Novel rifamycin derivatives of formula I (both hydroquinone and corresponding quinone (C.sub.1-C.sub.4) forms): ##STR00001## or their salts, hydrates or prodrugs thereof, wherein: a preferred R comprises hydrogen, acetyl; L is a linker, a preferred linker group elements selected from any combination of 1 to 5 groups shown FIG. 1, provided L is not ##STR00002## wherein R.sub.1 is H, methyl or alkyl. The inventive compounds exhibit valuable antibiotic properties. Formulations having these compounds can be used in the control or prevention of infectious diseases in mammals, both humans and non-humans. In particular, the compounds exhibit a pronounced antibacterial activity, even against multiresistant strains of microbes.

 
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> 6-O-substituted ketolides having antibacterial activity

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