Preparation method of impurity TS-2A

The invention discloses a preparation method of an impurity TS-2A, and relates to the technical field of chemical engineering, the technical scheme comprises the following steps: preparing materials including L-alanine, industrial oxalic acid, 95% ethanol, diethyl oxalate and benzene; s2, 75.00 g of oxalic acid is added into a 1L three-neck bottle, and 130.00 g of 95% ethanol and 45.00 g of L-alanine are added into the three-neck bottle; stirring, heating and refluxing, and keeping refluxing, stirring and reacting for 8 hours; s3, after the reaction is stopped, 147.60 g of diethyl oxalate, 120.00 g of ethyl alcohol (95%) and 130.00 g of benzene are added into reaction liquid; s4, stirring, heating, refluxing and separating water until no water is separated out, and stopping the reaction; the preparation method comprises the following steps: concentrating a TS-2A toluene solution at 60-65 DEG C under reduced pressure to obtain 40.05 g of black oily matter, and adding 150ml of dichloromethane to dissolve the bl