Studies on the Synthesis of Landomycin A. Synthesis of the Originally Assigned Structure of the Aglycone, Landomycinone, and Revision of Structure

Rapid spectrophotometric detection for optimized production of landomycins and characterization of their therapeutic potential

Microbial-derived natural products remain a major source of structurally diverse bioactive compounds and chemical scaffolds that have the potential as new therapeutics to target drug-resistant pathogens and cancers. In particular, genome mining has revealed the vast number of cryptic or low-yield biosynthetic gene clusters in the genus Streptomyces. However, low natural product yields-improvements to which have been hindered by the lack of high throughput methods-have slowed the discovery and development of many potential therapeutics. Here, we describe our efforts to improve yields of landomycins-angucycline family polyketides under investigation as cancer therapeutics-by a genetically modified Streptomyces cyanogenus 136. After simplifying the extraction process from S. cyanogenus cultures, we identified a wavelength at which the major landomycin products are absorbed in culture extracts, which we used to systematically explore culture medium compositions to improve total landomycin titers. Through correlational analysis, we simplified the culture optimization process by identifying an alternative wavelength at which culture supernatants absorb yet is representative of total landomycin titers. Using the subsequently improved sample throughput, we explored landomycin production during the culturing process to further increase landomycin yield and reduce culture time. Testing the antimicrobial activity of the isolated landomycins, we report broad inhibition of Gram-positive bacteria, inhibition of fungi by landomycinone, and broad landomycin resistance by Gram-negative bacteria that is likely mediated by the exclusion of landomycins by the bacterial membrane. Finally, the anticancer activity of the isolated landomycins against A549 lung carcinoma cells agrees with previous reports on other cell lines that glycan chain length correlates with activity. Given the prevalence of natural products produced by Streptomyces, as well as the light-absorbing moieties common to bioactive natural products and their metabolic precursors, our method is relevant to improving the yields of other natural products of interest.

Rapid spectrophotometric detection for optimized production of landomycins and characterization of their therapeutic potential

Microbial derived natural products remain a major source of structurally diverse bioactive compounds and chemical scaffolds that have potential as new therapeutics to target drug resistant pathogens and cancers. In particular, genome mining has revealed the vast number of cryptic or low yield biosynthetic gene clusters in the genus Streptomyces . Here, we describe our efforts to improve yields of landomycins - angucycline family polyketides under investigation as cancer therapeutics - by a genetically modified Streptomyces cyanogenus 136. After simplifying the extraction process from S. cyanogenus cultures, we identified a wavelength at which the major landomycin products absorb in culture extracts, which we used to systematically explore culture medium compositions to improve total landomycin titers. Through correlational analysis, we simplified the culture optimization process by identifying an alternative wavelength at which culture supernatants absorb yet is representative of total landomycin titers. Using the subsequently improved sample throughput, we explored landomycin production during the culturing process to further increase landomycin yield and reduce culture time. Testing the antimicrobial activity of the isolated landomycins, we report broad inhibition of Gram-positive bacteria, inhibition of fungi by landomycinone, and broad landomycin resistance by Gram-negative bacteria that is likely mediated by exclusion of landomycins by the bacterial membrane. Finally, the anticancer activity of the isolated landomycins against A549 lung carcinoma cells agrees with previous reports on other cell lines that glycan chain length correlates with activity. Given the prevalence of natural products produced by Streptomyces , as well as the light-absorbing moieties common to bioactive natural products and their metabolic precursors, our method is relevant to improving the yields of other natural products of interest.

Dearomatization/Spiroannulation of Halophenols Enables the Forging of Contiguous Quaternary Carbon Cyclohexadienones

A dearomatization/spiroannulation process has been successfully achieved between simple halophenols and α,β-unsaturated olefins under mild reaction conditions. This transformation addresses the chemoselectivity issue in the dearomatizative transformation of phenol scaffolds (6π-electron) caused by the SEAr process, enabling the construction of versatile cyclohexadienone frameworks containing contiguous quaternary all-carbon centers in high yields. Further studies have provided valuable insights into the process, revealing that debromination/spiroannulation occurs through the SRN1 pathway.

De novo asymmetric Achmatowicz approach to oligosaccharide natural products

The development and application of the asymmetric synthesis of oligosaccharides from achiral starting materials is reviewed. This de novo asymmetric approach centers around the use of asymmetric catalysis for the synthesis of optically pure furan alcohols in conjunction with Achmatowicz oxidative rearrangement for the synthesis of various pyranones. In addition, the use of a diastereoselective palladium-catalyzed glycosylation and subsequent diastereoselective post-glycosylation transformation was used for the synthesis of oligosaccharides. The application of this approach to oligosaccharide synthesis is discussed.

Dearomatization/Deiodination of o-Iodophenolic Compounds with α,β-Unsaturated Imines for Accessing Benzofuran Derivatives

A dearomatization/deiodination/rearomatization strategy for the [3 + 2] cyclization of o-iodophenolic substrates with α,β-unsaturated imines to construct various dihydrobenzofuran-related skeletons has been established. Tolerance to different functional groups has been tested. Mechanistic studies revealed that this domino reaction was possibly realized by the deiodination and tautomerization of the key dearomatized intermediate to generate a free phenolic O radical. Moreover, an anticancer agent 4 and an α-glucosidase inhibitor 5 with high bioactivities were successfully synthesized using this novel protocol.

Landomycins as glutathione-depleting agents and natural fluorescent probes for cellular Michael adduct-dependent quinone metabolism

Landomycins are angucyclines with promising antineoplastic activity produced by Streptomyces bacteria. The aglycone landomycinone is the distinctive core, while the oligosaccharide chain differs within derivatives. Herein, we report that landomycins spontaneously form Michael adducts with biothiols, including reduced cysteine and glutathione, both cell-free or intracellularly involving the benz[a]anthraquinone moiety of landomycinone. While landomycins generally do not display emissive properties, the respective Michael adducts exerted intense blue fluorescence in a glycosidic chain-dependent manner. This allowed label-free tracking of the short-lived nature of the mono-SH-adduct followed by oxygen-dependent evolution with addition of another SH-group. Accordingly, hypoxia distinctly stabilized the fluorescent mono-adduct. While extracellular adduct formation completely blocked the cytotoxic activity of landomycins, intracellularly it led to massively decreased reduced glutathione levels. Accordingly, landomycin E strongly synergized with glutathione-depleting agents like menadione but exerted reduced activity under hypoxia. Summarizing, landomycins represent natural glutathione-depleting agents and fluorescence probes for intracellular anthraquinone-based angucycline metabolism.

Highly regioselective and stereoselective synthesis of C-Aryl glycosides via nickel-catalyzed ortho-C-H glycosylation of 8-aminoquinoline benzamides

C-Aryl glycosides are of high value as drug candidates. Here a novel and cost-effective nickel catalyzed ortho-CAr-H glycosylation reaction with high regioselectivity and excellent α-selectivity is described. This method shows great functional group compatibility with various glycosides, showing its synthetic potential. Mechanistic studies indicate that C-H activation could be the rate-determining step.

Unified synthesis and cytotoxic activity of 8-O-methylfusarubin and its analogues

A simple and unified synthesis of four related pyranonaphthoquinone natural products, e.g. 8-O-methylfusarubin, 8-O-methylanhydrofusarubin, fusarubin and anhydrofusarubin, is reported. The key synthetic features include the precedented Diels-Alder cycloaddition to assemble the naphthalene skeleton, selective formylation and acetonylation and intramolecular acetalization to construct the pyran ring. Manipulation of the oxidation state of the naphthoquinone core was performed to construct the two analogues, fusarubin and anhydrofusarubin. This work also highlights an unprecedented directing effect of the hydroxymethylene group in the selective hypervalent iodine-mediated quinone oxidation. The four synthetic compounds were evaluated for their in vitro cytotoxic activities against six human cancer cells. 8-O-Methylfusarubin was the most potent analogue and displayed excellent cytotoxic activity against MCF-7 breast cancer cells with an IC₅₀ value of 1.01 μM with no cytotoxic effect on noncancerous Vero cells, which could potentially be a promising lead compound for anti-breast cancer drug discovery.

A general strategy for diverse syntheses of anhydrolandomycinone, tetrangulol, and landomycinone

A protecting group promoted-CH arylation strategy for the total synthesis of landomycinone and related core structures.

Palladium-Catalyzed One-Pot Conversion of Aldehydes and Ketones into 4-Substituted Homopropargyl Alcohols and 5-En-3-yn-1-ols †

Sequential treatment of 2,3-dichloropropene with magnesium and n-BuLi generated the equivalent of 1,3-dilithiopropyne, which adds regiospecifically to aldehydes and ketones to produce homopropargyl alcohols. The lithium acetylide intermediate formed in this protocol can be further reacted with aromatic and vinyl halides, under palladium catalysis, to produce 4-substituted homopropargyl alcohols and 5-en-3-yn-1-ols, respectively, in one-pot with good overall yields.

Synthesis of a landomycinone skeleton via Masamune–Bergmann cyclization

In this report, a synthetic study of landomycinone via Masamune–Bergmann cyclization is described.

Tailoring enzymes involved in the biosynthesis of angucyclines contain latent context-dependent catalytic activities

Comparison of homologous angucycline modification enzymes from five closely related Streptomyces pathways (pga, cab, jad, urd, lan) allowed us to deduce the biosynthetic steps responsible for the three alternative outcomes: gaudimycin C, dehydrorabelomycin, and 11-deoxylandomycinone. The C-12b-hydroxylated urdamycin and gaudimycin metabolites appear to be the ancestral representatives from which landomycins and jadomysins have evolved as a result of functional divergence of the ketoreductase LanV and hydroxylase JadH, respectively. Specifically, LanV has acquired affinity for an earlier biosynthetic intermediate resulting in a switch in biosynthetic order and lack of hydroxyls at C-4a and C-12b, whereas in JadH, C-4a/C-12b dehydration has evolved into an independent secondary function replacing C-12b hydroxylation. Importantly, the study reveals that many of the modification enzymes carry several alternative, hidden, or ancestral catalytic functions, which are strictly dependent on the biosynthetic context.

Delineation of gilvocarcin, jadomycin, and landomycin pathways through combinatorial biosynthetic enzymology

The exact sequence of events in biosyntheses of natural products is essential not only to understand and learn from nature's strategies and tricks to assemble complex natural products, but also for yield optimization of desired natural products, and for pathway engineering and muta-synthetic preparation of analogues of bioactive natural products. Biosyntheses of natural products were classically studied applying in vivo experiments, usually by combining incorporation experiments with stable-isotope labeled precursors with cross-feeding experiments of putative intermediates. Later genetic studies were dominant, which consist of gene cluster determination and analysis of gene inactivation experiments. From such studies various biosynthetic pathways were proposed, to a large extent just through in silico analyses of the biosynthetic gene clusters after DNA sequencing. Investigations of the complex biosyntheses of the angucycline group anticancer drugs landomycin, jadomycin and gilvocarcin revealed that in vivo and in silico studies were insufficient to delineate the true biosynthetic sequence of events. Neither was it possible to unambiguously assign enzyme activities, especially where multiple functional enzymes were involved. However, many of the intriguing ambiguities could be solved after in vitro reconstitution of major segments of these pathways, and subsequent systematic variations of the used enzyme mixtures. This method has been recently termed 'combinatorial biosynthetic enzymology'.

Angucyclines: Biosynthesis, mode-of-action, new natural products, and synthesis

Covering: 1997 to 2010. The angucycline group is the largest group of type II PKS-engineered natural products, rich in biological activities and chemical scaffolds. This stimulated synthetic creativity and biosynthetic inquisitiveness. The synthetic studies used five different strategies, involving Diels-Alder reactions, nucleophilic additions, electrophilic additions, transition-metal mediated cross-couplings and intramolecular cyclizations to generate the angucycline frames. Biosynthetic studies were particularly intriguing when unusual framework rearrangements by post-PKS tailoring oxidoreductases occurred, or when unusual glycosylation reactions were involved in decorating the benz[a]anthracene-derived cores. This review follows our previous reviews, which were published in 1992 and 1997, and covers new angucycline group antibiotics published between 1997 and 2010. However, in contrast to the previous reviews, the main focus of this article is on new synthetic approaches and biosynthetic investigations, most of which were published between 1997 and 2010, but go beyond, e.g. for some biosyntheses all the way back to the 1980s, to provide the necessary context of information.

Landomycins P-W, cytotoxic angucyclines from Streptomyces cyanogenus S-136

Streptomyces cyanogenus S-136 is the producer of previously reported landomycins A-D. An analysis of minor products of the strain led to isolation and structure elucidation of eight new congeners, named landomycins P-W (5, 6, 3, 17, 9, 10, 15, 7), along with 10 other known angucyclin(on)es. The structures of the new compounds were established from their NMR and mass spectrometry data. The activity of these angucyclin(on)es was determined using MCF-7 (estrogen responsive) and MDA-231 (estrogen refractory) breast cancer cell lines. Cell viability assays showed that anhydrolandomycinone (2), landomycinone (11), and landomycin A (16) showed the best combined activities in both MCF-7 and MDA-231 assays, with 2 being the most potent in the former and 11 and 16 in the latter. These data reveal that some of the aglycones are equipotent to the principle product 16, which contains the longest saccharide chain. Specifically, anhydrolandomycinone (2) was the most active against MCF-7 cells (IC(50) = 1.8 μM). Compounds with shorter saccharidal moieties were less potent against MCF-7. The fact that the most active landomycins have either long penta- or hexasaccharide chains or no sugars at all suggests that the large compounds may act by a different mode of action than their small sugar-free congeners. The results presented here provide more insights into the structure-activity relationship of landomycins.

Synthesis of pyrroles: reaction of chromium N-alkylaminocarbene complexes with α,β-unsaturated aldehydes

N-alkylaminocarbene complexes of chromium were found to react with alpha,beta-unsaturated aldehydes to give pyrroles in good yields.

Identification of the function of gene lndM2 encoding a bifunctional oxygenase-reductase involved in the biosynthesis of the antitumor antibiotic landomycin E by Streptomyces globisporus 1912 supports the originally assigned structure for landomycinone

The angucycline antibiotic family of the landomycins displays potent antitumor activity. To elucidate early post polyketide synthase (PKS) tailoring steps of the landomycin E biosynthetic pathway in Streptomyces globisporus 1912, the mutant S. globisporus M12 was prepared through gene replacement experiment of lndM2. It encodes an enzyme with putative oxygenase and reductase domains, according to sequencing of the gene and its counterpart lanM2 from S. cyanogenus S136 landomycin A biosynthetic gene cluster. The isolation of the novel shunt products 11-hydroxytetrangomycin and 4-hydroxytetrangomycin along with the well-known angucyclines tetrangomycin and tetrangulol from the culture of S. globisporus M12 provides evidence for the involvement of lndM2 in the early biosynthetic pathway of the landomycins, in particular in the formation of the alicyclic 6-hydroxy function of the landomycin aglycon. We therefore propose LndM2 to be responsible for both hydroxylation of the 6-position and its subsequent reduction. These reactions are necessary before the glycosylation reactions can occur. The results are in agreement with the originally published structure of landomycin but do not support the recently suggested revised structure.