Glenthmycins A-M: Macrocyclic Spirotetronate Polyketide Antibacterials from the Australian Pasture Plant-Derived Streptomyces sp. CMB-PB041.

Chemical investigation of Australian pasture plant-derived Streptomyces sp. CMB-PB041, supported by miniaturized cultivation profiling and molecular network analysis, led to the isolation and characterization of 13 new macrocyclic spirotetronates, glenthmycins A-M ( 1 - 13 ), with structures assigned by detailed spectroscopic analysis, chemical degradation and derivatization, and mechanistic and biosynthetic considerations. Hydrolysis of glenthmycin B ( 2 ) yielded the aglycone 14 , whose structure and absolute configuration were secured by X-ray analysis, along with the unexpected amino sugar residues glenthose lactams A ( 15 ) and B ( 16 ), with Mosher analysis of 15 facilitating assignment of absolute configurations of the amino sugar. While the glenthmycins proved to be acid stable, treatment of isomeric glenthmycins (i.e., 3 , 6 , and 8 ) with base catalyzed rapid intramolecular trans -esterification to regio-isomeric mixtures (i.e., 3 + 6 + 8 ). Exposure of 5 to base achieved the same intramolecular trans -esterification and was instrumental in detecting and tentatively identifying two additional minor co-metabolites, glenthmycins N ( 19 ) and O ( 20 ). A structure-activity relationship analysis carried out on 1 - 13 and the semisynthetic analogues 14 and 21 - 26 revealed a promising Gram +ve antibacterial pharmacophore, effective against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococci (VRE), but with no detectable cytotoxicity to eukaryotic cells (i.e., fungal and human carcinoma). Of particular note, the semisynthetic analogue glenthmycin K 9-valerate ( 26 ) was unique among glenthmycins in potently inhibiting growth of the full panel of Gram +ve pathogens (IC 50 0.2-1.6 μM). We conclude with an observation that any future evaluation of the antibacterial potential of glenthmycins and related macrocyclic spirotetronates may do well to include important soil-derived Gram +ve pathogens, such as Bacillus anthrax , Clostridium botulinum , and Rhodococcus equi , the causative agents of anthrax, botulism, and livestock pneumonia.