The present invention relates to a novel nucleotide analogue having the general formula (I) and pharmaceutically acceptable salts, esters, or salt of such esters: ##STR1## wherein n, X, Q U R1', R1, Z and R.sub.2 are defined here within. The compounds object of the present invention may be a single enantiomers or as mixtures of said enantiomers, the compounds may have a .alpha.D, .alpha.-L, .beta.-D, .beta.-L, R or S configuration at each chiral center or mixtures thereof. This invention also relates to pharmaceutical compositions containing them, alone or in combination with other therapeutic agents, and their use as antiviral agents, particularly against HIV and/or HBV infections in mammals.