2, 4-Diamino-6-hydroxypyrimidines substituted in position 5 by an allyl, benzyl, cyanomethyl, ethoxycarbonylmethyl, phenyl, cyclopropyl, or methyl group were prepared either by C5- alkylation or by formation of the pyrimidine ring by cyclization. Their alkylation with 2-[( diisopropoxyphosphoryl) methoxy] ethyl tosylate afforded N1-and O6-regioisomers that were separated and converted to the free phosphonic acids by treatment with ...