A series of pyrophosphoryl (Z)-(phosphonomethoxy) but-2-enyl derivatives of pyrimidines and purines 9a-d and the corresponding phosphonates 10a-d were synthesized. The prepared compounds contain the phosphonate group as an a-phosphate mimic as well as an acyclic residue emulating the sugar moiety in 2', 3'-dideoxy-2', 3'-didehydronucleoside 5'- triphosphates known as highly potent chain terminators of DNA polymerases. ...