Madurastatins with Imidazolidinone Rings: Natural Products or Side-Reaction Products from Extraction Solvents?

Madurastatins are a group of pentapeptides containing an oxazoline moiety, and, in a few cases, an imidazolidinone ring as an additional structural feature. In our search for new potential antiparasitic metabolites from natural sources, we studied the acetone extracts from a culture of Actinomadura sp. CA-135719. The LC/HRMS analysis of this extract identified the presence of the known madurastatins C1 (1), D1 (4), and D2 (5) together with additional members of the family that were identified as the new madurastatins H2 (2) and 33-epi-D1 (3) after isolation and spectroscopic analysis. The planar structures of the new compounds were established by HRMS, ESI-qTOF-MS/MS, and 1D and 2D NMR data, and their absolute configuration was proposed using Marfey's and bioinformatic analyses of the biosynthetic gene cluster (BGC). A revision of the absolute configuration of madurastatins D1 and D2 is proposed. Additionally, madurastatins containing imidazolidinone rings are proved to be artifacts originating during acetone extraction of the bacterial cultures.

The diversity and utility of arylthiazoline and aryloxazoline siderophores: challenges of total synthesis

Siderophores are unique ferric ion chelators produced and secreted by some organisms like bacteria, fungi and plants under iron deficiency conditions. These molecules possess immense affinity and specificity for Fe³⁺ and other metal ions, which attracts great interest due to the numerous possibilities of application, including antibiotics delivery to resistant bacteria strains. Total synthesis of siderophores is a must since the compounds are present in natural sources at extremely small concentrations. These molecules are extremely diverse in terms of molecular structure and physical and chemical properties. This review is focused on achievements and developments in the total synthesis strategies of naturally occurring siderophores bearing arylthiazoline and aryloxazoline units.

A review presents advances in total synthesis of thiazoline and oxazoline-bearing siderophores, unique ferric ion chelators found in some bacteria, fungi and plants.

Preliminary studies on the antibacterial mechanism of Yanglingmycin

The antibacterial mechanism of Yanglingmycin, a new dihydrooxazole antibiotic, was preliminarily investigated by symptomatology observation and physical and biochemical analysis. The electron microscopy observation exhibited that the bacterial cell became elongated, appeared breakage or even cavities on the cell surface after treated with Yanglingmycin. The content of reducing sugar and the activity levels of alanine transaminase and aspartate transaminase in treated group had a significant increase compared to control group. These results indicated that the integrity of bacteria cell membrane was damaged by the antibiotic. Furthermore, the activity of Accase and carboxyltransferase could be effectively inhibited by Yanglingmycin. Meanwhile, the addition of exogenous fatty acid resulted in the decrease or even loss of the antibacterial activity of Yanglingmycin. These findings implied that Yanglingmycin might take effect by inhibiting the activity of Accase, which resulted in the blockade of fatty acids and lipids biosynthesis.

Efficient Synthesis and Antibacterial Evaluation of (±)-Yanglingmycin and Its Analogues

An efficient synthetic route was developed for the large-scale preparation of (±)-Yanglingmycin and its analogues. Three series of derivatives of (±)-Yanglingmycin were synthesized and the structures of all compounds were elucidated by analyses of NMR and ESI-MS spectra data. Moreover, their antibacterial activities against seven species of bacteria were systematically evaluated by the micro-broth dilution method, most of which displayed considerable activity. It was worth noting that compounds 5b, 5c, 5d, 6g, and 7 were found to be the most promising leading candidates, with peak MIC values of 0.98 μg·mL⁻¹ for Bacillus subtilis, which is superior to positive controls (MIC = 3.91 μg·mL⁻¹). The above results might lay the firm foundation for the design and synthesis of novel antibacterial drugs based on (±)-Yanglingmycin.

Design and Synthesis of New 2-Aryl-4,5-Dihydro-thiazole Analogues: In Vitro Antibacterial Activities and Preliminary Mechanism of Action

Sixty 2-aryl-4,5-dihydrothiazoles were designed and synthesized in yields ranging from 64% to 89% from cysteine and substituted-benzonitriles via a novel metal- and catalyst-free method. The structures of the title compounds were confirmed mainly by NMR spectral data analysis. Antibacterial activity assays showed that the compounds (S)-2-(2′-hydroxyphenyl)-4-hydroxy-methyl-4,5-dihydrothiazole (7h) and (R)-2-(2′-hydroxyphenyl)-4-hydroxymethyl-4,5-dihydro-thiazole (7h′) exhibited significant inhibition against Ralstonia solanacearum, Pseudomonas syringae pv. actinidiae, Bacillus subtilis and Bacillus cereus, with minimum inhibitory concentrations (MICs) ranging from 3.91 to 31.24 μg·mL⁻¹. The effect of substituents showed that not only electron-withdrawing groups, but also electron-donating groups could abolish the antibacterial activities unless a 2′-hydroxy group was introduced on the 2-aryl substituent of the 4,5-dihydrothiazole analogues. The results of scanning electron microscope (SEM) and fatty acid exposure experiments indicated that these antibacterial compounds influence fatty acid synthesis in the tested bacteria.

Synthesis and antibacterial activities of Yanglingmycin analogues

The synthesis of Yanglingmycin and its enantiomer, along with eighteen Yanglingmycin analogues is reported. The structures were confirmed mainly by analyses of NMR spectral data. Antibacterial activity assays showed that Yanglingmycin and some of its analogues exhibited significant antibacterial activities against two important agricultural pathogenic bacteria, Ralstonia solanacearum and Pseudomonas syringae pv. actinidiae, with minimum inhibitory concentration (MIC) values ranging from 3.91 to 15.62 µg/mL. The antibacterial activities exhibited by Yanglingmycin and its analogues are promising, suggesting potential in the development of compounds for novel bactericides.