General procedure for the synthesis of (S)-3-methyl-2-phenylbutyric acid from 3-methyl-2-phenylbutyric acid: 2-isopropylphenylacetic acid (8.91 g), (+)-α-phenylethylamine (6.45 mL), and 63% aqueous ethanol solution (193 mL) were mixed, heated and refluxed, then slowly cooled. The precipitated salt was recrystallized three times from 63% aqueous ethanol solution. Subsequently, the salt was treated with 10% sulfuric acid at 0 °C, and the organic layer was extracted with ethyl acetate, dried and concentrated to give 3.52 g (39% yield) of (R)-(-)-2-isopropylphenylacetic acid with a specific optical rotation [α]25D of -58.6° (CHCl3, c=0.02).
