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

2',4'-Bridged nucleic acid (2',4'-BNA) analogues are used for therapeutic oligonucleotides, owing to their excellent hybridizing ability with complementary RNA and high resistance toward enzymatic degradation. We developed 2',4'-BNA analogues with oxygen atoms at 6'-positions (e.g., EoNA and EoDNAs) and demonstrated that the presence of 6'-oxygen atoms in the bridge structure could show positive effect on the properties of the modified oligonucleotides. Herein, we designed and synthesized 7'-methyl derivatives of methyleneoxy-bridged 2'-deoxyribonucleic acid (MoDNA), possessing a five-membered bridge with 6'-oxygen atom via radical cyclization for the bridge construction. The synthesized 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.

Synthesis and Properties of the 5-Methyluridine Derivative of 3,4-Dihydro-2H-pyran-Bridged Nucleic Acid (DpNA).

A novel 2'-O,4'-C-bridged nucleic acid, 3,4-dihydro-2H-pyran bridge moiety (DpNA), with a dioxabicyclo[3.2.1]oct-3-ene ring was designed. Construction of the dihydropyran bridge was achieved by dehydration of a six-membered hemiacetal ring, and the DpNA monomer was synthesized in 10 steps from 5-methyluridine (total yield 9%). The synthesized DpNA monomer was incorporated into oligonucleotides to examine the properties of the modified oligonucleotides. The DpNA-modified oligonucleotides possessed high affinity toward ssRNA and were more resistant to nucleases compared to the corresponding natural oligonucleotide.