Rifaximin Y ; [(7S,11S,12R,13S,14R,15S,16S,18R)-2,15,18,36-tetrahydroxy-11-methoxy-3,7,12,14,16,18,22,30-octamethyl-6,17,23-trioxo-8,37-dioxa-24,27,33-triazahexacyclo[23.10.1.14,7.05,35.026,34.027,32]heptatriaconta-1(35),2,4,9,19,21,25(36),26(34),28,30,32-undecaen-13-yl] acetate;