Compounds which are active against viruses have the following Formulas: ##STR1##

wherein B is a purine or pyrimidine heterocyclic ring and is preferably selected from the group consisting of 6-aminopurine (adenine), 2,6-diaminopurine, 2-amino-6-azidopurine, 2-amino-6-cyclopropylaminopurine, 6-hydroxypurine (hypoxanthine), 2-amino-6-halo substituted purines, 2-amino-6-alkoxy substituted purines, 2-amino-6-hydroxypurine (guanine), 3-deazapurines, 7-deaza-purines, 8-azapurines, cytosine, 5-halo substituted cytosines, 5-alkyl substituted cytosines, thymine, uracil and 6-azapyrimidines; X is O; and, R₁ and R₂ are alkyl or aryl groups. The compounds of the present invention also include the R- and S-enantiomers of the above compounds. The R₁X and/or R₂X can also be amino acid residues with X as NH.