Production method of polyoxyethylene derivative having plurality of hydroxyl groups at terminal

A production method of a polyoxyethylene derivative represented by the following formula (1) which comprises performing the following step (A), step (B), step (C) and step (D):

• (in the formula (1), L1 is a divalent linker, X is a functional group capable of reacting with a physiologically active substance, a is 1 or 2, and n is from 11 to 3,650);
• Step (A):
• a step wherein in a polyhydric alcohol having 5 or 7 hydroxyl groups, 4 or 6 hydroxyl groups are protected by cyclic benzylidene acetalization to obtain a compound having the hydroxyl group at a 1-position and a protective group of a cyclic benzylidene acetal structure;
• Step (B):
• a step wherein from 11 to 3,650 moles of ethylene oxide are polymerized to the compound obtained in the step (A) to obtain a polyoxyethylene derivative;
• Step (C):
• a step wherein the hydroxyl group at a terminal of the polyoxyethylene derivative obtained in the step (B) is converted to the functional group capable of reacting with a physiologically active substance;
• and
• Step (D):
• a step wherein the protective group of the polyoxyethylene derivative is deprotected.