Synthesis, Reactivity Studies, and X-ray Crystal Structure of (11R)-25-O-Deacetyl-11-deoxo-11-hydroxy-21,23-O-isopropylidenerifamycin S

Preparation and in vitro anti-staphylococcal activity of novel 11-deoxy-11-hydroxyiminorifamycins

We report herein the preparation and anti-staphylococcal activity of a series of novel 11-deoxy-11-hydroxyiminorifamycins. Many of the compounds synthesized exhibit potent activity against wild-type Staphylococcus aureus with MICs equivalent to, or better than, rifamycin reference agents. In addition, some of the compounds retain potent activity against an intermediate rifamycin-resistant strain of Staphylococcus aureus. For instance, compound 5k exhibits an MIC of 0.12 microg/mL against an intermediate rifamycin-resistant strain, while the rifamycin reference agents, rifampin and rifalazil, exhibit MICs of 16 microg/mL and 2 microg/mL, respectively, against the same strain.

New C25 carbamate rifamycin derivatives are resistant to inactivation by ADP-ribosyl transferases

A novel series of 3-morpholino rifamycins in which the C25 acetate group was replaced by a carbamate group were prepared and found to exhibit significantly improved antimicrobial activity than rifampin against Mycobacterium smegmatis. Further characterization of such compounds suggests that relatively large groups attached to the rifamycin core via a C25 carbamate linkage prevent inactivation via ribosylation of the C23 alcohol as catalyzed by the endogenous rifampin ADP-ribosyl transferase of M. smegmatis. SAR studies of the C25 carbamate rifamycin series against M. smegmatis and other bacteria are reported.

NMR spectroscopic analysis of new spiro-piperidylrifamycins

New spiro-piperidylrifamycin derivatives are presented. These compounds were synthesized from the reaction of 3-amino-4-iminorifamycin S and enantiomerically pure 4-piperidones, which generate diasteroisomeric rifabutin analogues with a new stereocentre at the spiranic carbon. The (1)H and (13)C NMR spectra of these new compounds, and also the configuration of the new stereogenic centres, were assigned using 2D NMR spectroscopic techniques. A preliminary study of the (1)H and (13)C NMR spectra of the starting compounds rifamycin S, 3-amino-4-iminorifamycin S and the related rifabutin was also carried out and as a result, their previously published (13)C NMR data were corrected.

Comprehensive study on structure-activity relationships of rifamycins: discussion of molecular and crystal structure and spectroscopic and thermochemical properties of rifamycin O

The mechanism of action of rifamycins against bacterial DNA-dependent RNA polymerase has been explained on the basis of the spatial arrangement of four oxygens which can form hydrogen bonds with the enzyme. Structural descriptors are derived from X-ray diffraction crystal structures of 25 active and nonactive rifamycins. Principal component analysis is used to find the combination of structural parameters which better discriminate between active and nonactive rifamycins. Two possible mechanisms of molecular rearrangement are described which can convert nonactive into active conformations. The energy involved for conformational rearrangements is studied by molecular modeling techniques. Methyl C34 is found to play a key role for determining the geometry of the pharmacophore. Rifamycin O, reported to be active, is obtained by oxidation of rifamycin B and is studied by X-ray single-crystal diffractometry, by solution IR and NMR spectroscopy, and by thermal analysis. Surprisingly the oxidation process is totally stereospecific, and an explanation is given based on solution spectroscopic evidence. The conformation found in the solid state is typical of nonactive compounds, and molecular mechanics calculations show that a molecular rearrangement to the active conformation would require about 15 kcal/mol. Thermal analysis confirms that rifamycin O has a sterically constrained conformation. Therefore, it is likely that the antibiotic activity of rifamycin O is due either to chemical modification prior to reaching the enzyme or to conformational activation.

Synthesis of Nucleosidyl Rifamycins as Inhibitors of Human Immunodeficiency Virus Type 1

In the search for potential nucleoside/non-nucleoside mixed type inhibitors of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase, we synthesized a new set of rifamycin S derivatives, containing AZT connected via its hydroxyl at 5′ C, through a spacer, to the third C of rifamycin S. The length of the spacer was eight, nine or 14 atoms. Rifamycin S was also used in its 21, 23-O, O-isopropylidene derivative form, and in one case thymidine replaced AZT. These nucleosidyl rifamycins were weak inhibitors of isolated HIV-1 reverse transcriptase. The inhibitory power was weak most probably because their large molecular volume hindered the inhibition process. With the exception of the thymidine derivative, the AZT derivatives, at concentrations in the range 0.04–0.07 μM, proved non-toxic and inhibited the replication of HIV-1 in C8166 T lymphocytes. This activity appears to be owing to AZT released by the derivatives upon hydrolysis in solution. The present compounds require further development as mixed type reverse transcriptase inhibitors and can be considered non-toxic lipophilic prodrugs of AZT.