Preparation method of lacosamide process impurity

The invention discloses a preparation method of a lacosamide process impurity. The preparation method of the lacosamide process impurity comprises the following steps: dissolving (R)-2-amino-N-benzyl-3-methoxyl propanamide in an aprotic solvent, simultaneously and dropwise adding an alkaline reagent and a methylation reagent at 0-25 DEG C, conducting reacting at 0-25 DEG C after dropwise adding is finished so as to generate an intermediate 1, and acetylating the intermediate 1 to generate a lacosamide process impurity C, wherein a molar ratio of the (R)-2-amino-N-benzyl-3-methoxyl propanamide to the methylation reagent is 1: (0.5-1.5). The impurity C prepared by the preparation method of the lacosamide process impurity provided by the invention is good in yield and relatively high in purity, can be used as an impurity reference substance, is suitable for large-scale preparation, and provides a qualified, cheap and easily available reference substance for quality control of a lacosamide bulk drug and a prepara