6-Hydroxypyrimidines substituted at positions 2 and 4 by hydrogen, methyl, amino, cyclopropylamino, dimethylamino, methylsulfanyl, or hydroxyl group afford by the reaction with diisopropyl 2-(chloroethoxy)methylphosphonate in the presence of NaH, Cs(2)CO(3), or DBU a mixture of N(1)- and O(6)-[2-(diisopropylphosphorylmethoxy)ethyl] isomers which were converted to the free phosphonic acids by treatment with bromotrimethylsilane followed by hydrolysis. Analogously, 2,4-diamino-6-hydroxypyrimidine gave on reaction with [(R)- and (S)-2-(diisopropylphosphorylmethoxy)propyl] tosylate, followed by deprotection, the enantiomeric 6-[2-(phosphonomethoxy)propoxy]pyrimidines. 2,4-Diamino-6-sulfanylpyrimidine gave, on treatment with diisopropyl 2-(chloroethoxy)methylphosphonate in the presence of NaH and subsequent deprotection, 2,4-diamino-6-[[2-(phosphonomethoxy)ethyl]sulfanyl]pyrimidine. 2-Amino-4-hydroxy-6-[2-(phosphonomethoxy)ethyl]pyrimidine was obtained from the appropriate 2-amino-4-chloropyrimidine derivative by alkaline hydrolysis and ester cleavage. Direct alkylation of 2-amino-4,6-dihydroxypyrimidine afforded a mixture of 2-amino-4,6-bis[2-(phosphonomethoxy)ethyl]- and 2-amino-1,4-bis[2-(phosphonomethoxy)ethyl]pyrimidine. None of the N(1)-[2-(phosphonomethoxy)ethyl] isomers exhibited any antiviral activity against DNA viruses or RNA viruses tested in vitro. On the contrary, the O(6)-isomers, namely the compounds derived from 2,4-diamino-, 2-amino-4-hydroxy-, or 2-amino-4-[2-(phosphonomethoxy)ethoxy]-6-hydroxypyrimidine, inhibited the replication of herpes viruses [herpes simplex type 1 (HSV-1) and type 2 (HSV-2), varicella-zoster virus (VZV), and cytomegalovirus (CMV)] and retroviruses [Moloney sarcoma virus (MSV) and human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2)], their activity being most pronounced against the latter. The antiviral activity was lower if the oxygen at the position 6 was replaced by a sulfur atom, as in 2,4-diamino-6-[2-(phosphonomethoxy)ethylsulfanyl]pyrimidine. In analogy to N(9)-[2-(phosphonomethoxy)propyl]-2,6-diaminopurine (PMPDAP), solely the (R)-2,4-diamino-6-[2-(phosphonomethoxy)propoxy]pyrimidine exerted antiviral activity, whereas its (S)-enantiomer was essentially inactive.