4-amino-5-fluoro-1-((2R,5S)-2-((((5-fluoro-1-((2R,5S)-2-(hydroxymethyl)-1,3-oxathiolan-5-yl)-2-oxo-1,2-dihydropyrimidin-4-yl)amino)methoxy)methyl)-1,3-oxathiolan-5-yl)pyrimidin-2(1H)-one