In particular, suitable monomers include compounds of the formula II: wherein B is purine or pyrimidine moiety, and each Y independently represents H, a group that protects a hydroxy group, a (PO3)m ???2 group wherein m is an integer of 1-3, a group reactive to link hydroxy groups, or a phosphodiester linkage to another monomer of said oligonucleotide or polynucleotide, and X is selected from the