General procedure for the synthesis of (R)-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 (193 mL) were mixed, and the reaction was slowed and cooled by reflux. The resulting salt was purified by three recrystallizations from 63% aqueous ethanol solution. Subsequently, the salt was treated with 10% sulfuric acid at 0 °C, the reaction mixture was extracted with ethyl acetate, and the organic phase was dried and concentrated to give 3.52 g of (R)-(-)-2-isopropylphenylacetic acid in 39% yield, [α]25D -58.6° (CHCl3, c = 0.02).
