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Sulfangolids are the first sulfate ester containing secondary metabolites from myxobacteria. The metabolites 1-4 and the structurally related kulkenon (5) were isolated from different strains of the species Sorangium cellulosum. In the course of isolation all metabolites proved to be rather sensitive due to their conjugated double bond systems and the strong acidic nature of the sulfate ester in sulfangolids. The relative configuration of sulfangolid C (3) was assigned by extensive 1D and 2D NMR analysis and molecular modelling. In addition, the biosynthesis of 3 was studied by feeding experiments., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Southern belles! Simplified analogues of tedanolide, a natural product with picomolar activity against a range of tumor cell lines, were synthesized and evaluated for potency in mammalian cancer cells. The truncated analogues were found to retain significant activity in vitro (23 μmol mL(-1) for the example shown) compared with the parent compound tedanolide (0.33 nmol mL(-1)), and represent potential leads for the development of novel anticancer agents., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The improved synthesis of the antitumor compound (+)-tedanolide is described through an aldol coupling of bis-ketone 7. This modification shortens the synthetic steps in the endgame and provides rapid access to this biologically important natural product. Additionally, it serves as a probe in order to unravel the conformational effects that impede or enable its successful synthesis. Having this way access to des-epoxy-tedanolide, its biological characterization surprisingly unravelled the mode of action to resemble candidaspongiolide rather than deoxytedanolide., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The Kobayashi aldol reaction has become a prominent transformation in polyketide syntheses. This methodology takes advantage of the directing effects of the Evans auxiliary and allows the stereoselective incorporation of a four carbon segment with two additional methyl branches establishing an anti-relationship between the two newly formed chiral centers. So far this transformation was restricted to anti-aldol products. We present here a modified protocol that provides the corresponding aldol product with high syn-selectivity.

The challenging synthesis of a quaternary center within the highly oxygenated setting of tedanolide C can be performed via a Kiyooka aldol reaction. Here, the diastereomeric analog of tedanolide C with the configurations between C10 and C20 opposite compared to the proposed structure was chosen as the synthetic target. The tetra-substituted silyl ketene acetal provides the southern hemisphere of tedanolide C in useful selectivities, and the absolute configuration of the newly generated quaternary center was determined by NOE experiments of the corresponding acetonide.

Unsaturated aldehydes are exquisite building blocks for further transformations in polyketide synthesis. Besides standard transformations that take advantage of the aldehyde functionality, the conjugate addition of hydrides followed by internal protonation allows access to alpha chiral aldehydes. Even though vinylogous Mukaiyama aldol reactions have been used in natural product syntheses before, the first enantioselective Mukaiyama aldol reaction of aldehyde-derived dienolates is described.

Epoxyphomalins A (1) and B (2) are highly potent cytotoxic fungal metabolites. During the course of purifying larger quantities of 1 and 2 from Paraconiothyrium sp. fermentation extracts, three new epoxyphomalins (3-5) were isolated and characterized, showing modifications to the oxidation pattern of the cyclohexene moiety or of C-9 of the decalin system. IC(50) values for cytotoxicity against a panel of 36 human tumor cell lines were determined for all new compounds. Compound 4 was found to be cytotoxic particularly toward prostate PC3M (IC(50) = 0.72 μM) and bladder BXF 1218 L (IC(50) = 1.43 μM) cancer cell lines. In addition, inhibition of chymotrypsin-, caspase-, and trypsin-like activity of purified 20S proteasomes was determined for epoxyphomalins A (1) and B (2). The results indicate that compounds 1 and 2 exert their cytotoxic effect through potent inhibition of the 20S proteasome.

The first synthesis of the highly biologically active chivosazole F is described. It features an intramolecular Stille coupling for the macrolactone formation and thereby circumvents the problem of isomerization associated with the tetraene segment. Additionally, the synthesis confirms the structure which has been proposed based solely on a combination of NMR/computational methods and genetic analysis.

During heme biosynthesis the oxygen-independent coproporphyrinogen III oxidase HemN catalyzes the oxidative decarboxylation of the two propionate side chains on rings A and B of coproporphyrinogen III to the corresponding vinyl groups to yield protoporphyrinogen IX. Here, the sequence of the two decarboxylation steps during HemN catalysis was investigated. A reaction intermediate of HemN activity was isolated by HPLC analysis and identified as monovinyltripropionic acid porphyrin by mass spectrometry. This monovinylic reaction intermediate exhibited identical chromatographic behavior during HPLC analysis as harderoporphyrin (3-vinyl-8,13,17-tripropionic acid-2,7,12,18-tetramethylporphyrin). Furthermore, HemN was able to utilize chemically synthesized harderoporphyrinogen as substrate and converted it to protoporphyrinogen IX. These results suggest that during HemN catalysis the propionate side chain of ring A of coproporphyrinogen III is decarboxylated prior to that of ring B.

The synthesis of the C10-C32 core structure of spirangien A is reported. The pivotal aldol coupling between both key intermediates provides a synthetic challenge in the synthesis of this complex natural product.

This Highlight covers the synthetic contributions leading to the syntheses of tedanolide and 13-deoxytedanolide or the corresponding macrolactones, respectively. Additionally, the first SAR data for 13-deoxytedanolide (published by Fusetani) are presented.

A reduction in the cellular levels of the cyclin kinase inhibitor p27(kip1) is frequently found in many human cancers and correlates directly with patient prognosis. In this work, we identify argyrin A, a cyclical peptide derived from the myxobacterium Archangium gephyra, as a potent antitumoral drug. All antitumoral activities of argyrin A depend on the prevention of p27(kip1) destruction, as loss of p27(kip1) expression confers resistance to this compound. We find that argyrin A exerts its effects through a potent inhibition of the proteasome. By comparing the cellular responses exerted by argyrin A with siRNA-mediated knockdown of proteasomal subunits, we find that the biological effects of proteasome inhibition per se depend on the expression of p27(kip1).

Tedanolide, which was isolated by Schmitz in 1984 from the marine sponge Tedania ignis, is a highly cytotoxic macrolide leading to strong growth inhibition of P338 tumor cells in bioassays. A unique structural feature of the known tedanolides is the primary hydroxyl group incorporated in the macrolactone. This unusual motif for macrolactones originated from PKS biosynthesis might arise through lactonizations others than those derived by the thioesterase reaction. First experimental data that support this hypothesis and reflect the inherent preference of PKS-induced macrolactonization were obtained during this synthesis. The inherent preference for the formation of a 14-membered macrocyclization is discussed together with the pivotal steps in the synthesis.

Importin 13 is a member of the importin beta superfamily of nuclear transport proteins and is expressed in multiple tissues at high levels both in humans and rodents, including fetal lung, brain, and heart. In order to elucidate potential functions of imp13 in the heart, we have used rat imp13 as bait to screen a human heart cDNA library and identified an interaction with the C-terminal peptide of myopodin (a.a. 360-698), an actin-bundling protein, associated with tumor-suppressor activity that localizes to both the cytoplasm and the nucleus. We have used GST-pull down assays and co-immunoprecipitation experiments to demonstrate an interaction between imp13 and full-length myopodin and observed that RanGTP dissociates the myopodin-imp13 complex. In studies of cultured cells, we show that both imp13 siRNA and a C-terminal fragment of imp13 protein prevent nuclear localization of myopodin. We, therefore, conclude that imp13 functions in myopodin import and we suggest that the regulation of these events is critical for normal and abnormal cellular differentiation.

delta-Hydroxy-alpha,beta-unsaturated carbonyl compounds were prepared in one step via the vinylogous Mukaiyama aldol reactions with O,O-silyl ketene acetals. Isopropyl alcohol as additive and tryptophane-based B-phenyloxazaborolidinone were required for obtaining the gamma-alkylated product in high enantioselectivities.

The first total synthesis of the (+)-tedanolide is described. Pivotal steps are the Felkin-selective aldol coupling between C12 and C13 and an efficient Mitsunobu macrolactonization. Selective protecting group transformations and subsequent oxidations generate the macrocyclic triketone. In the endgame of the synthesis, four TBS groups are removed in one reaction and a chemo- and stereoselective final step epoxidation generates (+)-tedanolide.

[reaction: see text] The tandem intramolecular Michael-aldol reaction was studied as a tool for the construction of the C-ring of hexacyclinic acid. By changing the reaction conditions it was possible to selectively obtain either the kinetic or the thermodynamic product. Retro-aldol reaction and subsequent epimerization provides four individual cyclopentane derivatives that can be incorporated as building blocks in natural product syntheses.

The formation of the Tat-protein/TAR-RNA complex is a crucial step in the regulation of human immunodeficiency virus (HIV)-gene expression. To obtain full-length viral transcripts the Tat/TAR complex has to recruit the positive transcription elongation factor complex (P-EFTb), which interacts with TAR through its cyclin T1 (CycT1) component. Mutational studies identified the TAR hexanucleotide loop as a crucial region for contacting CycT1. Interfering with the interaction between the Tat/CycT1 complex and the TAR-RNA is an attractive strategy for the design of anti-HIV drugs. Positively charged molecules, like aminoglycosides or peptidomimetics, bind the TAR-RNA, disrupting the Tat/TAR complex. Here, we investigate the complex between the HIV-2 TAR-RNA and a neooligoaminodeoxysaccharide by NMR spectroscopy. In contrast to other aminoglycosides, this novel aminoglycoside analogue contacts simultaneously the bulge residues required for Tat binding and the A35 residue of the hexanucleotide loop. Upon complex formation, the loop region undergoes profound conformational changes. The novel binding mode, together with the easy accessibility of derivatives for the neooligoaminodeoxysaccharide, could open the way to the design of a new class of TAR-RNA binders, which simultaneously inhibit the formation of both the Tat/TAR binary complex and the Tat/TAR/CycT1 ternary complex by obstructing both the bulge and loop regions of the RNA.

Among nature's ecosystems, the marine environment has been an extremely rich source of structurally complex and biologically active molecules. This review aims to cover the recent developments in the synthesis of marine natural products, also reflecting the trend of their increased use to address biological questions. The examples chosen should be viewed as representative of the different structural motifs on the one hand and the strategies and stimuli for their synthesis on the other.

Callystatin A is a prominent member of a class of natural products which display promising growth inhibition of cancer cells in their biological profile. The challenging structure and the interesting biological activity of (-)-callystatin A fueled our interest in the synthesis of this marine natural product. We achieved the total synthesis using a highly convergent approach joining four subunits together with a Wittig olefination, a selective Heck reaction and an aldol reaction as the pivotal steps. The aldol reaction as one of the final transformations during the synthesis opens fast access to a variety of structural analogues and circumvents tedious protecting group manipulations. Here we report an improved synthesis utilizing a modified vinyl iodide which shortens the synthesis by two steps. Additionally, first biological results will be reported.

[reaction: see text] The enantioselective synthesis of callystatin A is described. The pivotal step in the synthesis is the stereoselective aldol reaction that generates the beta-hydroxy ketone moiety. Utilizing the allylic strain within the ethyl ketone precursor, we were able to generate the all-syn configuration of callystatin A. For the construction of the two diene moieties, both a Heck coupling and a Wittig reaction were employed.

The first total synthesis of ratjadone was achieved using a highly convergent approach joining three subunits together with a Wittig olefination and a selective Heck reaction as the pivotal steps. Besides establishing a robust and reliable route for the synthesis of this orphan ligand, the configuration of unknown stereocenters could also be determined. This synthesis not only provides an additional access to a biologically important compound but also enables the synthesis of structural analogues for biological target identification.

Covalent linkage of the arginine-rich fragment of the Tat protein to 1,4,7,10-tetraazacyclododecane (cyclen) results in the selective cleavage of the TAR-RNA of HIV-1 (see picture; the biotin at the 5' end acts as a label for the subsequent analysis of the cleavage fragments). The cleavage occurs at room temperature and is diminished when Eu(III) ions are present-at a concentration of about 1/10 of the concentration of the peptide-cyclen conjugate. The pH dependence indicates that two ammonium ions are responsible for the cleavage reaction. The white arrows in the schematic diagram mark the cleavage sites in RNase T1, and the black arrows the sites in the peptide-cyclen conjugate.