1,721,057 research outputs found
A new aspect of the reactivity of sodium dithionite provides a facile route to the synthesis of 2-deoxy-alpha-glycosides
No full textGlycolipids from Sponges. 10. Plaxyloside from the Marine Sponge Plakortis simplex: an Improved Strategy for NMR Structural Studies of Carbohydrate Chains
No full textPlaxyloside, a glycolipid composed of a C35 linear polyisoprenoid alcohol aglycone and a linear carbohydrate chain made up of six â-xylopyranose units, has been isolated as its peracetate from the Caribbean sponge Plakortis simplex, and its structure determined on the basis of spectral and chemical data. Study of plaxyloside showed that: (a) carbohydrate chains with repetitive structures can be conveniently studied as their peracetates by NMR spectroscopy in C6D6, thanks to the peculiar conformational behavior of these derivatives, and (b) coupling constant analysis can be used to establish relative configurations in pyranose sugars even in the presence of conformational equilibria
The First 12-Methylhopanoid: 12-Methylbacteriohopanetetrol from the Marine Sponge Plakortis simplex
No full textStereoselective synthesis of a 2-deoxyglycosphingolipid
No full textStudying the chemistry of marine sponges of the genus Agelas and Axinella, our research group discovered alpha-galactosylglycosphingolipids, a new class of glycolipids characterized by the presence of an alpha-galactopyranose as the first sugar of the saccharide chain.1 Some of these compounds showed immunostimulating properties, being able to induce proliferation of T cells in vitro, others resulted to be inactive. By a comparison of the structure of active and inactive glycolipids, we proposed a structure-activity relationship for the immunomodulating activity: only glycosphingolipids with a non-glycosylated OH at position 2 of the first sugar are immunostimulating agents.2
In order to perform more detailed structure-activity studies, we decided to synthesize some analogues of natural -galactosylglycosphingolipids modified at position 2 of the first sugar. Our first target was 1-O-(2-deoxy--D-galactopyranosyl)-2-docosanoylamino-1,3,4-octadecanetriol (1).
This communication describes a stereoselective and efficient strategy to the total synthesis of 2-deoxy--glycosylceramides, using galactal as chiral starting material for both the sphingosine and the sugar units. In addition, this synthetic work led to the discover of some interesting new aspects of carbohydrate chemistry, that will also be discussed
Glycolipids from Sponges.XIII.Clarhamnoside, the first rhamnosylated a-Galactosylceramide from Agelas clathrodes. Improving spectral strategies for glycoconjugate structure determination
No full textReinvestigation of the glycosphingolipid composition of the marine sponge Agelas clathrodes revealed the presence of a new tetraglycosylated R-galactoglycosphingolipid (1a), containing an unusual L-rhamnose unit in the sugar head. The structure of the new compound was elucidated using extensive 2D NMR studies. Because of the strong overlapping of the signals of the sugar protons in the 1H spectrum, 13C-coupled and 13C-decoupled phase-sensitive HMQC spectra were used to study the multiplicity of the overlapping signals. In addition, the absolute configuration of sugars was determined using a simple and efficient, yet underutilized CD method
"A Biosynthetically Significant New Bacteriohopanoid Present in Large Amounts in the Caribbean Sponge Plakortis simplex "
No full textIn addition to the previously reported bacteriohopanoids, the marine sponge Plakortis simplex was shown to contain large amounts of (32R,33S,34S)-32,35-anhydrobacteriohopanetetrol. The structure of this new bacteriohopanoid was determined by extensive NMR analysis, and further supports the hypothesized biosynthetic pathway to bacteriohopanoids. Altogether, the amounts of bacteriohopanoids in P. simplex is as high as 50% of sterols in weight, and these compounds could play a structural role in the sponge cell membranes. © 2001 Elsevier Science Ltd
Synthesis of Analogues of Marine Glycosphingolipids
No full textStudying the chemistry of marine sponges of the genus Agelas and Axinella, our research group discovered α- galactosylglycosphingolipids, a new class of glycolipids characterized by the presence of an α-galactopyranose as the first sugar of the saccharide chain. Some of these compounds showed immunostimulating properties, being able to induce proliferation of T cells in vitro, others resulted to be inactive. By a comparison of the structure of active and inactive glycolipids, we proposed a structure-activity relationship for the immunomodulating activity: only glycosphingolipids with a non-glycosylated OH at position 2 of the first sugar are immunostimulating agents.
To confirm this hypothesis and to perform more detailed structure-activity studies, we needed a collection ot compounds with a greater structural variety than that provided by natural compounds. Therefore, we decided to synthesize some analogues of natural α-galactosylglycosphingolipids modified at position 2 of the first sugar. We set up an efficient procedure for the synthesis of glycosphingolipids with trihydroxylated sphingosines, that we used for the preparation of analogues. This synthetic study showed some interesting new aspects of carbohydrate chemistry, such as the use of sodium dithionite as an efficient and mild dehalogenating agent
J-coupling analysis for stereochemical assignments in furanosides: structure elucidation of vesparioside B, a glycosphingolipid from the marine sponge Spheciospongia vesparia
No full textReinvestigation of the glycosphingolipid composition of the marine sponge Spheciospongia vesparia revealed the presence of vesparioside B (2a), a new furanose-rich hexaglycosylated glycosphingolipid which is the most complex glycosphingolipid isolated from a marine sponge to date. The structure of the new compound was elucidated using extensive 2D NMR studies and chemical degradation. Particularly useful for structure elucidation of vesparioside B was a quantum mechanical computational study, showing that in furanosides a vicinal coupling constant < 2.0 Hz (for H-1/H-2 or H-3/H-4) or < 3.5 Hz (for H-2/H-3) is a proof of the trans orientation of the relevant protons. This general rule, combined with ROE data, allowed us to elucidate the relative stereochemistry (including anomeric configuration) of the three furanose five-membered rings
Immumomodulating marine glycosphingolipids: combining natural product isolation and chemical synthesis in search of chemical diversity
No full textIt is now well established that sponges of the genus Agelas and Axinella produce α-galactoglycosphingolipids (α-Gal-GSLs, e.g. 1–3), unique glycosphingolipids which are different from common β-glucosyl- or β-galactosyl ceramides from higher animals and plants in that they show an α-galactose as the first sugar of the carbohydrate chain. Natural α-Gal-GSLs possess interesting immunomodulating activities. They are potent ligands of the CD1d antigen presenting protein, and are capable to specifically activate natural killer T cells (N KT cells) in vivo. KRN7000 (4), a synthetic analogue the simplest α-Gal GSLs agelasphin (1) is under clinical trial as a novel anticancer agent, acting through stimulation of the immune system. As a part of our ongoing search of glycolipids from sponges, we discovered a number of more complex alpha-Gal-GSLs, which were tested on murine T-cell populations. The obtained data suggested that the immunostimulating activity is greatly reduced by glycosylation of the galactose 2-OH group, although some subsequent results obtained by other researchers seemed to be in contrast with these findings. This inconsistency was solved by a recent study, demonstrating that alpha-Gal-GSLs glycosylated at 2' or 3' positions are not active themselves, but are processed by the antigen presenting cells (APCs) with removal of the sugars linked to the galactose. In this scenario, we felt that the availability of analogs of alpha-Gal-GSLs, modified at the key position 2' in a way that cannot be affected by APC processing (e.g. 5), would be important to obtain clear-cut results. Therefore, we carried out the stereoselective total synthesis of the alpha-Gal-GSL analogs 5, and evaluated the influence of the structural modification on their immunostimulating activity. The synthetic procedures, the results of the assays, and their implications on the structural requirements for the immunostimulating activity will be discussed.Meanwhile, the analysis of natural alpha-Gal-GSLs from Agelas and Axinella sponges is far from being concluded, and these species are continuing to provide new glycosphingolipids with unprecedented glycosylation patterns. The latest results of this studies, which also contributed to clarify the structure-activity relationship of alpha-Gal-GSLs will be discussed
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