Diethyl-phosphoramidic acid (2R,3S,4R,5R)-5-(2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)-3,4-dihydroxy-tetrahydro-furan-2-ylmethyl ester (2R,3S,4R,5R)-5-(2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)-4-hydroxy-2-hydroxymethyl-tetrahydro-furan-3-yl ester