Additional congeners of the macrolide neaumycin: structure revision and biological activity

Stereoselective synthesis of the northern hemisphere of the proposed structure of neaumycin B

The stereoselective synthesis of the northern hemisphere (C20-C41) of the purported structure of the extremely potent anticancer natural product neaumycin B has been accomplished. Twelve out of nineteen asymmetry centers have been installed chemically. The key highlights of this synthesis include the Krische iridium catalyzed anti-diastereoselective carbonyl crotylation, the Crimmins aldol reaction, HWE olefination, CBS reduction, vanadium promoted stereoselective epoxidation, Evans methylation and spiroketalization.

Total Synthesis of the Proposed Structure of Neaumycin B

The total synthesis of the proposed structure of anti-glioblastoma natural product neaumycin B was achieved in 22 steps (longest linear sequence). The synthesis features HCl-mediated [6,6]-spiroketalization, a combination of Krische iridium-catalyzed crotylation, Marshall palladium-catalyzed propargylation, Fürstner nickel-catalyzed regio- and enantioselective vicinal monoprotected diol formation, Brown crotylation and asymmetric halide-aldehyde cycloaddition, so as to establish the challenging contiguous stereocenters.

Synthesis of the C28-C41 Side Chain of (Proposed) Neaumycin B: A Contiguous Stereohextet

The C28-C41 side-chain of the proposed structure of neaumycin B, which encompasses a contiguous stereohextet, is prepared in 8 steps (longest linear sequence). Convergency is maximized via Williams' Felkin-Anh-selective triorganozincate-mediated vinylation of an α,β-stereogenic aldehyde. The relative stereochemical assignment of the C33-C35 stereotriad was accomplished via the 13C NMR analysis of the related acetonide. Relative stereochemistry of C33 carbinol and C36-C37 glycidol was determined by comparative NOE analysis of the related diastereomeric furans.

Total Synthesis of the Reported Structure of Neaumycin B

The stereoselective total synthesis of structure 1 assigned to the macrolide natural product neaumycin B is reported in a 2.3% overall yield on 90 mg scale. The synthesis features a gram-scale nickel-catalyzed reductive cross-coupling/spiroketalization tactic to construct the spiroketal core of neaumycin B. The stereostructures of the C3-C6, C8-C14, and C20-C41 segments of synthetic neaumycin B were unambiguously verified by X-ray crystallography.

Ruthenium-Catalyzed Butadiene-Mediated Crotylation and Oxazaborolidine-Catalyzed Vinylogous Mukaiyama Aldol Reaction for The Synthesis of C1-C19 and C23-C35 of Neaumycin B

Neaumycin B is a femtomolar inhibitor of U87 human glioblastoma. Using a newly developed anti-diastereoselective ruthenium-catalyzed butadiene-mediated crotylation of primary alcohol proelectrophiles via hydrogen auto-transfer, as well as a novel variant of the catalytic asymmetric vinylogous Mukaiyama aldol (VMA) reaction applicable to linear aliphatic aldehydes and terminally methylated dienyl ketene acetals, preparation of the key C1-C19 and C23-C35 substructures of neaumycin B is achieved in 12 and 7 steps (LLS), respectively.

Stereoselective Synthesis of the Southern Hemisphere Acyclic Domain of Neaumycin B

A stereocontrolled synthetic entry to the southern hemisphere C3-C17 acyclic domain of neaumycin B, a highly potent cytotoxic macrolide natural product, has been developed. The present synthesis is based on (i) a tandem olefin cross-metathesis/hemiacetalization/intramolecular oxa-Michael addition, (ii) a regioselective reductive acetal opening for differential protection of the C14 hydroxy group, (iii) a Horner-Wadsworth-Emmons reaction for the stereoselective formation of the C8-C9 olefin, and (iv) a Corey-Bakshi-Shibata asymmetric reduction to create the C7 stereogenic center.

Blocks in the pseudouridimycin pathway unlock hidden metabolites in the Streptomyces producer strain

We report a metabolomic analysis of Streptomyces sp. ID38640, a soil isolate that produces the bacterial RNA polymerase inhibitor pseudouridimycin. The analysis was performed on the wild type, on three newly constructed and seven previously reported mutant strains disabled in different genes required for pseudouridimycin biosynthesis. The results indicate that Streptomyces sp. ID38640 is able to produce, in addition to lydicamycins and deferroxiamines, as previously reported, also the lassopeptide ulleungdin, the non-ribosomal peptide antipain and the osmoprotectant ectoine. The corresponding biosynthetic gene clusters were readily identified in the strain genome. We also detected the known compound pyridindolol, for which we propose a previously unreported biosynthetic gene cluster, as well as three families of unknown metabolites. Remarkably, the levels of most metabolites varied strongly in the different mutant strains, an observation that enabled detection of metabolites unnoticed in the wild type. Systematic investigation of the accumulated metabolites in the ten different pum mutants identified shed further light on pseudouridimycin biosynthesis. We also show that several Streptomyces strains, able to produce pseudouridimycin, have distinct genetic relationship and metabolic profile with ID38640.

A cyclopeptide and three oligomycin-class polyketides produced by an underexplored actinomycete of the genus Pseudosporangium

Aside from the well-studied conventional actinomycetes such as Streptomyces, the less investigated genera of actinomycetes also represent a promising source of natural products. Genome mining indicated that members of the underexplored genus Pseudosporangium, from which no secondary metabolites have been reported to date, may harbor the biosynthetic machinery for the formation of novel natural products. The strain RD062863, that is available at a public culture collection, was obtained and subjected to metabolite analysis, which resulted in the discovery of a novel cyclopeptide, pseudosporamide (1), along with three new oligomycin-class polyketides, pseudosporamicins A–C (2–4). The unusual structure of compound 1, featured by a biaryl-bond bridging across a tripeptide scaffold, N-acetyl-ʟ-Tyr-ʟ-Pro-ʟ-Trp, was determined by a combination of spectroscopic analyses, chemical derivatization, ECD calculation, and DFT-based theoretical chemical shift calculation, revealing the presence of an (S_a)-axial chirality around the biaryl bond. Compounds 2–4 lacked hydroxylation on the side chain of the spiroacetal rings, which showed clear contrast to other oligomycin congeners and related polyketides with ring-truncation or expansion. The new macrolides 2–4 displayed potent antimicrobial activity against the Gram-positive bacterium Kocuria rhizohpila and the plant pathogenic fungus Glomerella cingulata. All compounds showed moderate cytotoxicity against P388 murine leukemia cells with IC₅₀ values in the micromolar to submicromolar ranges. These results exemplified the validity of phylogeny-focused strain selection combined with biosynthetic gene-directed genome mining for the efficient discovery of new natural products.

Integration of Genomic Data with NMR Analysis Enables Assignment of the Full Stereostructure of Neaumycin B, a Potent Inhibitor of Glioblastoma from a Marine-Derived Micromonospora

The microbial metabolites known as the macrolides are some of the most successful natural products used to treat infectious and immune diseases. Describing the structures of these complex metabolites, however, is often extremely difficult due to the presence of multiple stereogenic centers inherent in this class of polyketide-derived metabolites. With the availability of genome sequence data and a better understanding of the molecular genetics of natural product biosynthesis, it is now possible to use bioinformatic approaches in tandem with spectroscopic tools to assign the full stereostructures of these complex metabolites. In our quest to discover and develop new agents for the treatment of cancer, we observed the production of a highly cytotoxic macrolide, neaumycin B, by a marine-derived actinomycete bacterium of the genus Micromonospora. Neaumycin B is a complex polycyclic macrolide possessing 19 asymmetric centers, usually requiring selective degradation, crystallization, derivatization, X-ray diffraction analysis, synthesis, or other time-consuming approaches to assign the complete stereostructure. As an alternative approach, we sequenced the genome of the producing strain and identified the neaumycin gene cluster ( neu). By integrating the known stereospecificities of biosynthetic enzymes with comprehensive NMR analysis, the full stereostructure of neaumycin B was confidently assigned. This approach exemplifies how mining gene cluster information while integrating NMR-based structure data can achieve rapid, efficient, and accurate stereostructural assignments for complex macrolides.