TiCl4 and SnCl4 promote the formation of dioxolane nucleosides with racemization in the coupling of enantiomerically pure 2'-deoxy-3'oxaribosides with silylated N-acetylcytosine. The use of TMSOTf, TMSI or TiCl2(Oi-Pr)2 furnishes enantiomerically pure cytosine dioxolane nucleosides in low diastereoselectivity.