Krystian Pyta

Synthesis, crystal structures and antibacterial activity studies of aza-derivatives of phytoalexin from cotton plant - gossypol.

Using Gossypol (GOS) extracted from cotton seeds, a series of its Schiff bases (1-18) and hydrazones (19-27) have been synthesized and evaluated for their antimicrobial activities against Gram-positive bacteria, Gram-negative bacteria and yeast-like organisms. Of the 27 aza-derivatives of gossypol, 11 have been found active against the microorganisms tested, similar to gossypol. Crystal structures of the new Schiff base (compound 7) and hydrazone (compound 25) of gossypol reveal the presence of two different tautomers within two types of the aza-derivatives studied. The newly synthesized aza-derivatives of gossypol are characterized by the FT-IR, NMR and MS methods.

Antifungal activity of alkyl and heterocyclic aza-derivatives of gossypol as well as their complexes with NaClO4 against Fusarium oxysporum f. sp. lupini

Eight alkyl and six heterocyclic aza-derivatives of gossypol (2-15) have been synthesized using gossypol (1) extracted from Gossypium Herbaceum cottonseeds. The ability of gossypol aza-derivatives to form complexes with NaClO(4) has been investigated by electrospray ionisation (ESI) mass spectra recorded in the positive and negative ion detection modes. The gossypol aza-derivatives have been characterized by FT-IR, (1)H and (13)C NMR spectroscopic methods and subsequently tested for their antifungal properties against Fusarium oxysporum. Four alkyl aza-derivatives (2-5), present in the enamine-enamine tautomeric form, have shown activity comparable or higher than that of gossypol against this fungus. To improve the antifungal activity the complexes of the most active compounds 2-5 with NaClO(4) were prepared. Complexes of 2 and 5 with NaClO(4) have shown antifungal activity higher than that of the uncomplexed compounds.

13C and 15N CP/MAS, 1H–15N SCT CP/MAS and FTIR spectroscopy as tools for qualitative detection of the presence of zwitterionic and non-ionic forms of ansa-macrolide 3-formylrifamycin SV and its derivatives in solid state

13C, 15N CP/MAS, including 1H–13C and 1H–15N short contact time CP/MAS experiments, and FTIR methods were applied for detailed structural characterization of ansa‐macrolides as 3‐formylrifamycin SV (1) and its derivatives (2–6) in crystal and in powder forms. Although HPLC chromatograms for 2/CH3OH and 2/CH3CCl3 were the same for rifampicin crystals dissolved in respective solvents, the UV–vis data recorded for them were different in 300–375 nm region. Detailed solid state 13C and 15N CP/MAS NMR and FTIR studies revealed that rifampicin (2), in contrast to 3‐formylrifamycin SV (1) and its amino derivatives (3–6), can occur in pure non‐ionic or zwitterionic forms in crystal and in pure these forms or a mixture of them in a powder. Multinuclear CP/MAS and FTIR studies demonstrated also that 3–6 derivatives were present exclusively in pure zwitterionic forms, both in powder and in crystal. On the basis of the solid state NMR and FTIR studies, two conformers of 3‐formylrifamycin SV were detected in powder form due to the different orientations of carbonyl group of amide moiety. The PM6 molecular modeling at the semi‐empirical level of theory, allowed visualization the most energetically favorable non‐ionic and zwitterionic forms of 1–6 antibiotics, strongly stabilized via intramolecular H‐bonds. FTIR studies indicated that the originally adopted forms of these type antibiotics in crystal or in powder are stable in standard laboratory conditions in time. The results presented point to the fact that because of a possible presence of two forms of rifampicin (compound 2), quantification of the content of this antibiotic in relevant pharmaceuticals needs caution. Copyright

Redetermination of rifampicin pentahydrate revealing a zwitterionic form of the antibiotic.

Rifampicin belongs to the family of naphthalenic ansamycin antibiotics. The first crystal structure of rifampicin in the form of the pentahydrate was reported in 1975 [Gadret, Goursolle, Leger & Colleter (1975). Acta Cryst. B 31, 1454-1462] with the rifampicin molecule assumed to be neutral. Redetermination of this crystal structure now shows that one of the phenol -OH groups is deprotonated, with the proton transferred to a piperazine N atom, confirming earlier spectroscopic results that indicated a zwitterionic form for the molecule, namely (2S,12Z,14E,16S,17S,18R,19R,20R,21S,22R,23S,24E)-21-acetyloxy-6,9,17,19-tetrahydroxy-23-methoxy-2,4,12,16,18,20,22-heptamethyl-8-[(E)-N-(4-methylpiperazin-4-ium-1-yl)formimidoyl]-1,11-dioxo-1,2-dihydro-2,7-(epoxypentadeca[1,11,13]trienimino)naphtho[2,1-b]furan-5-olate pentahydrate, C(43)H(58)N(4)O(12)·5H(2)O. The molecular structure of this antibiotic is stabilized by a system of four intramolecular O-H···O and N-H···N hydrogen bonds. Four of the symmetry-independent water molecules are arranged via hydrogen bonds into helical chains extending along [100], whereas the fifth water molecule forms only one hydrogen bond, to the amide group O atom. The rifampicin molecules interact via O-H···O hydrogen bonds, generating chains along [001]. Rifampicin pentahydrate is isostructural with recently reported rifampicin trihydrate methanol disolvate.

Impact of metal cation complexation and protonation on tautomeric and resonance forms of the oxaalkyl Schiff bases derived from 5-substituted salicylaldehyde and 2-hydroxy-1-naphthlaldehyde

Structures of the main resonance and tautomeric forms of three Schiff bases of 2-hydroxy-1-naphthaldehyde (1OXA), 5-bromosalicylaldehyde (2OXA) and 5-nitrosalicylaldehyde (3OXA) with 1,8-diamine-3,6-dioxaoctane, before and after protonation and complexation of monovalent metal cations, have been studied by FTIR, 1H, 13C and 15N NMR methods. The spectroscopic investigations provided clear evidence that the Schiff bases exist in acetonitrile solution as three different tautomers: 1OXA in enamine-oxo, 2OXA in imine-hydroxy and 3OXA in a low-barrier O···H···N imine-oxo forms. It was found on the basis of multinuclear NMR studies, that in solid state, the enamine-oxo and imine-hydroxy tautomers are formed exclusively, but not the untypical imine-oxo tautomer, which requires strong stabilisation by solvent molecules in solution. MOG-PM6 calculations of the different tautomers allowed visualisation of their energetically the most favourable structures. Protonation of 1OXA–3OXA Schiff bases leads to formation of common forms, i.e. protonated imine-hydroxy structure, irrespectively of the structure of tautomer before protonation. In turn, coordination of monovalent metal cations implies common formation of zwitterionic forms within all studied ligands in solution. Application of FT-IR and NMR titrations in combination with ESI MS spectrometry revealed the nature and the structure of OXA complexes, whose formation is impeded by the intra- and intermolecular interactions. MOG-PM6 calculations allowed visualisation of Li+ and Na+ metal coordination sphere geometries within structure of all investigated complexes, stabilised by intermolecular interactions with solvent molecules.

16‐Membered Macrolide Lactone Derivatives Bearing a Triazole‐Functionalized Arm at the Aglycone C13 Position as Antibacterial and Anticancer Agents

A series of new C13‐triazole‐bridged and C13‐ether leucomycin analogues with a reduced aldehyde group were synthesized. Derivatives with the highest antibacterial [MIC values (S. epidermidis, S. pneumoniae): ∼2–4 μg mL−1; 2.55–5.09 μm] and cytotoxic [IC50 values (HeLa, KB, MCF‐7, A549, HepG2 cells): ∼1.35–3.70 μm] potencies were those with the best aqueous solubility and bearing a saccharide‐triazole arm at the C13 position of the aglycone. These derivatives preferentially bind at the ribosomal tunnel and show the most attractive selectivity indexes [SI; calculated relative to the human dermal fibroblast (HDF) cell line], even higher than that of the reference compound cytarabine. Results of molecular docking studies of this type of macrolide antibiotics at the ribosomal tunnel, together with experimentally determined lipophilicity and aqueous solubility values, as well as biological assay data revealed the importance of the introduced functional group at the aglycone C13 arm to the future design of anticancer and antibacterial drug candidates. Our results clearly indicate that the high antibacterial and anticancer activities of these types of macrolides do not necessarily depend on the presence of the aldehyde group at the aglycone lactone ring.

Regio- and Stereoselective Functionalization of 16-Membered Lactone Aglycone of Spiramycin via Cascade Strategy

Functionalization of 16-membered aglycone of spiramycin with the use of intramolecular cascade strategy yielded access to novel types of diastereopure bicyclic spiramycin derivatives containing tetrahydrofuran ring. Experimental results shows that a specific sequence of regio- and stereoselective transformations, based on the intramolecular transesterification, E1cB tandem eliminations, 1,2-addition to carbonyl, and 1,6-conjugate addition at the spiramycin aglycone, proceeds with the inversion of absolute configuration at C(5) stereogenic center. Performed cascade and multistep transformations have opened new possibilities in divergent modifications, not only spiramycin but also the whole family of leucomycin type antibiotics having a similar structure of 16-membered aglycone lactone ring.

The effect of complexation of 3-formylrifamycin SV macrocyclic ether derivatives with metal cations and small nitrogen-containing organic molecules on antibacterial activity against S. aureus and S. epidermidis.

Spectroscopic studies of ether rifamycins (1-9) have shown that all these compounds tend to be zwitterions with different localizations of intramolecularly transferred proton, which influences their solubility and logP values. According to ESI MS studies, rifamycins 3 and 4 form complexes with Li(+) or Na(+), while the other ones (7-9) coordinate small organic molecules, which can be further replaced by Na(+) cation. Biological assays revealed that the use of 7-9 in the form of complexes with small organic molecules improves their antibacterial potency as a result of changed: logP, solubility and binding mode with bacterial RNA polymerases.

Synthesis, structure and antimicrobial evaluation of a new gossypol triazole conjugates functionalized with aliphatic chains and benzyloxy groups.

Synthetic limitations in the copper-catalyzed azide alkyne cycloaddition (CuAAC) on gossypols skeleton functionalized with alkyne (2) or azide (3) groups have been indicated. Modified approach to the synthesis of new gossypol-triazole conjugates yielded new compounds (24-31) being potential fungicides. Spectroscopic studies of triazole conjugates 24-31 have revealed their structures in solution, i.e., the presence of enamine-enamine tautomeric forms and π-π stacking intramolecular interactions between triazole arms. Biological evaluation of the new gossypol-triazole conjugates revealed the potency of 30 and 31 derivatives, having triazole-benzyloxy moieties, comparable with that of miconazole against Fusarium oxysporum. The results of HPLC evaluation of ergosterol content in different fungi strains upon treatment of gossypol and its derivatives enabled to propose a mechanism of antifungal activity of these compounds.

Regioselective Long‐Range Proton Transfer in New Rifamycin Antibiotics: A Process in which Crown Ethers Act as Stronger Brønsted Bases than Amines

Water-mediated proton transfer in six new derivatives of 3-formylrifamycin SV that contain crown, aza-crown, and benzo-crown ether rings were investigated by FTIR and NMR spectroscopy. (1)H-(1)H COSY couplings provide evidence for the formation of zwitterionic structures of the aza-crown and crown ether derivatives of rifamycin, in which a proton from one of the phenolic groups is transferred to tertiary and secondary nitrogen atoms. The increased intensity of the continuous absorption in the mid-infrared region together with the NMR data indicate proton transfer from the phenol group of the rifamycin core to the cavity of the benzo-crown ether ring. This proton transfer is achieved by formation of hydronium (H3O(+)) or Zundel ions (H5O2(+)), which form intermolecular hydrogen bonds with the oxygen atoms of the crown ether. DFT calculations are in agreement with the spectroscopic data and allow visualization of the structures of all new rifamycin derivatives, characterized by different intramolecular protonation sites.