The Myxobacterial Antibiotic Myxovalargin: Biosynthesis, Structural Revision, Total Synthesis, and Molecular Characterization of Ribosomal Inhibition.

Resistance of bacterial pathogens against antibiotics is declared by WHO as a major global health threat. As novel antibacterial agents are urgently needed, we re-assessed the broad-spectrum myxobacterial antibiotic myxovalargin and found it to be extremely potent against Mycobacterium tuberculosis. To ensure compound supply for further development, we studied myxovalargin biosynthesis in detail enabling production via fermentation of a native producer. Feeding experiments as well as functional genomics analysis suggested a structural revision, which was eventually corroborated by the development of a concise total synthesis. The ribosome was identified as the molecular target based on resistant mutant sequencing, and a cryo-EM structure revealed that myxovalargin binds within and completely occludes the exit tunnel, consistent with a mode of action to arrest translation during a late stage of translation initiation. These studies open avenues for structure-based scaffold improvement toward development as an antibacterial agent.

Cystobactamids 920-1 and 920-2: Assignment of the Constitution and Relative Configuration by Total Synthesis.

Total synthesis of cystobactamid 920-1 and its epimer has allowed an unambiguous assignment of the relative and absolute configuration of the natural product. A careful structural analysis of each isomer using both NMR and computational techniques also prompted a constitutional revision of the structures originally reported for cystobactamids 920-1 and 920-2, and has provided further insight into the unique conformational preferences of the cystobactamid family.

Discovery and Total Synthesis of Natural Cystobactamid Derivatives with Superior Activity against Gram-Negative Pathogens.

Antibiotic discovery and development is challenging as chemical scaffolds of synthetic origin often lack the required pharmaceutical properties, and the discovery of novel ones from natural sources is tedious. Herein, we report the discovery of new cystobactamids with a significantly improved antibacterial profile in a detailed screening of myxobacterial producer strains. Some of these new derivatives display antibacterial activities in the low-µg mL-1 range against Gram-negative pathogens, including clinical isolates of Klebsiella oxytoca, Pseudomonas aeruginosa, and fluoroquinolone-resistant Enterobacteriaceae, which were not observed for previously reported cystobactamids. Our findings provide structure-activity relationships and show how pathogen resistance can be overcome by natural scaffold diversity. The most promising derivative 861-2 was prepared by total synthesis, enabling further chemical optimization of this privileged scaffold.

Studies on the synthesis of peptides containing dehydrovaline and dehydroisoleucine based on copper-mediated enamide formation.

The preparation of peptide fragments containing dehydrovaline and dehydroisoleucine moieties present in the antibiotic myxovalargin is reported. Peptide formation is based on a copper-mediated C-N cross-coupling protocol between an acyl amide and a peptidic vinyl iodide. The presence of a neighboring arginine in the vinyl iodide posed a challenge with respect to the choice of the protecting group and the reaction conditions. It was found that ornithine - a suitable precursor - is better suited than arginine for achieving good yields for the C-N cross-coupling reaction. The optimized conditions were utilized for the synthesis of peptides 32, 33, 39 and 40 containing a neighboring ornithine as well as for the tripeptide 44 containing dehydroisoleucine with the correct stereochemistry.