtained in a yield of 0.29 g (25 percent of theory), and the isomeric 2-chloro-3-methoxy-pyrazine in a yield of 0.38 g (33 percent of theory). 'H NMR (300 MHz, CDC13) of the title compound: 8.143 ppm (s, 1H) ; 8.131 ppm (s, 1H) ; 3.988 ppm (s, 3H). 'H NMR (300 MHz, CDC13) of 2-chloro-3-methoxy-pyrazine : 8.031 ppm (d (J=2.7 Hz), 1 H) ; 7.937 ppm (d (J=2.7 Hz), 1 H) ; 4.057 ppm (s, 3H).
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