(2S,16Z,18E,20S,21S,22R,23R,24R,25S,26R,27S,28E)-5,6,21,23- trihydroxy-16-(hydroxymethyl)-27-methoxy-2,4,11,20,22,24,26-heptamethyl1,15-dioxo-1,2-dihydro-2,7-(epoxypenta deca[1,11,13]trienonitrilo)[1]benzofuro-[4,5-e]pyrido[1,2-a]benzimidazol-25-yl acetate,Rifaximin Impurity