Chemical modifications of the aliphatic bridge of ansamycins. 3. Synthesis and activity of 21-epi-rifamycin S.

Rifamycins inhibit bacterial DNA-dependent RNA polymerase through the formation of non-covalent bonds by the oxygenated groups at C(1), C(8), C(21), and C(23). These must be unhindered and underivatized, with the antibiotic in a proper overall molecular conformation. The present study shows that contrary to previous conclusions the availability of the hydroxyl group at C(21) is not as important as that of the other three groups. In support of this is the observation that 21-epi-rifamycin S is partially active, both on the isolated DNA-dependent RNA polymerase and on some Gram-positive bacterial strains.