A marine bacterial enzymatic cascade degrades the algal polysaccharide ulvan

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Synthesis of bioactive 3-amino-glycyrrhetinic acid derivatives

Zsfassung in dt. SpracheGlycyrrhizin (GN), ein Saponin, und sein Aglykon Glycyrrhetinsäure (GA) sind die bestuntersuchten biologisch aktiven Substanzen des Extrakts der Süßholzwurzel mit breiter Verwendung in der traditionellen Medizin. Unter anderem sind entzündungshemmende Wirkung, antivirale Aktivität sowie auch inhibierende Wirkung der Glukocorticoid-regulierenden 11[beta]-Hydroxysteroid Dehydrogenasen beschrieben. Auf Basis von GN und GA als Lead-Verbindungen wurde eine Vielzahl pharmakologisch relevanter Derivate der 3-Amino-3-deoxy-glycyrrhetinsäure (Amino-GA) ausgewählt, synthetisiert und für pharmakologische Tests zur Verfügung gestellt. Die Substanzen wurden einerseits auf ihre antivirale Wirkung in mit Influenza A Virus (IAV) infizierten Zellen hin untersucht sowie auch als potentielle selektive 11[beta]-Hydroxysteroid Dehydrogenase Inhibitoren (11[beta]-HSD1- und 11[beta]-HSD2-Subtypen) getestet. Es wurden sowohl Strukturen mit als auch ohne Zuckerfunktionalitäten hergestellt. So wurden Analoga einerseits des GN und andererseits auch der GA und dereen Hemisuccinat Carbenoxolon erhalten. Durch Variation der N-Substitution beider C3-epimerer Amine wurden Derivate der GA erzeugt, die sich stark in Acidität (respektive Nukleophilie) der Aminfunktionalität unterscheiden. Des Weiteren wurden zusätzliche Carbonsäurefunktionalitäten sowie Zuckerstrukturen eingeführt um Glycyrrhizin zu mimikrieren. Im 11[beta]-HSD1/-HSD2-Inhibierungsexperiment zeigte eine große Anzahl N-Acyl- und N-Sulfonylderivate erfreuliche Aktivität wie auch Selektivität für den 11[beta]-HSD2 Subtyp. Auf Basis dieser ersten positiven Ergebnisse wurden dieser Verbindungen formal mit dem C30 Hydroxamsäuremotiv - einem weiteren erfolgreichen Strukturelement - kombiniert, um eine zweite Generation von 3-Aminoglycyrrhetinsäureverbindungen zu erhalten.Der zweite Teil der Arbeit befasste sich mit der Synthese N-basierter Neoglycoside der Glycyrrhetinsäure. Es wurde eine ganze Subfamilie von überbrückten Thioglucuroniden synthetisiert, die alle wesentlichen Strukturelemente des Glycyrrhizin aufweisen, abgesehen von der natürlichen O-glykosidischen Bindung, die durch eine physiologisch stabilere thioglycosidische Bindung ersetzt wurde. Die ganze Substanzklasse war inaktiv in den Enzym-Tests wies aber vielversprechende Ergebnisse im IAV-Test auf. Zuletzt beschäftigte sich die Arbeit mit der möglichen Anwendung der Methode der direct glycosylation von O-substitutierten Hydroxylaminen mit reduzierenden Zuckern auf den Grundkörper der Glycyrrhetinsäure. Die ursprünglich angestrebten direkten 3[beta]-Methoxyaminoglycoside der GA konnten zwar nicht mittels direct glycosylation hergestellt werden, jedoch wurde eine prototypische Verbindung mittels konventioneller Glycosylierungschemie hergestellt um die Stabilität dieser Verbindungsklasse zu belegen. Im Gegensatz dazu konnten überbrückte Methoxyaminoglycoside der GA mittels direct glycosylation erfolgreich synthetisiert werden. Im Sinne eines Proof-of-concepts wurden zwei Beispielverbindungen hergestellt, um diese Substanzklasse im Rahmen einer Diplomarbeit weiterzubetreuen.The saponin glycyrrhizin (GN) and its aglycon glycyrrhetinic acid (GA), the main biological active compounds of licorice root, have a long tradition in natural medicine. The broad spectrum of reported activities comprises anti-inflammatory, cytotoxic, antiulcer, antiproliferative, anti-oxidative and endocrine activities. Starting from GN and GA as lead structures a variety of pharmacologically interesting compounds based on the 3-amino-3-deoxy-glycyrrhetinic acid (both diastereomers) has been envisioned, prepared, characterized and submitted for pharmacological screening. The compounds have been evaluated as antiviral compounds (influenza A virus, IAV) and as selective 11[beta]-hydroxysteroid dehydrogenase inhibitors. The synthesized compounds include structures with and without sugar moieties, thus mimicking the parent structures, glycyrrhizin, the corresponding aglycon glycyrrhetinic acid and its hemisuccinate ester carbenoxolone, a licensed drug with anti-inflammatory activities. Target compounds were selected in order to vary the relative stereochemistry at the 3-position, the acidity and nucleophilicity of the NH-group by installation of different residues at the amino moiety. Furthermore, additional carboxylic acid functionalities and monosaccharides were attached to the core structure to mimic glycyrrhizin and carbenoxolone. A large number of "sugar-free" 3-amino-GA derivatives, including "amido-carbenoxolone" and "sulfonamido-carbenoxolone", gave promising results in the 11[beta]-HSD1/11[beta]-HSD2 enzyme assay as potent and selective 11[beta]-HSD2 inhibitors. This positive response from biology triggered the selection of several candidates and their decoration with the C30 hydroxamic acid motif, another promising structural modification developed within the ASPEX-project, bringing forward a second generation of 11[beta]-HSD inhibitors.The second part of the thesis was focused on neoglycosides of the glycyrrhetinic acid. A small compound family of spacered aminoethylthioglucuronides was prepared, having all structural features of glycyrrhizin except for the natural O-glycosidic bond which was formally replaced by the physiologically more stable thioglycosidic bond. The whole set of compounds was inactive in the enzymatic assay but gave promising results in the antiviral screening against influenza A virus. The last two fields of interest were dealing with the applicability of the direct glycosylation methodology of alkoxyamines with unprotected, unactivated reducing sugars on methoxyamino-derivatives of the glycyrrhetinic acid. The initially targeted 3[beta]-methoxyaminoglycosides of GA could not be prepared by means of direct glycosylation of the 3[beta]-methoxyamino-GA, but one prototype was synthesized via conventional glycosylation chemistry to prove the principal stability of such compounds. In contrast, the chemistry towards ethylamino-spacered methoxyaminoglycosides, was established in a small scope (glucose and glucuronic acid) as a proof-of-concept study and was extended into a supervised diploma thesis.19

Convergent Synthesis of 4‑<i>O</i>‑Phosphorylated l-<i>glycero</i>-d-<i>manno</i>-Heptosyl Lipopolysaccharide Core Oligosaccharides Based on Regioselective Cleavage of a 6,7‑<i>O</i>‑Tetraisopropyldisiloxane-1,3-diyl Protecting Group

The structurally conserved lipopolysaccharide core region of many Gram-negative bacteria is composed of trisaccharides containing 4-<i>O</i>-phosphorylated l-<i>glycero</i>-d-<i>manno</i>-heptose (l,d-Hep) units, which act as ligands for antibodies and lectins. The disaccharides Glc-(1→3)-Hep4P Hep-(1→3)-Hep4P and Hep-(1→7)-Hep4P and the branched trisaccharide Glc-(1→3)-[Hep-(1→7)]-Hep4P, respectively, have been synthesized from a methyl heptopyranoside acceptor in less than 10 steps. The synthetic strategy was based on the early introduction of a phosphotriester at position 4 of heptose followed by a regioselective opening of a 6,7-<i>O</i>-(1,1,3,3-tetraisopropyl-1,3-disiloxane-1,3-diyl) group allowing for a straightforward access to glycosylation at position 7. Perbenzylated <i>N</i>-phenyl trifluoroacetimidate glucosyl and heptosyl derivatives served as α-selective glycosyl donors

Straight Forward and Versatile Differentiation of the l-glycero and d-glycero-d-manno Heptose Scaffold

Bacterial lipopolysaccharides (LPS) are important bio-medical structures, playing a major role in the interaction with human immune systems. Their core regions, containing multiple units of L-glycero-D-manno heptoses (L,D-heptose), are highly conserved structurally (with O3 and O7 glycosidic bonds), making them an epitope of high interest for the potential development of new antibiotics and vaccines. Research in this field has always been restricted by the limited availability of the parent L,D-heptose as well as its biochemical epimeric precursor D-glycero-D-manno heptose (D,D-heptose). This problem of availability has recently been solved by us, through a rapid and efficient practical synthesis of L,D-manno-heptose peracetate demonstrated at scale. Herein we report an optimized, technically simple and versatile synthetic strategy for the differentiation of both the L-glycero and D-glycero-D-manno heptose scaffolds. Our approach is based on an orthoester methodology for the differentiation of all three positions of the sugar core using a O6, O7-tetraisopropyl disiloxyl (TIPDS) protecting group for the exocyclic positions. Furthermore, the regioselective opening toward 7-OH acceptors (6O-FTIPDS ethers) differentiates the exocyclic diol which has been demonstrated with a broader set of substrates and for both manno-heptoses for the first time

Total synthesis of [13C2]-labelled phytosiderophores of the mugineic and avenic acid families

We herein report the synthesis of 13C2-labelled natural products from the mugineic acid and avenic acid family. These phytosiderophores (“plant iron carriers”) are built up from non-proteinogenic amino acids and play a key role in micronutrient uptake in gramineous plants. In this work two central building blocks are prepared from labelled starting materials (13C2-bromoacetic acid, 13C2-glycine) and further employed in our recently reported divergent, branched synthetic strategy delivering eight isotopically labelled phytosiderophores. The required labelled building blocks (13C2-L-allylglycine and a related hydroxylated derivative), were prepared via enantioselective phase-transfer catalysis and enantio- and diastereoselective aldol condensation with a chiral auxiliary respectively, both potentially valuable themselves for other synthetic routes towards labelled (natural) products

Synthesis and antiviral activities of spacer-linked 1-thioglucuronide analogues of glycyrrhizin Full Research Paper Open Access

Abstract The influenza virus infection remains a significant threat to public health and the increase of antiviral resistance to available drugs generates an urgent need for new antiviral compounds. Starting from the natural, antivirally active compound glycyrrhizin, spacerbridged derivatives were generated with improved antiviral activity against the influenza A virus infection. Simplified analogues of the triterpene saponin glycyrrhizin containing 1-thio-β-D-glucuronic acid residues have been prepared in good yields by alkylation of 3-amino and 3-thio derivatives of glycyrrhetinic acid with a 2-iodoethyl 1-thio-β-D-glucopyranosiduronate derivative. The spacer-connected 3-amino derivatives were further transformed into N-acetylated and N-succinylated derivatives. The deprotected compounds containing these carboxylic acid appendices mimic the glycon part of glycyrrhizin as well as the hemisuccinate derivative of glycyrrhetinic acid, carbenoxolone. Antiviral activities of the compounds were determined in a biological test based on influenza A virus-infected cells, wherein the 3-(2-thioethyl)-N-acetylamino-and 3-(2-thioethyl)-thio-linked glucuronide derivatives were effective inhibitors with IC 50 values as low as 54 µM. 70

The Indium and Zinc-Mediated Acyloxyallylation of Protected and Unprotected Aldotetroses - Revealing a Pronounced Diastereodivergence and a Fundamental Difference in the Performance of the Mediating Metal

The acyloxyallylation of unprotected aldoses was first demonstrated more than a decade ago as a potentially elegant two-carbon homologation of reducing sugars (upon ozonolysis); however, its application in real case syntheses remained scarce. Following up on such a successful showcase and to answer several pending questions about this attractive transformation, we engaged in an in depth methodological reinvestigation. The epimeric tetroses l-erythrose and d-threose in unprotected and protected form were successfully applied to the indium and also zinc-mediated acyloxyallylation, with the latter being a first for an unprotected sugar. The investigation largely benefited from the choice of these more exotic starting materials as it allowed unambiguous identification/quantification of the hexose-products which are available as authentic reference materials. The observed diastereoselectivities indicate a strong substrate control (stereochemistry at O2), and the influence of the reagent’s structure on the selectivity was investigated in great detail. A strong facial diastereodivergence between related protected and unprotected structures was demonstrated and an unexpected, pronounced principle difference in performance between indium and zinc was revealed

Synthesis of novel 3-amino and 29-hydroxamic acid derivatives of glycyrrhetinic acid as selective 11?-hydroxysteroid dehydrogenase 2 inhibitors

Glycyrrhetinic acid, the metabolite of the natural product glycyrrhizin, is a well known nonselective inhibitor of 11?-hydroxysteroid dehydrogenase (11?-HSD) type 1 and type 2. Whereas inhibition of 11?-HSD1 is currently under consideration for treatment of metabolic diseases, such as obesity and diabetes, 11?-HSD2 inhibitors may find therapeutic applications in chronic inflammatory diseases and certain forms of cancer. So far, no selective 11?-HSD2 inhibitor has been developed and neither animal studies nor clinical trials have been reported based on 11?-HSD2 inhibition. Starting from the lead compound glycyrrhetinic acid, novel triterpene type derivatives were synthesized and analyzed for their biological activity against overexpressed human 11?-HSD1 and 11?-HSD2 in cell lysates. Several hydroxamic acid derivatives showed high selectivity for 11?-HSD2. The most potent and selective compound is active against human 11?-HSD2 in the low nanomolar range with a 350-fold selectivity over human 11?-HSD1

Synthesis of new glycyrrhetinic acid derived ring A azepanone, 29-urea and 29-hydroxamic acid derivatives as selective 11?-hydroxysteroid dehydrogenase 2 inhibitors

Glycyrrhetinic acid, the metabolite of the natural product glycyrrhizin, is a well known nonselective inhibitor of 11?-hydroxysteroid dehydrogenase (11?-HSD) type 1 and type 2. Whereas inhibition of 11?-HSD1 is currently under consideration for treatment of metabolic diseases, such as obesity and diabetes, 11?-HSD2 inhibitors may find therapeutic applications in chronic inflammatory diseases and certain forms of cancer. Recently, we published a series of hydroxamic acid derivatives of glycyrrhetinic acid showing high selectivity for 11?-HSD2. The most potent and selective compound is active against human 11?-HSD2 in the low nanomolar range with a 350-fold selectivity over human 11?-HSD1. Starting from the lead compounds glycyrrhetinic acid and the hydroxamic acid derivatives, novel triterpene type derivatives were synthesized and analyzed for their biological activity against overexpressed human 11?-HSD1 and 11?-HSD2 in cell lysates. Here we describe novel 29-urea- and 29-hydroxamic acid derivatives of glycyrrhetinic acid as well as derivatives with the Beckman rearrangement of the 3-oxime to a seven-membered ring, and the rearrangement of the C-ring from 11-keto-12-ene to 12-keto-9(11)-ene. The combination of modifications on different positions led to compounds comprising further improved selective inhibition of 11?-HSD2 in the lower nanomolar range with up to 3600-fold selectivity

Microbial decomposition of 13C- labeled phytosiderophores in the rhizosphere of wheat: Mineralization dynamics and key microbial groups involved

AbstractBeing low molecular weight carbon (LMW-C) compounds, phytosiderophores (PS) released by strategy II plants are highly susceptible to microbial decomposition. However, to date very little is known about the fate of PS in soil. Using in-house synthesized 13C4-2′-deoxymugineic acid (DMA), the main PS released by wheat, we investigated DMA mineralization dynamics, including microbial incorporation into phospholipid fatty acids (PLFA), in the wheat rhizosphere and bulk soil of two alkaline and one acidic soil. Half-lives of the intact DMA molecule (3–8 h) as well as of DMA-derived C-compounds (8–38 days) were in the same order of magnitude as those published for other LMW-C compounds like sugars, amino acids and organic acids. Combining mineralization with PLFA data showed that between 40 and 65% of the added DMA was either respired or incorporated into soil microbial biomass after 24 h, with the largest part of total incorporated DMA-13C being recovered in gram negative bacteria. Considering root growth dynamics and that PS are mainly exuded from root tips, the significantly slower mineralization of DMA in bulk soil is of high ecological importance to enhance the Fe scavenging efficiency of PS released into the soil