Oligonucleotide synthesis method using highly dispersible liquid-phase support

A method for synthesizing an oligonucleotide comprising:

• sequentially condensing and oxidizing a nucleoside phosphoramidite compound in the presence of an acid/azole complex compound using a starting raw material comprising a hydrophobic group-bonded nucleoside represented by Formula (1):
• where R1 represents an alkylene group having 1 to 12 carbon atoms;
• R2 represents an alkylene group having 1 to 22 carbon atoms;
• R3 and R4 each independently represent an alkyl group having 1 to 22 carbon atoms, or R3 and R4 represent a divalent group in which they are bonded to each other to form a heterocycle together with adjacent nitrogen atoms and R2, the heterocycle being represented by Formula (2):
• where R7 represents an alkyl group having 1 to 6 carbon atoms;
• m represents an integer of 0 or 1;
• and y represents an integer of 0 to 3;
• R5 represents a single bond or an alkylene group having 1 to 22 carbon atoms;
• R6 each independently represents an alkyl group having 6 to 30 carbon atoms;
• n represents an integer of 2 to 6;
• X represents a hydrogen atom, a hydroxyl group, or a hydroxyl group protected by a protecting group;
• and Z represents an adenyl group, a guanyl group, a cytosyl group, a thyminyl group, a uracil group, or a derivative of the nucleic acid base having a polar group optionally protected by a protecting group;
• wherein a condensation reaction is performed by preliminarily dissolving the hydrophobic group-bonded nucleoside or hydrophobic group-bonded oligonucleotide and the nucleoside phosphoramidite compound in a non-polar solvent, and contacting the resulting solution with the acid/azole complex compound or a solution containing the complex compound.