Isopropyl ((((2R,3S,4R,5R)-5-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)-3,4-dihydroxytetrahydrofuran-2-yl)methoxy)(((S)-1-methoxy-1-oxopropan-2-yl)amino)phosphoryl)-L-alaninate