[F:1][C:2]1[CH:3]=[C:4]([CH:7]=[CH:8][C:9]=1[F:10])[CH:5]=O.[C:11]([O:15]C(=O)CC)(=[O:14])[CH2:12][CH3:13].C([O-])(=O)CC.[Na+]>O>[F:1][C:2]1[CH:3]=[C:4]([CH:7]=[CH:8][C:9]=1[F:10])[CH:5]=[C:12]([CH3:13])[C:11]([OH:15])=[O:14]A mixture of 2.88 g (0.02 mol) of 3,4-difluorobenzaldehyde, 3.24 g (0.025 mol) of propionic anhydride and 0.92 g (0.02 mol) of sodium propionate under nitrogen is heated at 135° C. with a magnetic stirrer for 20 hours. The reaction mixture is poured onto 50 ml of water. A solid precipitates that dissolves when 50 ml of saturated K2CO3 is added with stirring. The basic solution is extracted with ether (2×100 ml). The aqueous phase is then poured into an excess of concentrated HCl and ice. The precipitated white solid is filtered and dried to give 3,4-difluoro-α-methylcinnamic acid, M.P. 122-125° C.
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