Preparation method of L-methyldopa

The invention discloses a preparation method of L-methyldopa. The key points of the method are as follows: veratone is recycled from the L-methyldopa production waste liquor, the mother liquor synthesized from DL-aminopropionitrile or the racemic mother liquor of D-aminopropionitrile is mixed with an organic solvent, the mixture is heated to 40-110 DEG C to react for 1-24 hours, and the obtained solution is stood and layered after the reaction to obtain a water layer and an organic layer; the organic solvent is a non-solvent of water and a good solvent of veratone; or alkali and water are added in the splitting waste liquor of DL-aminopropionitrile, the pH is adjusted to 7-14, the solution is heated to 40-110 DEG C to react for 1-24 hours, and the obtained solution is stood and layered after the reaction to obtain a water layer and an organic layer; and the solvent is recycled from the obtained organic layer, and high vacuum distillation is performed under the condition that the pressure is no more than 5kPa to obtain veratone for reuse. By adopting the preparation method of L-methyldopa, the main raw material veratone can be recycled from the L-methyldopa production waste liquor, the chemical oxygen demand (COD) of wastewater can be reduced and the concept of green chemistry can be realized.