• Contiguous Quaternary Carbons: A Selection of Total Syntheses.

      Eggert, Alina; Etling, Christoph; Lübken, Dennis; Saxarra, Marius; Kalesse, Markus (2020-08-24)
      Contiguous quaternary carbons in terpene natural products remain a major challenge in total synthesis. Synthetic strategies to overcome this challenge will be a pivotal prerequisite to the medicinal application of natural products and their analogs or derivatives. In this review, we cover syntheses of natural products that exhibit a dense assembly of quaternary carbons and whose syntheses were uncompleted until recently. While discussing their syntheses, we not only cover the most recent total syntheses but also provide an update on the status quo of modern syntheses of complex natural products. Herein, we review (±)-canataxpropellane, (+)-waihoensene, (-)-illisimonin A and (±)-11-O-debenzoyltashironin as prominent examples of natural products bearing contiguous quaternary carbons.
    • Desymmetrization of C -Symmetric Bis(Boronic Esters) by Zweifel Olefinations.

      Linne, Yannick; Schönwald, Axel; Weißbach, Sebastian; Kalesse, Markus; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (Wiley, 2020-06-03)
      anti-Configured 1,3-dimethyl deoxypropionate motifs are important sub structures in natural products. Herein, we describe a bidirectional approach for the rapid construction of natural products featuring such motifs by using C2 -symmetrical 1,3-bis(boronic esters). As for its application in convergent syntheses it was important to establish a selective mono-Zweifel olefination we describe the scope and limitations by using different 1,3-bis(boronic esters) and nucleophiles. This protocol takes advantage of the combination of the Hoppe-Matteson-Zweifel chemistry, which was elegantly put into practice by Aggarwal et al. In order to show its applicability the total syntheses of two natural products, serricornin and (+)-invictolide, were performed.
    • Mode of action of epoxyphomalins A and B and characterization of related metabolites from the marine-derived fungus Paraconiothyrium sp.

      Mohamed, Ietidal E; Kehraus, Stefan; Krick, Anja; König, Gabriele M; Kelter, Gerhard; Maier, Armin; Fiebig, Heinz-Herbert; Kalesse, Markus; Malek, Nisar P; Gross, Harald; et al. (2010-12-27)
      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.
    • Recent Applications of the Diels-Alder Reaction in the Synthesis of Natural Products (2017-2020)

      Sara, Alexandru A.; Um-E-Farwa, Um E.Farwa; Saeed, Aamer; Kalesse, Markus; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (Thieme, 2021-06-21)
      The Diels-Alder reaction has long been established as an extremely useful procedure in the toolbox of natural product chemists. It tolerates a wide spectrum of building blocks of different complexity and degrees of derivatization, and enables the formation of six-membered rings with well-defined stereochemistry. In recent years, many total syntheses of natural products have been reported that rely, at some point, on the use of a [4+2]-cycloaddition step. Among classic approaches, several modifications of the Diels-Alder reaction, such as hetero-Diels-Alder reactions, dehydro-Diels-Alder reactions and domino-Diels-Alder reactions, have been employed to extend the scope of this process in the synthesis of natural products. Our short review covers applications of the Diels-Alder reaction in natural product syntheses between 2017 and 2020, as well as selected methodologies which are inspired by, or that can be used to access natural products. 1 Introduction 2 Syntheses from 2017 3 Syntheses from 2018 4 Syntheses from 2019 5 Syntheses from 2020 6 Conclusion. © 2021 Georg Thieme Verlag. All rights reserved
    • Resolving Inflammation: Synthesis, Configurational Assignment, and Biological Evaluations of RvD1.

      Tungen, Jørn Eivind; Gerstmann, Lisa; Vik, Anders; De Matteis, Roberta; Colas, Romain Alexandre; Dalli, Jesmond; Chiang, Nan; Serhan, Charles Nicholas; Kalesse, Markus; Hansen, Trond Vidar; et al. (Wiley, 2018-12-20)
      New drugs that can resolve inflammation without immunosuppressive effects are at the medicinal chemistry frontier. Pro-resolving endogenously formed small molecules, that is, the resolvins, are excellent candidates displaying such bioactions. The first total synthesis of the specialized pro-resolving mediator RvD1n-3 DPA has been achieved using the underutilized sp3 -sp3 Negishi cross coupling reaction and an alkyne hydrosilylation-protodesilylation protocol. Biological evaluations revealed that this novel mediator displays low nanomolar pro-resolving properties and potently activates the human DRV1/GPR32 receptor. As such, this endogenous natural product is a lead compound for the development of novel immunoresolvents.
    • Ribosome-Targeting Antibiotics Impair T Cell Effector Function and Ameliorate Autoimmunity by Blocking Mitochondrial Protein Synthesis.

      Almeida, Luís; Dhillon-LaBrooy, Ayesha; Castro, Carla N; Adossa, Nigatu; Carriche, Guilhermina M; Guderian, Melanie; Lippens, Saskia; Dennerlein, Sven; Hesse, Christina; Lambrecht, Bart N; et al. (Elsevier (Cell Press), 2020-11-24)
      While antibiotics are intended to specifically target bacteria, most are known to affect host cell physiology. In addition, some antibiotic classes are reported as immunosuppressive for reasons that remain unclear. Here, we show that Linezolid, a ribosomal-targeting antibiotic (RAbo), effectively blocked the course of a T cell-mediated autoimmune disease. Linezolid and other RAbos were strong inhibitors of T helper-17 cell effector function in vitro, showing that this effect was independent of their antibiotic activity. Perturbing mitochondrial translation in differentiating T cells, either with RAbos or through the inhibition of mitochondrial elongation factor G1 (mEF-G1) progressively compromised the integrity of the electron transport chain. Ultimately, this led to deficient oxidative phosphorylation, diminishing nicotinamide adenine dinucleotide concentrations and impairing cytokine production in differentiating T cells. In accordance, mice lacking mEF-G1 in T cells were protected from experimental autoimmune encephalomyelitis, demonstrating that this pathway is crucial in maintaining T cell function and pathogenicity.
    • RNA polymerase motions during promoter melting.

      Feklistov, Andrey; Bae, Brian; Hauver, Jesse; Lass-Napiorkowska, Agnieszka; Kalesse, Markus; Glaus, Florian; Altmann, Karl-Heinz; Heyduk, Tomasz; Landick, Robert; Darst, Seth A; et al. (2017-05-26)
      All cellular RNA polymerases (RNAPs), from those of bacteria to those of man, possess a clamp that can open and close, and it has been assumed that the open RNAP separates promoter DNA strands and then closes to establish a tight grip on the DNA template. Here, we resolve successive motions of the initiating bacterial RNAP by studying real-time signatures of fluorescent reporters placed on RNAP and DNA in the presence of ligands locking the clamp in distinct conformations. We report evidence for an unexpected and obligatory step early in the initiation involving a transient clamp closure as a prerequisite for DNA melting. We also present a 2.6-angstrom crystal structure of a late-initiation intermediate harboring a rotationally unconstrained downstream DNA duplex within the open RNAP active site cleft. Our findings explain how RNAP thermal motions control the promoter search and drive DNA melting in the absence of external energy sources.
    • Synthesis of Desepoxy-Tedanolide C.

      Lücke, Daniel; Kalesse, Markus; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (Wiley, 2021-04-09)
      The synthesis of desepoxy-tedanolide C was accomplished and provided experimental evidence on the configuration of tedanolide C. The reported chemical shifts and coupling constants point to a configuration different from the published structure and analogous to the structures of the other members of this family of natural products. The key step is a Kiyooka aldol protocol for the stereoselective synthesis of the tertiary alcohol flanked by three additional oxygenated carbon atoms. Furthermore, two additional aldol reactions and a Julia-Kocienski olefination were used to assemble the carbon framework.
    • Total Synthesis and Structure Revision of Halioxepine.

      Poock, Caroline; Kalesse, Markus; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (Wiley, 2020-11-20)
      The first total synthesis of halioxepine is accomplished using a 1,4-addition for constructing the quaternary center at C10 and a halo etherification for the generation of the tertiary ether at C7. The correct structure of halioxepine was determined by assembling different enantiomeric building blocks and by changing the relative configuration between C10 and C15.
    • The Total Synthesis of Chondrochloren A.

      Linne, Yannick; Bonandi, Elisa; Tabet, Christopher; Geldsetzer, Jan; Kalesse, Markus; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (Wiley-VCH, 2021-02-25)
      The first total synthesis of chondrochloren A is accomplished using a 1,2-metallate rearrangement addition as an alternative for the Nozaki-Hiyama-Kishi reaction. This transformation also avoids the inherent challenges of this polyketide segment and provides a new, unprecedented strategy to assemble polyketidal frameworks. The formation of the Z-enamide is accomplished using a Z-selective cross coupling of the corresponding amide to a Z-vinyl bromide.
    • Total Synthesis of Nannocystin Ax.

      Poock, Caroline; Kalesse, Markus; Hemholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2017-09-01)
      The total synthesis of nannocystin Ax in an overall yield of 11% with 14 steps as the longest linear sequence is reported. Nannocystin Ax is a cytotoxic 21-membered depsipeptide and was isolated from the myxobacterial genus Nannocystis sp. The synthesis uses a vinylogous Horner-Wadsworth-Emmons reaction (HWE) and a vinylogous Mukaiyama aldol reaction (VMAR) as the key steps for the construction of the polyketide fragment. The macrocycle was closed via a macrolactamization reaction using COMU.
    • Towards the total synthesis of chondrochloren A: synthesis of the (Z)-enamide fragment.

      Geldsetzer, Jan; Kalesse, Markus (2020-04-14)
      The stereoselective synthesis of the (Z)-enamide fragment of chondrochloren (1) is described. A Buchwald-type coupling between amide 3 and (Z)-bromide 4 was used to generate the required fragment. The employed amide 3 comprising three chiral centers was obtained through a seven-step sequence starting from ᴅ-ribonic acid-1,4-lactone. The (Z)-vinyl bromide 4 is accessible in four steps from 4-hydroxybenzaldehyde. The pivotal cross coupling between both fragments was achieved after extensive experimentation with copper(I) iodide, K2CO3 and N,N'-dimethylethane-1,2-diamine.
    • Tris(acetylacetonato) Iron(III): Recent Developments and Synthetic Applications

      Lübken, Dennis; Saxarra, Marius; Kalesse, Markus; HZI,Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7,38124 Braunschweig, Germany. (Thieme, 2018-11-27)
      Tris(acetylacetonato) iron(III) [Fe(acac)3] is an indispensable reagent in synthetic chemistry. Its applications range from hydrogen atom transfer to cross-coupling reactions and to use as a Lewis acid. Consequently, the exceptional utility of Fe(acac)3 has been demonstrated in several total syntheses. This short review summarizes the applications of Fe(acac)3 in methodology and catalysis and highlights its use for the synthesis of medicinally relevant structures and in natural product syntheses.