Effective syntheses of 2′,4′-BNANC monomers bearing adenine, guanine, thymine, and 5-methylcytosine, and the properties of oligonucleotides fully modified with 2′,4′-BNANC

We efficiently synthesized 2'-O,4'-C-aminomethylene-bridged nucleic acid (2',4'-BNA(NC)) monomers bearing the four nucleobases, guanine, adenine, thymine, and 5-methylcytosine and incorporated these monomers into oligonucleotides. Initially, we carried out the transglycosylation reaction on several 2'-O-substituted 5-methyluridines to evaluate the effects of 2'-substitutions on this reaction. Under the optimized conditions, purine nucleobases were successfully introduced, and 2',4'-BNA(NC) monomers bearing adenine or guanine were obtained over several steps. In addition, the improved synthesis of the 2',4'-BNA(NC) monomers bearing thymine or 5-methylcytosine was also achieved. The obtained 2',4'-BNA(NC) monomers were subsequently incorporated into oligonucleotides and the duplex-forming abilities of the modified oligonucleotides were investigated. Duplexes containing 2',4'-BNA(NC) monomers in both or either strands were found to possess excellent thermal stabilities.