tert-Butyl ((((4aR,6R,7R,7aS)-6-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)-7-hydroxy-2-oxidotetrahydro-4H-furo[3,2-d][1,3,2]dioxaphosphinin-2-yl)oxy)methyl) carbonate