Polyene Side Chain Research Articles

Maduraflavacins A-E, Unusual Phenyl Polyene Metabolites Produced by a Rare Marine-Derived Actinomadura sp.

Phenyl polyenes comprise a small family of bacterial natural products with broad and potent bioactivities, primarily found in actinobacteria. Here we report the discovery of five new phenyl polyene metabolites, maduraflavacins A-E (1-5), from a rare, marine-derived actinobacteria strain Actinomadura glauciflava NA03286. The structures of these natural products were determined by NMR spectroscopy, HRESIMS, LC-MS/MS, and chemical derivatization. All of these new maduraflavacins feature methyl substitutions at the polyene side chain, and maduraflavacins A-C (1-3) possessed a 1-N-β-d-glucosamine-(3 → 1)-O-β-d-glucopyranosyl-(3 → 1)-O-β-d-glucopyranosyl-(6 → 1)-O-β-d-glucopyranoside tetrasaccharide moiety via an amido linkage with a phenyl polyene skeleton. Compounds 1 and 2 showed weak antibacterial activities against the Gram-positive bacteria Staphylococcus aureus Sau 16339 and Micrococcus luteus, respectively.

Advances Toward Amphidinolides C, F and U: Isolations, Synthetic Studies and Total Syntheses.

Amphidinolides C, F, and U, including C2-C4 analogs, are highly cytotoxic marine macrolides, mainly isolated from dinoflagellates of the genus Amphidinium. All these polyketides share a 75 % or more similar structure, highlighted by a macrolactone ring, at least one trans-2,5-substituted-THF motif and a characteristic polyenic side chain. From their isolation and absolute configurational assignment, the total synthesis of these marine macrolides represented an intense challenge to the organic synthesis community over the last 15 years, with around 14 research groups engaged in this inspiring task. In the first part of this review, we present the different approaches to the isolation and characterization of these natural products, including the most recent analogs, which may cast doubt on the biogenetic origin of these compounds. The various synthetic approaches to the total synthesis of C, F, and U amphidinolides are presented in a second part, focusing on key reactions and/or innovative strategies. The review concludes in a third section summarizing the successful approaches leading to the total synthesis of one of the members of this amphidinolide subfamily.

Carotenoids, β-Apocarotenoids, and Retinoids: The Long and the Short of It.

Naturally occurring retinoids (retinol, retinal, retinoic acid, retinyl esters) are a subclass of β-apocarotenoids, defined by the length of the polyene side chain. Provitamin A carotenoids are metabolically converted to retinal (β-apo-15-carotenal) by the enzyme β-carotene-15,15′-dioxygenase (BCO1) that catalyzes the oxidative cleavage of the central C=C double bond. A second enzyme β-carotene-9′-10′-dioxygenase cleaves the 9′,10′ bond to yield β-apo-10′-carotenal and β-ionone. Chemical oxidation of the other double bonds leads to the generation of other β-apocarotenals. Like retinal, some of these β-apocarotenals are metabolically oxidized to the corresponding β-apocarotenoic acids or reduced to the β-apocarotenols, which in turn are esterified to β-apocarotenyl esters. Other metabolic fates such as 5,6-epoxidation also occur as for retinoids. Whether the same enzymes are involved remains to be understood. β-Apocarotenoids occur naturally in plant-derived foods and, therefore, are present in the diet of animals and humans. However, the levels of apocarotenoids are relatively low, compared with those of the parent carotenoids. Moreover, human studies show that there is little intestinal absorption of intact β-apocarotenoids. It is possible that they are generated in vivo under conditions of oxidative stress. The β-apocarotenoids are structural analogs of the naturally occurring retinoids. As such, they may modulate retinoid metabolism and signaling. In deed, those closest in size to the C-20 retinoids—namely, β-apo-14′-carotenoids (C-22) and β-apo-13-carotenone (C-18) bind with high affinity to purified retinoid receptors and function as retinoic acid antagonists in transactivation assays and in retinoic acid induction of target genes. The possible pathophysiologic relevance in human health remains to be determined.

Asperginols A and B, Diterpene Pyrones, from an Aspergillus sp. and the Structure Revision of Previously Reported Analogues.

Two new diterpene pyrones, asperginols A (1) and B (2), and four known analogues (3-6) were isolated from the endophytic fungus Aspergillus sp. HAB10R12. The structures and absolute configurations of these compounds were elucidated based on the analysis of their NMR, MS, and X-ray diffraction data. The revision of the absolute configurations at C-10, C-11, and C-14 of the known diterpene pyrones (3-6) and the determination of the configuration at the polyene side chain for compounds (4-6) were made using chemical methods and vibrational circular dichroism analysis. This group of diterpene pyrone compounds showed unique structural features including a 7/6/6 tricyclic diterpene moiety with an unusual trans-syn-trans stereochemical arrangement. Compound 6 showed moderate activity against the HT-29 colon cancer cell line.

Synthesis of One Double Bond-Inserted Retinal Analogs and Their Binding Experiments with Opsins: Preparation of Novel Red-Shifted Channelrhodopsin Variants.

In optogenetics, red-shifted channelrhodopsins (ChRs) are eagerly sought. We prepared six kinds of new chromophores with one double bond inserted into the polyene side chain of retinal (A1) or 3,4-didehydroretinal (A2), and examined their binding efficiency with opsins (ReaChR and ChrimsonR). All analogs bound with opsins to afford new ChRs. Among them, A2-10ex (an extra double bond is inserted at the C10-C11 position of A2) showed the greatest red-shift in the absorption spectrum of ChrimsonR, with a maximum absorbance at 654 nm (67 nm red-shifted from that of A1-ChrimsonR). Moreover, a long-wavelength spectral boundary of A2-10ex-ChrimsonR was extended to 756 nm, which reached into the far-red region (710-850 nm).

New Insights into the Biosynthesis Pathway of Polyketide Alkaloid Argimycins P in Streptomyces argillaceus.

Argimycins P are a recently identified family of polyketide alkaloids encoded by the cryptic gene cluster arp of Streptomyces argillaceus. These compounds contain either a piperideine ring, or a piperidine ring which may be fused to a five membered ring, and a polyene side chain, which is bound in some cases to an N-acetylcysteine moiety. The arp cluster consists of 11 genes coding for structural proteins, two for regulatory proteins and one for a hypothetical protein. Herein, we have characterized the post-piperideine ring biosynthesis steps of argimycins P through the generation of mutants in arp genes, the identification and characterization of compounds accumulated by those mutants, and cross-feeding experiments between mutants. Based in these results, a biosynthesis pathway is proposed assigning roles to every arp gene product. The regulation of the arp cluster is also addressed by inactivating/overexpressing the positive SARP-like arpRI and the negative TetR-like arpRII transcriptional regulators and determining the effect on argimycins P production, and through gene expression analyses (reverse transcription PCR and quantitative real-time PCR) of arp genes in regulatory mutants in comparison to the wild type strain. These findings will contribute to deepen the knowledge on the biosynthesis of piperidine-containing polyketides and provide tools that can be used to generate new analogs by genetic engineering and/or biocatalysis.

Total Synthesis of Polycavernosides A and B, Two Lethal Toxins from Red Alga.

Polycavernosides A and B are glycosidic macrolide natural products isolated as the toxins causing fatal human poisoning by the edible red alga Gracilaria edulis (Polycavernosa tsudai). Total synthesis of polycavernosides A and B has been achieved via a convergent approach. The synthesis of the macrolactone core structure is highlighted by the catalytic asymmetric syntheses of the two key fragments using hetero-Diels-Alder reaction and Kiyooka aldol reaction as the key steps, their union through Suzuki-Miyaura coupling, and Keck macrolactonization. Finally, glycosylation with the l-fucosyl-d-xylose unit and construction of the polyene side chain through Stille coupling completed the total synthesis of polycavernosides A and B.

Total syntheses of both enantiomers of amphirionin 4: A chemoenzymatic based strategy for functionalized tetrahydrofurans

The total syntheses of (−)-amphirionin-4 and (+)-amphirionin-4 have been achieved in a convergent and enantioselective manner. The tetrahydrofuranol cores of amphirionin-4 were constructed in optically active form by enzymatic resolution of racemic cis-3-hydroxy-5-methyldihydrofuran-2(3H)-one. The polyene side chain was efficiently synthesized using Stille coupling. The remote C8-stereocenter was constructed using the Nozaki-Hiyama-Kishi coupling reaction. Detailed 1H NMR studies of Mosher esters of (−)-amphirionin-4 and (+)-amphirionin-4 were carried out to support the assignment of the absolute configurations of the C-4 and C-8 asymmetric centers of amphirionin-4.

Identification by Genome Mining of a Type I Polyketide Gene Cluster from Streptomyces argillaceus Involved in the Biosynthesis of Pyridine and Piperidine Alkaloids Argimycins P.

Genome mining of the mithramycin producer Streptomyces argillaceus ATCC 12956 revealed 31 gene clusters for the biosynthesis of secondary metabolites, and allowed to predict the encoded products for 11 of these clusters. Cluster 18 (renamed cluster arp) corresponded to a type I polyketide gene cluster related to the previously described coelimycin P1 and streptazone gene clusters. The arp cluster consists of fourteen genes, including genes coding for putative regulatory proteins (a SARP-like transcriptional activator and a TetR-like transcriptional repressor), genes coding for structural proteins (three PKSs, one aminotransferase, two dehydrogenases, two cyclases, one imine reductase, a type II thioesterase, and a flavin reductase), and one gene coding for a hypothetical protein. Identification of encoded compounds by this cluster was achieved by combining several strategies: (i) inactivation of the type I PKS gene arpPIII; (ii) inactivation of the putative TetR-transcriptional repressor arpRII; (iii) cultivation of strains in different production media; and (iv) using engineered strains with higher intracellular concentration of malonyl-CoA. This has allowed identifying six new alkaloid compounds named argimycins P, which were purified and structurally characterized by mass spectrometry and nuclear magnetic resonance spectroscopy. Some argimycins P showed a piperidine ring with a polyene side chain (argimycin PIX); others contain also a fused five-membered ring (argimycins PIV-PVI). Argimycins PI-PII showed a pyridine ring instead, and an additional N-acetylcysteinyl moiety. These compounds seem to play a negative role in growth and colony differentiation in S. argillaceus, and some of them show weak antibiotic activity. A pathway for the biosynthesis of argimycins P is proposed, based on the analysis of proposed enzyme functions and on the structure of compounds encoded by the arp cluster.

Enantioselective Total Synthesis of (+)-Amphirionin-4.

An enantioselective total synthesis of (+)-amphirionin-4 has been accomplished in a convergent manner. The synthesis features an efficient enzymatic lipase resolution to access the tetrahydrofuranol core in optically active form. The functionalized tetrahydrofuran derivative was synthesized via an oxocarbenium ion-mediated highly diastereoselective syn-allylation reaction. The polyene side chain was synthesized using Stille coupling reactions. Nozaki-Hiyama-Kishi coupling was utilized to construct the C-8 stereocenter and complete the synthesis of (+)-amphirionin-4.

Total Synthesis of Amphirionin-4.

The first total synthesis of amphirionin-4 has been achieved using a combination of cross-coupling strategies to access the polyene side chain and a chlorohydrin-based approach to construct the tetrahydrofuranol core. The remote C9-stereocenter was introduced through a Nozaki-Hiyama-Kishi coupling that proceeded with remote stereoinduction.

Synthesis and SAR of Lehualide B: a marine-derived natural product with potent anti-multiple myeloma activity.

We report a concise and convergent laboratory synthesis of the rare marine natural product lehualide B that has led to the discovery that (1) this compound has low nanomolar activity against human multiple myeloma cells and (2) the anticancer effects of lehualide B and its analogues are selective (i.e., they are approximately 2-3 orders of magnitude less toxic to human breast cancer cells). Synthetic lehualide B is shown to be an effective inhibitor of complex I of the mitochondrial electron transport chain, with potency similar to that observed for the terrestrial natural products piericidin A1 and rotenone, an observation that led to the discovery that piericidin A1 is also selectively cytotoxic toward human multiple myeloma cells. Interestingly, synthetic derivatives of lehualide B that resemble verticipyrone (an established complex I inhibitor composed of a γ-pyrone and a simple monounsaturated hydrophobic chain) lack the potent antimyeloma activity of the natural product. Finally, the synthesis and evaluation of a collection of lehualide-inspired analogues led to the elucidation of structure-activity relationships for this rare natural product that established important roles for the substituted γ-pyrone headgroup and the skipped polyene side chain.

Modular synthesis of polyene side chain analogues of the potent macrolide antibiotic etnangien by a flexible coupling strategy based on hetero-bis-metallated alkenes

An efficient procedure for the concise synthesis of hetero-bis-metallated alkenes as useful building blocks for the modular access to highly elaborate polyenes and stabilized analogues is reported. By applying these bifunctional olefins in convergent Stille/Suzuki-Miyaura couplings, novel, carefully selected side chain analogues of the potent RNA polymerase inhibitor etnangien were synthesized by a modular late stage coupling strategy and evaluated for antibacterial and antiproliferative activities.

ChemInform Abstract: Efficient Synthesis of Diverse Hetero‐Bis‐Metalated Alkenes as Modular Reagents Towards Highly Conjugated and Isolated Olefinic Systems.

AbstractA convergent, palladium‐catalyzed fragment coupling strategy is applied to the concise and modular construction of the polyene side chains of the potent polyketide antibiotic etnangien.

Efficient synthesis of diverse hetero-bis-metallated alkenes as modular reagents towards highly conjugated and isolated olefinic systems

An efficient synthesis of diverse hetero-bis-metallated alkenes with conjugated and isolated olefin subunits is reported. Relying on those useful tin/boron reagents a convergent, palladium-catalyzed fragment coupling strategy has been developed as an elegant methodology to construct highly conjugated polyenes and stabilized olefinic analogues thereof. The utility of these reagents was further demonstrated in a concise and modular construction of extended polyene side chains of the potent polyketide antibiotic etnangien.

Synthesis and Biological Evaluation of 9Z-Retinoic Acid Analogs Having 2-Substituted Benzo[b]furan

Palladium-catalyzed cross-coupling reactions of a 2-substituted 3-iodobenzo[b]furans and stannanyl ester afforded the stereoselective production of 9Z-retinoic acid ester analogs in good yields. These esters were then converted to the corresponding acids via basic hydrolysis in excellent yields, and their biological activities were evaluated. The analog changed the connected position of polyene side chain from 2-position to 3-position of benzo[b]furan decreased the biological activities dramatically, and the introduction of various substituents at 2-position afforded almost no effect on the activities.

Interaction of ladder-shaped polyethers with transmembrane α-helix of glycophorin A as evidenced by saturation transfer difference NMR and surface plasmon resonance

Ladder-shaped polyether (LSP) compounds are thought to interact with transmembrane α-helices, but direct evidence has scarcely obtained for these interactions. We adopted a transmembrane α-helix of glycophorin A, and quantitatively evaluated its interaction with LSPs such as yessotoxin (YTX), desulfated YTX and artificial LSPs, using surface plasmon resonance and saturation transfer difference NMR. As a result, dissociation constants (KD) of YTX and desulfated YTX to a transmembrane domain peptide of glycophorin A were determined to be in the submillimolar range. Furthermore, in saturation transfer difference NMR, the signals at the polyene side chain and the angular methyl groups of YTX were significantly attenuated, which probably comprised an interacting interface of LSPs with a transmembrane α-helix. These results suggest that hydrophobic interaction plays an important role in molecular recognition of the α-helix peptide by LSPs.

A rapid approach to lipid profiling of mycobacteria using 2D HSQC NMR maps

Mycobacteria, including Mycobacterium tuberculosis, are characterized by a unique cell wall rich in complex lipids, glycolipids, polyketides, and terpenoids. Many of these metabolites have been shown to play important roles in mycobacterial virulence and their inherent resistance to many antibiotics. Here, we report the development of a new simple method for global analysis of these metabolites using two-dimensional (1)H-(13)C heteronuclear single quantum coherence nuclear magnetic resonance. The major advantages of this method are as follows: the small amount of sample and the minimal sample manipulation required; a relatively short procedural time; and the ability to rapidly attain a qualitative and quantitative lipid profile of a mycobacterial sample in which the majority of the clinically relevant lipids can be observed simultaneously. The effectiveness of this method is demonstrated in four different areas of major concern to the mycobacterial research community: i) adaptive changes in cell wall lipids as a result of drug treatment; ii) analysis of gene function; iii) characterization of new mycobacterial species; and iv) analysis of the production of virulence factors in clinical isolates of M. tuberculosis. This method is complementary to mass spectrometry-based lipidomic technologies and provides an urgently needed tool to gain a better understanding of the role of lipids in mycobacteria pathogenesis.

Stereoselective Synthesis of Cyercene A and the Placidenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stereoselective Synthesis of Cyercene A and the Placidenes

Members of a family of alpha-methoxy-gamma-pyrone-containing polypropionate natural products have been stereoselectively synthesized. Two key iodovinyl pyrone building blocks were coupled to appropriately selected vinyl stannanes to assemble the highly substituted polyene side chains of the natural products. [structure: see text]