Preparation method of N-(t-butoxy carbonyl)-L-leucine-N'-methoxy-N'-methylformamide intermediate

The invention provides a preparation method of N-(t-butoxy carbonyl)-L-leucine-N'-methoxy-N'-methylformamide intermediate. The method includes the steps of: A) reacting a compound A and a compound B in dichloromethane under the action of a Carter condensation agent; B) concentrating to remove 20%-40% of the dichloromethane; C) adding a second solvent, extracting, concentrating the second solvent to obtain the N-(t-butoxycarbonyl)-L-leucine-N'-methoxy-N'-methylformamide intermediate, wherein the second solvent is one or more of petroleum ether, n-heptane, n-hexane, cyclohexane and octane, and in the extraction, the weight ratio of the second solvent to remaining dichloromethane after concentration is (0.6-0.9):1. The method uses two specific mutual solvents, which can be layered in specific proportion; and the impurities and the products in the reaction system are respectively dissolved in different solvents, thereby reaching the effects of separation and purification.