Synthesis of 2'-O-[2-(N-methylcarbamoyl)ethyl]ribonucleosides using oxa-Michael reaction and chemical and biological properties of oligonucleotide derivatives incorporating these modified ribonucleosides.

To develop oligonucleotides containing new 2'-O-modified ribonucleosides as nucleic acid drugs, we synthesized three types of ribonucleoside derivatives modified at the 2'-hydroxyl group with 2-(methoxycarbonyl)ethyl (MOCE), 2-(N-methylcarbamoyl)ethyl (MCE), and 2-(N,N-dimethylcarbamoyl)ethyl (DMCE) groups, as key intermediates, via the oxa-Michael reaction of the appropriately protected ribonucleoside (U, C, A, and G) derivatives. Among them, the 2'-O-MCE ribonucleosides were found to be the most stable under basic conditions. To study the effects of the 2'-O-modification on the nuclease resistance of oligonucleotides incorporating the 2'-O-modified ribonucleosides and their hybridization affinities for the complementary RNA and DNA strands, 2'-O-MCE-ribonucleoside phosphoramidite derivatives were successfully synthesized and subjected to the synthesis of 2'-O-MCE-oligonucleotides and 2'-O-methyl-oligonucleotides incorporating 2'-O-MCE-ribonucleosides. The 2'-O-MCE-oligonucleotides and chimeric oligomers with 2'-O-MCE and 2'-O-methyl groups thus obtained demonstrated complementary RNA strands and much higher nuclease resistances than the corresponding 2'-O-methylated species. Finally, we incorporated the 2'-O-MCE-ribonucleosides into antisense 2'-O-methyl-oligoribonucleotides to examine their exon-skipping activities in splicing reactions related to pre-mRNA of mouse dystrophin. The exon-skipping assay of these 2'-O-methyl-oligonucleotide incorporating 2'-O-MCE-uridines showed better efficacies than the corresponding 2'-O-methylated oligoribonucleotide phosphorothioate derivatives.