Propionic anhydride → 3-(3,4-Difluorophenyl)-2-methylprop-2-enoic acid

A mixture of 3,4-difluorobenzaldehyde (2.88 g, 0.02 mol), propionic anhydride (3.24 g, 0.025 mol) and sodium propionate (0.92 g, 0.02 mol) under nitrogen is heated to 135° C. for 20 hours. The reaction mixture is poured into water (50 ml). A solid precipitates, which dissolves when saturated K2CO3 (50 ml) is added with stirring. The basic solution is washed with ether (2×100 ml). The aqueous phase is 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.