This invention relates to novel substituted 1,3-oxathiolanes and substituted 1,3-dithiolanes of formula (1) wherein X is S, S=O, or SO2; Y is O, S, S=O, or SO2; R1 is hydrogen, trisubstituted silyl, substituted or unsubstituted C1-6 alkyl, substituted or unsubstituted aralkyl, or substituted or unsubstituted C1-16 acyl; and R2 is a purine or pyrimidine base or an analogue or derivative thereof; and pharmaceutically acceptable derivatives thereof. This invention also relates to processes for preparing these compounds, intermediates useful in their preparation, to pharmaceutical compositions containing them and to the use of these compounds as antiviral agents.