Stereochemical Determination and Complex Biosynthetic Assembly of Etnangien, a Highly Potent RNA Polymerase Inhibitor from the Myxobacterium Sorangium cellulosum

Abstract

A potent novel analogue of the natural macrolide antibiotic etnangien, a structurally unique RNA polymerase inhibitor from myxobacteria, is reported. It may be readily obtained from fermentation broths of Sorangium cellulosum and shows high antibiotic activity, comparable to that of etnangien. However, it is much more readily available than the notoriously labile authentic natural product itself. Importantly, it is stable under neutral conditions, allowing for elaborate NMR measurements for assignment of the 12 hydroxyl- and methyl-bearing stereogenic centers. The full absolute and relative stereochemistries of these complex polyketides were determined by a combination of extensive high-field NMR studies, including J-based configuration analysis, molecular modeling, and synthetic derivatization in combination with an innovative method based on biosynthetic studies of this polyketide which is also presented here. A first look into the solution conformation and 3D structure of these promising macrolide antibiotics is reported. Finally, the complete biosynthetic gene cluster was analyzed in detail, revealing a highly unusual and complex trans-AT type polyketide biosynthesis, which does not follow colinearity rules, most likely performs programmed iteration as well as module skipping, and exhibits HMG-CoA box-directed methylation.

Total synthesis of antibiotics: recent achievements, limitations, and perspectives

“…A complete assignment of configuration for all stereocenters in the etnangien molecule was accomplished by a combination of high-field NMR experiments, including 3 J CH coupling constants analysis, molecular modeling and genetic analysis of polyketide synthase cluster. (Menche, 2008) The structure of etnangien features a 22-membered macrolactone with a rather rare tetra-syn,anti-stereohexad and an exceptionally long and sensitive polyenic side chain terminated with a carboxylic acid. In their retrosynthetic analysis of etnangien, (Li, 2009) Menche group dissected the target molecule at the C13-C14, C31-C32 double bonds and at the lactone functionality in three fragments of approximately equal complexity.…”

Stereoselective Total Synthesis of Etnangien and Etnangien Methyl Ester

“…1,2 Etnangien together with the more readily available methyl ester 2, 3 present highly potent antibiotics, with average IC 50 values in the submicromolar range, both in vitro and in vivo. [1][2][3] They are effective against a broad panel of Gram-positive bacteria, especially those belonging to the actinomycetes. 1 On a molecular level, they constitute inhibitors of RNA-polymerase.…”

“…However, preclinical advancement is severely hampered by the notorious instability of etnangien (1), which has been associated to the polyene side chain. 1,3 The unique architecture of the etnangiens is characterized by a 22-membered macrolactone ring with two alkenes and a polyunsaturated side chain with seven E-configured double bonds. In total, they contain twelve methyl and hydroxyl-bearing stereogenic centers.…”

“…Although originally reported as a planar structure, the absolute and relative stereochemistry has been determined in our group in cooperation with the group of Rolf Müller by a combination of J-based configurational analysis with molecular modeling, chemical derivatization and an innovative method based on gene-cluster analysis. 3 First SAR data suggest that a free acid is not essential for biological activity and that it is possible to modify and stabilize the etnangien structure, yet still retain activity. Truncation of the side chain, however, leads to significant loss of activity, which suggests this region to be part of the pharmacophore region.…”

Design, synthesis and biological evaluation of simplified side chains of the macrolide antibiotic etnangien

“…5 Furthermore, a first and so far only total synthesis of etnangien (1) and its equipotent methyl ester derivative (2) have been accomplished in our group. 6,7 In connection with the total synthesis a few simplified etnangien analogues have been obtained and biologically evaluated against a range of Gram-positive and negative bacteria Herein, we report the synthesis of the first side chain analogues of etnangien and their evaluation for antibacterial activity against Gram-positive bacteria and one Gram-negative bacterium.…”