reaction toluene was distilled off under reduced pressure to obtain 2 50 g of yellow oil to the oily substance 40 ml of ethanol and 10 g of 20 aqueous sodium hydroxide solution were added the mixture was reacted under reflux after the completion of reaction ethanol was removed under reduced pressure ph value of the system was adjusted to 1 2 by adding 10 aqueous hydrochloric acid the precipitated crystals were separated by filteration and dried to obtain 1 61 g of 2 hydroxy 4 phenylbutanoic acid the obtained crystals were revealed to contain s 2 hydroxy 4 phenylbutanoic acid and r 2 hydroxy 4 phenylbutanoic acid in an area ratio of 50 4 to 49 6 by the same analytical conditions as example 15 an example of preparing processes of optically active 2 hydroxy 4 phenylbutanoic acid esters which can be used as a starting material for the present invention is described as follows in a 300 ml flask 134 g of 3 phenylpropanal and 1 4 g of 10 aqueous sodium acetate solution were placed to which 27 g of hydrocyanic acid was added in dropwise maintaining the reaction temperature at 20 u00b0 to 25 u00b0 c thereafter the reaction mixture was kept standing.