Synthesis of the Methyl Analog of 2′-O,4′-C-Ethylene-Bridged 5-Methyluridine via Intramolecular Radical Cyclization and Properties of Modified Oligonucleotides

The synthesis of 6'S-Me-2'-O,4'-C-ethylene-bridged 5-methyluridine (6'S-Me-ENA-T) was achieved using visible light-mediated stereoselective radical cyclization as a key step. This is the first example of a method for constructing a 2',4'-bridged structure from a 4'-carbon radical intermediate. The 6'S-Me-ENA-T monomer was successfully incorporated into oligonucleotides, and their properties were examined. The oligonucleotides containing 6'S-Me-ENA-T exhibited a highly selective hybridization affinity toward single-stranded RNA and an excellent enzymatic stability, compared to the corresponding LNA- and ENA-modified oligonucleotides.