Two nucleoside derivatives of the pyrimidine bases T and m 5 o x C have been prepared with flexible acyclic carbohydrate linkers. A new procedure, beginning with (R)-(-)-2,2-dimethyl-1,3-dioxolane-4-methanol permits the preparation of the stereochemically pure acyclic derivatives of both protected nucleoside analogues without contamination by a problematic rearrangement product. By simply increasing the flexibility of the carbohydrate portion of the am 5 o x C nucleoside derivative, 15-mer triplexes containing five contiguous G-C base pairs exhibit a 7-8 °C increase in T m value.