Synthesis of 2′-C,4′-C-Methyleneoxy-Bridged Thymidine Derivatives and Properties of Modified Oligonucleotides

2ʹ,4ʹ-Bridged nucleic acid (2ʹ,4ʹ-BNA) analogs are used for therapeutic oligonucleotides owing to their excellent hybridizing ability with complementary RNA and high resistance toward enzymatic degradation. We developed 2ʹ,4ʹ-BNA analogs with oxygen atoms at 6ʹ-positions (e.g. EoNA and EoDNAs), and demonstrated that the presence of 6ʹ-oxygen atoms in bridge structure could show posi-tive effect on the properties of the modified oligonucleotides. Herein, we designed and synthesized 7ʹ-methyl derivatives of methyleneoxy-bridged 2ʹ-deoxyribonucleic acid (MoDNA) possessing a five-membered bridge with 6ʹ-oxygen atom via radical cyclization for the bridge construction. The synthe-sized monomers were incorporated into the oligonucleotides by solid-phase oligonucleotide synthesis. The MoDNA-modified oligonucleotides showed high affinity toward single-stranded RNA and double-stranded DNA, as well as excellent resistance toward nuclease compared with the corresponding natural oligonucleotide.