Simple solid-phase synthesis and biological properties of carbohydrate-oligonucleotide conjugates modified at the 3'-terminus.

A novel synthesis method for oligonucleotides possessing a functional moiety at the 3'-terminus was established based on solid-phase synthesis. In order to install the functional group at the 3'-terminus of the oligonucleotide, a solid support possessing the functional group was prepared. A carbohydrate was employed in this study for the functionalization of the oligonucleotide. To prepare a glycosylated solid support, a novel glycosyl acceptor (2) was synthesized using 4,4-dihydroxymethyl-cyclopenta-1-ene as the starting compound. The glycosylation reaction proceeded smoothly (yield = 95%) to yield the suitable glycosylated compound (3). After 8 was immobilized on the solid support, it was subjected to solid-phase oligonucleotide synthesis by the standard phosphoramidite coupling method. An oligonucleotide possessing a sugar moiety at the 3'-terminus was obtained after the products were deprotected and cleaved from the solid support. The stability of the carbohydrate-modified oligonucleotide was greatly increased even in the serum buffer, indicating that the sugar moiety at the 3'-position improved the resistance against enzymatic degradation. This technique was also applied to RNA synthesis. Galactose-ended siRNA was prepared and was confirmed to possess enough ability, at a concentration of 10 nM, to regulate the expression of the target gene.