Chivosazol A – Aufklärung der absoluten und relativen Konfiguration

Design and Synthesis of Simplified Polyketide Analogs: New Modalities beyond the Rule of 5

Natural products provide important lead structures for development of pharmaceutical agents or present attractive tools for medicinal chemistry. However, structurally complex and thus less accessible metabolites defying conventional drug-like properties, as expressed by Pfizer's rule of five, have received less attention as medicinal leads. Traditionally, research focus has been on realizing total syntheses rather than developing more readily available analogs to resolve the critical supply issue. However, very recent studies with complex myxobacterial polyketides have demonstrated that considerable structural simplification may be realized with retention of biological potencies. The context, underlying rationale and importance of tailored synthetic strategies of three such case studies are presented, which may inspire further related activities and may eventually help exploiting the largely untapped biological potential of complex metabolites in general.

Modular Fragment Synthesis and Bioinformatic Analysis Propose a Revised Vancoresmycin Stereoconfiguration

Elaborate fragments of the proposed stereostructure of the complex polyketide antibiotic vancoresmycin have been synthesized in a stereoselective fashion based on a modular and convergent approach. Significant nuclear magnetic resonance differences in one of these subunits compared with the natural product question the proposed stereoconfiguration. Consequently, an extensive bioinformatics analysis of the biosynthetic gene cluster was carried out, leading to a revised stereoconfigurational proposal for this highly potent antibiotic.

An Expedient Total Synthesis of Chivosazole F: an Actin-Binding Antimitotic Macrolide from the Myxobacterium Sorangium Cellulosum

A unified strategy for the chemical synthesis of the chivosazoles is described. This strategy is based on two closely related approaches involving the late‐stage installation of the isomerization‐prone (2Z,4E,6Z,8E)‐tetraenoate motif, and an expedient fragment‐assembly procedure. The result is a highly convergent total synthesis of chivosazole F through the orchestration of three mild Pd/Cu‐mediated Stille cross‐coupling reactions, including the use of a one‐pot, site‐selective, three‐component process, in combination with controlled installation of the requisite alkene geometry.

Isolation, Structure Elucidation, and (Bio)Synthesis of Haprolid, a Cell-Type-Specific Myxobacterial Cytotoxin

Myxobacteria are well-established sources for novel natural products exhibiting intriguing bioactivities. We here report on haprolid (1) isolated from Byssovorax cruenta Har1. The compound exhibits an unprecedented macrolactone comprising four modified amino acids and a polyketide fragment. As configurational assignment proved difficult, a bioinformatic analysis of the biosynthetic gene cluster was chosen to predict the configuration of each stereocenter. In-depth analysis of the corresponding biosynthetic proteins established a hybrid polyketide synthase/nonribosomal peptide synthetase origin of haprolid and allowed for stereochemical assignments. A subsequent total synthesis yielded haprolid and corroborated all predictions made. Intriguingly, haprolid showed cytotoxicity against several cell lines in the nanomolar range whereas other cells were almost unaffected by treatment with the compound.

An Amidinohydrolase Provides the Missing Link in the Biosynthesis of Amino Marginolactone Antibiotics

Desertomycin A is an aminopolyol polyketide containing a macrolactone ring. We have proposed that desertomycin A and similar compounds (marginolactones) are formed by polyketide synthases primed not with γ-aminobutanoyl-CoA but with 4-guanidinylbutanoyl-CoA, to avoid facile cyclization of the starter unit. This hypothesis requires that there be a final-stage de-amidination of the corresponding guanidino-substituted natural product, but no enzyme for such a process has been described. We have now identified candidate amidinohydrolase genes within the desertomycin and primycin clusters. Deletion of the putative desertomycin amidinohydrolase gene dstH in Streptomyces macronensis led to the accumulation of desertomycin B, the guanidino form of the antibiotic. Also, purified DstH efficiently catalyzed the in vitro conversion of desertomycin B into the A form. Hence this amidinohydrolase furnishes the missing link in this proposed naturally evolved example of protective-group chemistry.

Paleo-soraphens: chemical total syntheses and biological studies

To provide a picture of the hypothetical evolutionary optimization of soraphen four additional paleo-soraphens and their biological profiles are described.

Disciformycins A and B: 12-membered macrolide glycoside antibiotics from the myxobacterium Pyxidicoccus fallax active against multiresistant staphylococci

Two macrolide glycosides with a unique scaffold were isolated from cultures of the myxobacterium Pyxidicoccus fallax. Their structures, including absolute configurations, were elucidated by a combination of NMR, MS, degradation, and molecular modeling techniques. Analysis of the proposed biosynthetic gene cluster led to insights into the biosynthesis of the polyketide and confirmed the structure assignment. The more active compound, disciformycin B, potently inhibits methicillin- and vancomycin-resistant Staphylococcus aureus.

Structure and biosynthetic assembly of gulmirecins, macrolide antibiotics from the predatory bacterium Pyxidicoccus fallax

The gulmirecins constitute a new class of glycosylated macrolides that were isolated from the predatory bacterium Pyxidicoccus fallax HKI 727. Their structures were solved by a combination of NMR spectroscopic experiments and chemical derivatization. Analysis of the annotated gulmirecin gene cluster complemented the configurational assignment and provided insights into the stereochemical course of the biosynthetic assembly. The gulmirecins exhibit strong activity against staphylococci, including methicillin-resistant Staphylococcus aureus, but no cytotoxic effects on human cells.

Biosynthetic code for divergolide assembly in a bacterial mangrove endophyte

Divergolides are structurally diverse ansamycins produced by a bacterial endophyte (Streptomyces sp.) of the mangrove tree Bruguiera gymnorrhiza. By genomic analyses a gene locus coding for the divergolide pathway was detected. The div gene cluster encodes genes for the biosynthesis of 3-amino-5-hydroxybenzoate and the rare extender units ethylmalonyl-CoA and isobutylmalonyl-CoA, polyketide assembly by a modular type I polyketide synthase (PKS), and enzymes involved in tailoring reactions, such as a Baeyer-Villiger oxygenase. A detailed PKS domain analysis confirmed the stereochemical integrity of the divergolides and provided valuable new insights into the formation of the diverse aromatic chromophores. The bioinformatic analyses and the isolation and full structural elucidation of four new divergolide congeners led to a revised biosynthetic model that illustrates the formation of four different types of ansamycin chromophores from a single polyketide precursor.