Oligonucleotide analogs are disclosed having pyrimidine monomeric sub-units therein that are modified at the 2' and 5' positions. Monomeric sub-units having these modifications may be further modified at the 2' position. Improved processes for the synthesis of 2'-O-substituted pyrimidine nucleosides are also provided. The processes feature alkylation of a 2,2'-anhydropyrimidine nucleoside or a 2S,2'-anhydropyrimidine nucleoside with a weak nucleophile in the presence of a Lewis acid.