An asymmetric process for the preparation of enantiomerically pure .beta.-L-(-)-1,3-oxathiolane-nucleosides that includes the initial preparation of the key chiral intermediates (2R,5R) and (2R,5S)-5-(O-protected)-2-(protected-oxymethyl)-1,3-oxathiolane from 1,6-thioanhydro-L-gulose. The 2R,5(R,S)-5-(O-protected)-2-(protected-oxymethyl)-1,3-oxathiolane is condensed with a desired heterocyclic base, typically a purine or pyrimidine base, to provide the product nucleoside. The synthesis can be used to prepare the pharmaceutically important compound, .beta.-L-(-)-1-[(2.beta.,4.beta.)-2-(hydroxymethyl)-4-(1,3-thioxolane)]cytosine (.beta.-L-(-)BCH-189).