Method for producing monomer for single-stranded nucleic acid molecule

A method for producing a compound represented by formula (3):

• or an enantiomer thereof, comprising:
• reacting a compound represented by formula (1):
• or an enantiomer thereof with 6-hydroxyhexanoic acid in a solvent in the presence of an additive and a condensing agent selected from the group consisting of 1-hydroxybenzotriazole, 1-hydroxy-7-azabenzotriazole, N-hydroxysuccinimide, ethyl(hydroxyimino)cyanoacetate, N,N′-disuccinimidyl carbonate, N-hydroxyphthalimide, N-hydroxypiperidine, 3-hydroxy-4-oxo-3,4-dihydro-1,2,3-benzotriazine and N-hydroxy-5-norbornene-2,3-dicarboxylic acid imide, and then mixing the resultant reaction mixture, water, and a base selected from the group consisting of alkali metal hydroxide, alkali metal carbonate, a bicyclic amidine compound, alkali metal alkoxide, and quaternary ammonium hydroxide to produce a compound represented by formula (2):
• or an enantiomer thereof;
• and
• reacting the compound represented by formula (2) and obtained in the production step or an enantiomer thereof with 2-cyanoethyl-N,N,N′,N′-tetraisopropylphosphorodiamidite in a solvent in the presence of a coupling activator to obtain the compound represented by formula (3) or an enantiomer thereof.