7-bromo-6((e)-2-bromovinyl)-2-((2s,4s,5r)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl)-8-(1-((2s,4s.5r)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl)-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)-2,4-diazabicyclo(4.2.0)octane-3,5-dione,Brivudine Impurity