A solid-phase synthesis for phosphate-branched oligonucleotides is described. The method is based on coupling of a single nucleoside phosphorodiamidite to terminal hydroxyl functions of two solidsupported oligonucleotides. After oxidation of the phosphite triester obtained to a phosphate triester, the third branch is assembled by conventional phosphoramidite chemistry.
Phosphorodialkylamidites 1-3 may in principle be used in two different ways to obtain phosphate-branched oligonucleotides. Either two solid-supported oligonucleotides are reacted through their terminal hydroxyl functions with a single phosphorodialkylamidite monomer (Reaction A in Scheme 1), or an internucleosidic phosphoramidite linkage of a solid-supported oligonucleotide is converted to a phosphite triester by nucleophilic attack of a monomeric nucleoside hydroxyl function (Reaction B in Scheme 1). The rest of the branches are then assembled, after oxidation of the phosphite triester to a
phosphate ester, by conventional phosphoramidite chemistry. As mentioned above, five different nucleoside phoshorodialkylamidite building blocks (1-3) were prepared, and the coupling reactions indicated in Scheme 1 were studied.