Disclosed are compounds of the formula ##STR00001## wherein R.sub.1 is hydrogen or an acyl group having 1 to 16 carbon atoms; R.sub.2 is a purine or pyrimidine base or an analogue or derivative thereof; Z is O, S, S.dbd.O or SO.sub.2; and pharmaceutically acceptable derivatives thereof. Also described are processes for and intermediates of use in their preparation, pharmaceutical compositions containing these compounds, and the use of these compounds in the antiviral treatment of mammals.