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Chemical modifications of the aliphatic bridge of ansamycins. Synthesis and activity of 25-deacetoxy-25-epi-hydroxyrifamycin S.

Rifamycins are supposed to bind to, and inhibit the bacterial DNA-dependent RNA polymerase (DDRP) by the formation of hydrogen bonds through O (1), O (2), O (9), O (10). Therefore, with the aim of increasing the intrinsic activity of rifamycin S (1), the 25-deacetoxy-25-epi-hydroxyrifamycin S (8), was synthesized, which displays an additional hydroxyl available for the inhibiting interaction with the bacterial enzyme. The configuration and conformation of the new compound were as expected, but the biological evaluation did not confirm the hypothesis.