Gorjana Lazarevski

Erythromycin series. Part 11. Ring expansion of erythromycin A oxime by the Beckmann rearrangement

The synthesis of 10-dihydro-10-deoxo-11-azaerythromycin A (11) by the Beckmann rearrangement of erythromycin A oxime (2) and reduction of the imino ether so obtained (5) is described. The structure elucidation of the new ring-expanded semisynthetic erythromycins (5) and (11) has been established on the basis of their analytical and spectral data and acid-catalysed degradation into the aglycones (7) and (13), respectively. Finally, the complete structure of ring-expanded erythronolides (7) and (13) has been determined by X-ray crystallography.

Conformational analysis of azithromycin by nuclear magnetic resonance spectroscopy and molecular modelling

Abstract The conformation of azithromycin 1 in the solution was determined by NMR spectroscopy and molecular mechanics calculations and compared with its crystal structure and with some erythromycin derivatives. In solution 1 exists predominantly in a “folded-in” conformation in the C-3 to C-5 region, whereas its crystal state conformation is “folded-out”.

Synthesis and Antibacterial Activity of Isomeric 15-Membered Azalides

A series of 3-keto and 3-O-acyl derivatives of both 6-O-alkyl-8a-aza-8a-homoerythromycin A and 6-O-alkyl-9a-aza-9a-homo-erythromycin A were synthesised and tested against Gram-positive and Gram-negative bacteria. Derivatives of 8a-aza-8a-homoerythromycin A have potent antibacterial activity against not only azithromycin-susceptible strains, but also efflux (M) and inducible macrolide-lincosamide-streptogramin (iMLSB) resistant Gram-positive pathogens, while the corresponding 9a-isomers were less active. Introduction of an additional ring such as 11,12-cyclic carbonate reduced antibacterial activity of both series. 3-Keto and 3-O-(4-nitrophenyl)acetyl derivatives of 6-O-methyl-8a-aza-8a-homo-erythromycin A show typical macrolide pharmacokinetics in preliminary in vivo studies in mice, and their in vivo efficacy is demonstrated.

Free and bound state structures of 6-O-methyl homoerythromycins and epitope mapping of their interactions with ribosomes.

The solution and solid state conformations of several 6-O-methyl homoerythromycins 1-4 were studied using a combination of X-ray crystallography, NMR spectroscopy and molecular modelling calculations. In the solid state 1 was found to exist as the two independent molecules with similar structures termed 3-endo-folded-out. In solution a significant conformational flexibility was noticed especially in the C2 to C5 region. The compounds 1 and 2 unlike 14-membered macrolides adopted the 3-endo-folded-out conformation while 3 and 4 existed in the classical folded-out conformation. TrNOESY and STD experiments showed that 1 and 2 bound to the Escherichia coli ribosome while 3 and 4, lacking the cladinose sugar, did not exhibit binding activities, this being in accordance with biochemical data. The bound conformations were found to be very similar to the free ones, some small differences were observed and discussed. The STD experiments provided evidence on binding epitopes. The structural parts of 1 and 2 in close contact with ribosome were similar, however the degree of saturation transfer was higher for 2. The differences between tr-NOE data and STD enhancements in 1 and 2 arouse as a consequence of structural changes upon binding and a closer proximity of 2 to the ribosome surface. An understanding of the molecular mechanisms involved in the interaction of macrolides with ribosomes can help in developing strategies aiming at design of potential inhibitors.

Conformational analysis of oleandomycin and its 8-methylene-9-oxime derivative by NMR and molecular modelling

Conformations of the 14-membered macrolide antibiotic oleandomycin and its 8-methylene-9-oxime derivative were determined in various solvents. The experimental NMR data--coupling constants and NOE contacts--were compared with the results of molecular modelling--molecular mechanics calculations and molecular dynamics simulations. The conformational changes, on the right-hand side of the 14-membered ring, affected mostly the 3JH2,H3 values and NOE crosspeaks H3 or H4 to H11. Oleandomycin was found to be present predominantly in the C3-C5 folded-in conformations in DMSO-d6 solution, whereas in buffered D2O, acetone-d6 and CDCl3, there was a mixture of folded-in and folded-out conformational families. The predominant conformation of the 8-methylene-oleandomycin-9-oxime derivative in solution was a folded-out one although different amounts of folded-in conformation were also present depending on the solvent. Oleandrose and desosamine sugar moieties adopted the usual and expected chair conformation. The conformation around the glycosidic bonds, governing the relative orientation of sugars vs. the lactone ring, showed a certain flexibility within two conformationally close families. We believe that by combining the experimental NMR data and the molecular modelling techniques, as reported in this paper, we have made significant progress in understanding the conformational behaviour and properties of macrolides. Our belief is based on our own current studies on oleandomycins as well as on the previously reported results and best practices concerning other macrolides. A rational for macrolide conformational studies and advances in methodology has been suggested accordingly.

Synthesis and biological properties of 4″- O -acyl derivatives of 8a-Aza-8a-homoerythromycin

A series of 4″-O-acyl derivatives of 8a-aza-8a-homoerythromycins A were synthesized and tested against Gram-positive and Gram-negative bacteria. Derivatives of 8a-aza-8a-homoerythromycin A have potent anti-bacterial activity against not only azithromycin-susceptible strains, but also efflux (M) and inducible macrolide–lincosamide–streptogramin-resistant Gram-positive pathogens. These compounds show moderate to high clearance and low oral bioavailability in preliminary in vivo pharmacokinetic studies in rat.

Impact of stereochemistry on the biological activity of novel oleandomycin derivatives.

A set of 8-methylene-, 8-methyl-, and 8-methyl-9-dihydro-oleandomycin derivatives having different combinations of stereochemistries at positions C-8 and/or C-9 have been prepared in a chemoselective and stereoselective manner and tested in vitro for antibacterial activity and inhibition of IL-6 production. Configurations of the stereocenters at C-8 and C-9 were determined using 2D NMR techniques. We have shown that change of stereochemistry at these positions can exert a major influence on antibacterial activity as well as IL-6 inhibition, providing novel macrolide derivatives with diminished antibacterial and potent anti-inflammatory activity. In addition, the anti-inflammatory activity observed in vitro was confirmed in an in vivo model of lipopolysaccharide-induced inflammation.

Conformational analysis of 9-deoxo-9a-aza-9a- and 9-deoxo-8a-aza-8a-homoerythromycin A 6,9-cyclic iminoethers

Abstract The conformation of 9a- 4 and 8a- 5 bicyclic iminoethers, the key synthetic intermediates of 15-membered AZALIDES was determined. The 2D NMR data and molecular modeling techniques were used to establish the predominant solution-state conformation. Conformational analysis suggests that in reduction of the iminoethers the smaller ring in both bicyclic compounds, as well as steric effects around the imidate group are reactivity-determined factors. The invariance in coupling constants indicates that 4 and 5 do not change conformation, when they go from a polar to a hydrophobic environment.