Abstract: Problem The purpose of the present invention is to provide: a stereo isomer of a novel CpG oligonucleotide, which has excellent stability; and a CpG oligonucleotide which has a capability of producing interferon-? (IFN?). Solution The present invention relates to an oligonucleotide which contains two to four sequences each represented by the formula 5?-X1X2CpGX3X4-3? (formula (I)) and has a length of 14 to 32 nucleotides. In formula (I), CpG represents a non-methylated CpG residue having a phosphate skeleton modification, X1X2 represents any one of AA, AT, GA and GT, and X3X4 represents any one of TT, AT, AC and CG. The oligonucleotide has at least one phosphate skeleton modification at an S-form stereoisomer located at a site other than the CpG.

Abstract: Described herein are methods of syntheses of phosphorous atom-modified nucleic acids comprising chiral X-phosphonate moieties. The methods described herein provide backbone-modified nucleic acids in high diasteteomeric purity via an asymmetric reaction of an achiral molecule comprising a chemically stable H-phophonate moiety with a nucleoside/nucleotide.