Preparation method for intermediate of auristatin derivative

A method for producing an intermediate of auristatin derivative, wherein the intermediate of auristatin derivative is a compound represented by formula (I) or an enantiomer, a racemate or a pharmaceutically acceptable salt thereof:

• which is produced via a route of:
• wherein:
• R1, R2, and R3 are independently selected from the group consisting of H, C1-C8 alkyl, C1-C8 heteroalkyl, C2-C8 alkenyl, C2-C5 alkynyl, C3-C8 heterocycle, C3-C8 carbocycle, aryl, C1-C8 alkaryl;
• C1-C8 alkyl carbocycle, and C1-C8 alkyl heterocycle;
• R4 is an amino protecting group;
• R5 is O or S;
• R6 is selected from the group consisting of H, —OH, C1-C8 alkyl, C3-C8 carbocycle, and —O—(C1-C8 alkyl);
• and
• the method comprises steps of:
• step 1:
• dissolving compound a in an appropriate amount of organic solvent 1;
• step 2:
• adding an aqueous solution of inorganic base to the solution obtained in step 1, and performing reaction;
• and
• step 3:
• performing post-process on the reaction system obtained in step 2, and obtaining the compound represented by formula (I);
• wherein the organic solvent 1 is one or more selected from the group consisting of N,N-dimethylformamide, N-methylpyrrolidone, methanol, ethanol, isopropanol, acetonitrile, acetone, tetrahydrofuran and 1,4-dioxane.