Symmetric dithiodigalactoside: strategic combination of binding studies and detection of selectivity between a plant toxin and human lectins

Thioglycosides offer the advantage over O-glycosides to be resistant to hydrolysis. Based on initial evidence of this recognition ability for glycosyldisulfides by screening dynamic combinatorial libraries, we have now systematically studied dithiodigalactoside on a plant toxin (Viscum album agglutinin) and five human lectins (adhesion/growth-regulatory galectins with medical relevance e.g. in tumor progression and spread). Inhibition assays with surface-presented neoglycoprotein and in solution monitored by saturation transfer difference NMR spectroscopy, flanked by epitope mapping, as well as isothermal titration calorimetry revealed binding properties to VAA (Ka: 1560 ± 20 M-1). They were reflected by the structural model and the affinity on the level of toxin-exposed cells. In comparison, galectins were considerably less reactive, with intrafamily grading down to very minor reactivity for tandem-repeat-type galectins, as quantitated by radioassays for both domains of galectin-4. Model building indicated contact formation to be restricted to only one galactose moiety, in contrast to thiodigalactoside. The tested lycosyldisulfide exhibits selectivity between the plant toxin and the tested human lectins, and also between these proteins. Therefore, glycosyldisulfides have potential as chemical platform for inhibitor design

Fluorinated Carbohydrates as Lectin Ligands: Biorelevant Sensors with Capacity to Monitor Anomer Affinity in 19F-NMR-Based Inhibitor Screening

11 páginas, 4 figurasd, 2 esquemas -- PAGS nros. 4354-4364Our recent description of pulse sequences for the intramolecular relay of 1H polarization to the 19F reporter of a carbohydrate ligand after saturation transfer from a cognate lectin prompted us to test the applicability of this technique for inhibitor screening. By strategically combining synthetic organic chemistry and cell assays with 19F-NMR-based competition experiments, we document the validity of this approach. Two mannose-specific leguminous agglutinins as receptors, the α-methyl derivative of 2-deoxy-2-fluoro-D-mannopyranoside as sensor and synthetic mannosides selected to represent different inhibitory capacities were used to establish a test panel. Signal amplitudes were found to vary among the two related lectins, and their precipitation by glycodendrimers enabled inherent limits to be set. The obtained experimental basis was then broadened by assaying a galactopyranoside-binding plant toxin, using 6-deoxy-6-fluoro-D-galactopyranose as a sensor molecule. The easy identification of the two anomeric signals by 19F NMR spectroscopy enabled ready detection of the preference of this lectin for the α-anomer and allowed the two individual inhibition profiles to be deduced. These 19F NMR spectroscopic data were in accord with the activities of inhibitors used to protect cells from toxicity. Our results therefore provide an experimental basis for 19F-NMR-based inhibitor screeningThis work was generously supported by the European Community (Marie Curie Research training grant MCRTN-CT-2005–19561), the Ministerio de Ciencia e Innovación (MICINN) (grant numbers CTQ2009-08536 and BFU2009-10052), and the Spanish Institute of Health Carlos III (ISCIII), under the initiative CIBER of Respiratory Diseases (CIBERES). The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement number 260600 (“GlycoHIT”)Peer reviewe

Synthesis of C-linked Disaccharides and Analogues as Biomimetics of the TF Epitope

Glycoproteins and glycolipids are major components of the outer surface of mammalian cells. Their carbohydrate moieties, which are directed into the outer environment, serve as ligands for receptor proteins, such as lectins, pathogen proteins and antibodies. Recognition processes between the receptor proteins and the carbohydrates have a significant impact on different aspects of cell biology as they turn structure into biological response in a selective way. Cellular carbohydrate structures are found to change dramatically during embryonic development or under pathological conditions including malignant transformation. One example is the Thomsen-Friedenreich (TF) epitope, a disaccharide consisting of galactose-β-(1→3)-N-acetyl-galactosamine α-linked to a serine or threonine of a carrier protein. The TF epitope is considered as a cancer-specific carbohydrate antigen that is found on various epithelial carcinomas of human adults and can be recognized by the anti-TF antibody. It has been demonstrated that synthetic glycoconjugates of the TF epitope can act as anti-cancer vaccines by stimulating the immune system against tumor growth and proliferation. Native carbohydrates are liable to chemical and enzymatic hydrolysis of their glycosidic bonds and therefore short-lived in the blood. Their stable C-analogues in which the glycosidic oxygens have been replaced by methylene or substituted methylene units are particularly interesting as therapeutic agents. We report herein our efforts to synthesize non-hydrolysable methylene and fluorinated methylene linked disaccharides analogues of the TF epitope. Our synthesis follows a methodology previously developed in our laboratory for the preparation of C(1→3)-disaccharides. The key step is an Oshima-Nozaki coupling between a β-C-galactosyl carbaldehyde and an enone, isolevoglucosenone. The hydroxyl methylene linker that is created by this coupling can be deoxygenated or can be substituted by one fluorine atom (or two fluorine atoms via the corresponding ketone). 1,4-Addition of an amine to the enone derived from the Oshima-Nozaki coupling, ketone reduction and some protecting group manipulations gave an hydroxyl methylene linked disaccharide that could smoothly undergo Barton-McCombie deoxygenation. On the contrary, the attempts of nucleophilic fluorination of this substrate failed to give the desired outcome probably due to an intramolecular rearrangement. With the deoxygenated product in hand and after some protecting group transformations, we attempted C-allylation of the anomeric centre and at the same time cleavage of the 1,6-anhydro bridge. Some preliminary results are presented. An allyl-C-disaccharide can lead to a C-glycosyl amino acid according to the literature and therefore it can be used for preparing a C,C-analogue of the TF epitope. In our search for biomimetics of the TF epitope with less synthetic requirements, we attempted to replace the second saccharidic unit by a phenyl ring. Simple phenyl CH2- and CF2-galactosides were prepared starting from α- and β-C-galactosyl carbaldehydes. NMR studies in combination with theoretical calculations revealed that the conformational behaviour of these galactosides in solution depends on the substitution at pseudoanomeric center. Similarities and differences to their O-analogues are discussed. Additionally, NMR spectroscopy was used to detect binding to a lectin protein. The β-galactosyl carbaldehyde –common starting material for the synthesis of C-linked disaccharides and phenyl C-galactosides mentioned above – was also used to prepare a small library of CH2-galactosyl aryl ethers bearing different aromatic amides. The affinities of these derivatives towards some galactoside-specific lectins, the galectins, were measured. None of the synthesized C-galactosyl analogues appeared to be high-affinity ligand of galectins. However, such studies are useful for the future design of sugar analogues with the minimum of functionalities that assure strong binding to carbohydrate recognition proteins

Synthesis of the marine furanoditerpene (-)-marginatone

A synthesis of the marine labdane furanoditerpene (-)-marginatone 1 has been accomplished by a short sequence of reactions starting from (+)-coronarin E 5. The key step is the stereocontrolled-intramolecular electrophilic cyclisation of the (+)-dihydrocoronarin E 6, to the tetracyclic marginatane skeleton 7, which is subsequently functionalized by allylic oxidation to give 1. As (+)-coronarin E 5 was previously synthesized from (-)-sclareol 10, the herein reported preparation constitutes the first formal total synthesis of (-)-marginatone 1, by which its absolute configuration has been confirmed. © 2005 Elsevier Ltd. All rights reserved

Amide Synthesis by Transamidation of Primary Carboxamides

International audienceThe amide functionality is one of the most important and widely used groups in nature and in medicinal and industrial chemistry. Because of its importance and as the actual synthetic methods suffer from major drawbacks, such as the use of a stoichiometric amount of an activating agent, epimerization and low atom economy, the development of new and efficient amide bond forming reactions is needed. A number of greener and more effective strategies have been studied and developed. The transamidation of primary amides is particularly attractive in terms of atom economy and as ammonia is the single byproduct. This review summarizes the advancements in metal-catalyzed and organocatalyzed transamidation methods. Lewis and Bronsted acid transamidation catalysts are reviewed as a separate group. The activation of primary amides by promoter, as well as catalyst- and promoter-free protocols, are also described. The proposed mechanisms and key intermediates of the depicted transamidation reactions are shown

Anticancer Activities of Marine-Derived Phenolic Compounds and Their Derivatives

Since the middle of the last century, marine organisms have been identified as producers of chemically and biologically diverse secondary metabolites which have exerted various biological activities including anticancer, anti-inflammatory, antioxidant, antimicrobial, antifouling and others. This review primarily focuses on the marine phenolic compounds and their derivatives with potent anticancer activity, isolated and/or modified in the last decade. Reports on the elucidation of their structures as well as biosynthetic studies and total synthesis are also covered. Presented phenolic compounds inhibited cancer cells proliferation or migration, at sub-micromolar or nanomolar concentrations (lamellarins D (37), M (38), K (39), aspergiolide B (41), fradimycin B (62), makulavamine J (66), mayamycin (69), N-acetyl-N-demethylmayamycin (70) or norhierridin B (75)). In addition, they exhibited anticancer properties by a diverse biological mechanism including induction of apoptosis or inhibition of cell migration and invasive potential. Finally, phlorotannins 1–7 and bromophenols 12–29 represent the most researched phenolic compounds, of which the former are recognized as protective agents against UVB or gamma radiation-induced skin damages. Finally, phenolic metabolites were assorted into six main classes: phlorotannins, bromophenols, flavonoids, coumarins, terpenophenolics, quinones and hydroquinones. The derivatives that could not be attributed to any of the above-mentioned classes were grouped in a separate class named miscellaneous compounds

The Isolation and Synthesis of Neodolastane Diterpenoids

The neodolastane diterpenoids comprise a group of 44 compounds including guanacastepenes, heptemerones, plicatilisins, radianspenes, 2,15-epoxy-5,13-dihydroxyneodolast-3-en-14-one and sphaerostanol. These fungal and marine natural products are characterized by a tricyclic neodolastane skeleton that consists of fused five-, seven- and six-membered rings. Their reported antibiotic activities against antibiotic-resistant bacteria together with strong antifungal and anticancer activities and their novel structures render these compounds interesting synthetic targets. The aim of this account is to summarise the progress in the isolation, characterisation and synthesis of these diterpenoids as well as to review their biogenetic origins and diverse biological activities since their discovery in 2000

Secondary Metabolites from Gorgonian Corals of the Genus Eunicella: Structural Characterizations, Biological Activities, and Synthetic Approaches

International audienceGorgonian corals, which belong to the genus Eunicella, are known as natural sources of diverse compounds with unique structural characteristics and interesting bioactivities both in vitro and in vivo. This review is focused primarily on the secondary metabolites isolated from various Eunicella species. The chemical structures of 64 compounds were divided into three main groups and comprehensively presented: a) terpenoids, b) sterols, and c) alkaloids and nucleosides. The observed biological activities of depicted metabolites with an impact on cytotoxic, anti-inflammatory, and antimicrobial activities were reviewed. The most promising biological activities of certain metabolites point to potential candidates for further development in pharmaceutical, cosmetic, and other industries, and are highlighted. Total synthesis or the synthetic approaches towards the desired skeletons or natural products are also summarized