Chimeric RNA with modified backbones: alternating methylene(methylimino) linked phosphodiester backbone oligonucleotides with 2'-OH and 2'-OMe groups.

Synthesis of a novel ribo-MMI dimer with 2'-OH and 2'-OMe in 5'- and 3'-nucleosides, respectively is presented. The synthesis was accomplished by reductive coupling of 3'-deoxy-3'-C-formyluridine and 2'-O-methyl-5'-O-methylaminouridine via a thioacetal as the key intermediate for the top part of the dimer. Incorporation of ribo-MMI dimers into oligonucleotides increased binding affinity for target RNA.