Method for preparing n-acetyl-d-galactosamine tripolymer precursor

A method for preparing an N-acetyl-D-galactosamine tripolymer precursor, and the method comprises:

• wherein n represents an integer ranging from 2 to 6, and m represents an integer ranging from 1 to 20,
• wherein a method for preparing a compound 4 comprises:
• adding a compound 3, a 4 Å molecular sieve powder, and a reaction solvent into a reactor at a room temperature;
• performing stirring at an inert gas atmosphere;
• firstly, adding an enol, followed by slowly dropping trimethylsilyl trifluoromethanesulfonate;
• after a reaction, quenching the reaction with an alkali solution;
• filtering to remove the 4 Å molecular sieve powder, washing the organic phase with saturated salt solution for three times, removing the solvent in vacuo;
• performing recrystallization with a non-polar organic solvent;
• and performing filtration with suction to obtain a filter cake, that is, the compound 4,
• wherein the reaction solvent is selected from any one of dichloromethane, 1,2-dichloroethane, trichloromethane, and tetrachloromethane;
• the inert gas is selected from either nitrogen gas or argon gas;
• the alkali solution is selected from any one of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, sodium methoxide and sodium ethoxide;
• and the non-polar organic solvent is selected from any one or more of n-pentane, n-hexane, cyclohexane, cyclopentane, petroleum ether, heptane, and tetrahydrofuran, and
• wherein a method for preparing a compound 5 comprises:
• adding the compound 4 and the organic solvent into a reactor at the room temperature, followed by stirring;
• slowly adding a NaIO4 solution, and followed by stirring;
• adding RuCl2, followed by stirring;
• and evaporating under reduced pressure to obtain the compound 5: