Synthesis of propyl and 2-aminoethyl glycosides of alpha-D-galactosyl-(1-->3')-beta-lactoside

NMR chemical shift prediction and structural elucidation of linker-containing oligo- and polysaccharides using the computer program CASPER

Carbohydrate structures containing alkyl groups as aglycones are useful for investigating enzyme activity and glycan-protein interactions. Moreover, linker-containing oligosaccharides with a spacer group are commonly used to print glycan microarrays or to prepare protein-conjugates as vaccine candidates. The structural accuracy of these synthesized glycans are essential for interpretation of results from biological experiments in which the compounds have been used and NMR spectroscopy can unravel and confirm their structures. An approach for efficient 1H and 13C NMR chemical shift assignments employed a parallel NOAH-10 measurement followed by NMR spin-simulation to refine the 1H NMR chemical shifts, as exemplified for a disaccharide with an azidoethyl group as an aglycone, the NMR chemical shifts of which have been used to enhance the quality of CASPER (http://www.casper.organ.su.se/casper/). The CASPER program has been further developed to aid characterization of linker-containing oligo- and polysaccharides, either by chemical shift prediction for comparison to experimental NMR data or as structural investigation of synthesized glycans based on acquired unassigned NMR data. The ability of CASPER to elucidate structures of linker-containing oligosaccharides is demonstrated and comparisons to assigned or unassigned NMR data show the utility of CASPER in supporting a proposed oligosaccharide structure. Prediction of NMR chemical shifts of an oligosaccharide, corresponding to the repeating unit of an O-antigen polysaccharide, having a linker as an aglycone and a non-natural substituent derivative thereof are presented to exemplify the diversity of structures handled. Furthermore, NMR chemical shift predictions of synthesized polysaccharides, corresponding to bacterial polysaccharides, containing a linker are described showing that in addition to oligosaccharide structures also polysaccharide structures having an aglycone spacer group can be analyzed by CASPER.

Application of a Janus aglycon with dual function in benzyl-free synthesis of spacer-armed oligosaccharide fragments of polysaccharides from rhizobacterium Azospirillum brasilense sp7

Both protective and pre-spacer features of 4-(2-chloroethoxy)phenyl (CEP) aglycon, which belong to the class of Janus aglycons, were engaged in a benzyl-free synthesis of oligosaccharide fragments of polysaccharides from rhizobacterium Azospirillum brasilense sp7. Introduction of α-1,4-linked L-fucose residue was performed using 3,4-di-O-benzoyl-2-O-triisopropylsilyl-α-L-fucopyranosyl N-phenyltrifluoroacetimidate in excellent stereoselectivity and high yields. The obtained deprotected di-, tri- and tetrasaccharides contain 4-(2-azidoethoxy)phenyl (AEP) spacer aglycon, which allows straightforward preparation of neoglycoconjugates that will be used for the study of the role of lipopolysaccharide of rhizobacterium A. brasilense sp7 in plant-microbe symbiosis. The intermediate protected oligosaccharide building blocks with cleavable CEP/AEP aglycons have a strong potential for further application in the synthesis of more complex oligosaccharides.

The design and synthesis of an α-Gal trisaccharide epitope that provides a highly specific anti-Gal immune response

Carbohydrate antigens displaying Galα(1,3)Gal epitopes are recognized by naturally occurring antibodies in humans. These anti-Gal antibodies comprise up to 1% of serum IgG and have been viewed as detrimental as they are responsible for hyperacute organ rejections. In order to model this condition, α(1,3)galactosyltransferase-knockout mice are inoculated against the Galα(1,3)Gal epitope. In our study, two α-Gal trisaccharide epitopes composed of either Galα(1,3)Galβ(1,4)GlcNAc or Galα(1,3)Galβ(1,4)Glc linked to a squaric acid ester moiety were examined for their ability to elicit immune responses in KO mice. Both target epitopes were synthesized using a two-component enzymatic system using modified disaccharide substrates containing a linker moiety for coupling. While both glycoconjugate vaccines induced the required high anti-Gal IgG antibody titers, it was found that this response had exquisite specificity for the Galα(1,3)Galβ(1,4)GlcNAc hapten used, with little cross reactivity with the Galα(1,3)Galβ(1,4)Glc hapten. Our findings indicate that while homogenous glycoconjugate vaccines provide high IgG titers, the carrier and adjuvanting factors can deviate the specificity to an antigenic determinant outside the purview of interest.

Studies related to Norway spruce galactoglucomannans: chemical synthesis, conformation analysis, NMR spectroscopic characterization, and molecular recognition of model compounds

Galactoglucomannan (GGM) is a polysaccharide mainly consisting of mannose, glucose, and galactose. GGM is the most abundant hemicellulose in the Norway spruce (Picea abies), but is also found in the cell wall of flax seeds, tobacco plants, and kiwifruit. Although several applications for GGM polysaccharides have been developed in pulp and paper manufacturing and the food and medical industries, attempts to synthesize and study distinct fragments of this polysaccharide have not been reported previously. Herein, the synthesis of one of the core trisaccharide units of GGM together with a less-abundant tetrasaccharide fragment is described. In addition, detailed NMR spectroscopic characterization of the model compounds, comparison of the spectral data with natural GGM, investigation of the acetyl-group migration phenomena that takes place in the polysaccharide by using small model compounds, and a binding study between the tetrasaccharide model fragment and a galactose-binding protein (the toxin viscumin) are reported.

Chemoenzymatic synthesis of α2-3-sialylated carbohydrate epitopes

Sialic acids are common terminal carbohydrates on cell surface. Together with internal carbohydrate structures, they play important roles in many physiological and pathological processes. In order to obtain α2-3-sialylated oligosaccharides, a highly efficient one-pot three-enzyme synthetic approach was applied. The P. multocida α2-3-sialyltransferase (PmST1) involved in the synthesis was a multifunctional enzyme with extremely flexible donor and acceptor substrate specificities. Sialyltransferase acceptors, including type 1 structure (Galβ1-3GlcNAcβProN(3)), type 2 structures (Galβ1-4GlcNAcβProN(3) and 6-sulfo-Galβ1-4GlcNAcβProN(3)), type 4 structure (Galβ1-3GalNAcβProN(3)), type 3 or core 1 structure (Galβ1-3GalNAcαProN(3)) and human milk oligosaccharide or lipooligosaccharide lacto-N-tetraose (LNT) (Galβ1-3GlcNAcβ1-3Galβ1-4GlcβProN(3)), were chemically synthesized. They were then used in one-pot three-enzyme reactions with sialic acid precursor ManNAc or ManNGc, to synthesize a library of natural occurring α2-3-linked sialosides with different internal sugar units. The sialylated oligosaccharides obtained are valuable probes for their biological studies.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update covering the period 2001-2002

This review is the second update of the original review on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates that was published in 1999. It covers fundamental aspects of the technique as applied to carbohydrates, fragmentation of carbohydrates, studies of specific carbohydrate types such as those from plant cell walls and those attached to proteins and lipids, studies of glycosyl-transferases and glycosidases, and studies where MALDI has been used to monitor products of chemical synthesis. Use of the technique shows a steady annual increase at the expense of older techniques such as FAB. There is an increasing emphasis on its use for examination of biological systems rather than on studies of fundamental aspects and method development and this is reflected by much of the work on applications appearing in tabular form.

Organic azides: an exploding diversity of a unique class of compounds

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aubé rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Pentaerythritol as the core of multivalent glycolipids: synthesis of a glycolipid with three SO3Lea ligands

A glycolipid containing three SO(3)Le(a) ligands was synthesized with pentaerythritol as the core. The glycolipid was used to prepare glycoliposomes that showed stability similar to that of DSPC liposomes without glycolipid. The easily prepared derivatives of pentaerythritol proved to be useful scaffolds for multivalent displaying of carbohydrates in the form of glycolipids and clustered glycoliposomes. [structure: see text]

Synthesis and molecular recognition of carbohydrate-centered multivalent glycoclusters by a plant lectin RCA120

Water soluble and lectin-recognizable carbohydrate-centered glycoclusters were prepared efficiently by the Huisgen 1,3-cycloaddition reaction of methyl-2,3,4,6-tetra-O-propargyl beta-D-galactopyranoside with 2-azidoethyl glycosides of lactose and N-acetyllactosamine. Their binding by a plant lectin RCA120 was examined by capillary affinity electrophoresis using fluorescence-labeled asialoglycans from human alpha1-acid glycoprotein. The glycoclusters showed 400-fold stronger inhibitory effect than free lactose, manifesting strong multivalency effect.

Regioselective and Diastereoselective Amination with Use of Chlorosulfonyl Isocyanate: A Short and Efficient Synthesis of (−)-Cytoxazone

The total synthesis of (-)-cytoxazone 1 was achieved in six linear steps (34% overall yield) from p-anisaldehyde. The key steps in this route are the regioselective and stereoselective introduction of a N-protected amine group, using the CSI reaction of the anti-1,2-dimethyl ether 3, and the subsequent regioselective cyclization of the N-protected amino diol 13 to give the 2-oxazolidinone unit of (-)-cytoxazone 1. [reaction: see text]

A General Strategy for Stereoselective Glycosylations

The principal challenge that the synthesis of oligosaccharides of biological importance presents is the development of a general approach for the stereoselective introduction of a glycosidic linkage. It is shown here that a (1S)-phenyl-2-(phenylsulfanyl)ethyl moiety at C-2 of a glycosyl donor can perform neighboring group participation to give a quasi-stable anomeric sulfonium ion. Due to steric and electronic factors, the sulfonium ion is formed as a trans-decalin ring system. Displacement of the sulfonium ion by a hydroxyl leads to the stereoselective formation of alpha-glycosides. NMR experiments were employed to show convincingly the presence of the beta-linked sulfonium ion intermediate. The (1S)-phenyl-2-(phenylsulfanyl)ethyl moiety could be introduced by reaction of a sugar alcohol with acetic acid (1S)-phenyl-2-(phenylsulfanyl)ethyl ester in the presence of BF(3)-OEt(2). Furthermore, it could be removed by conversion into acetate by treatment with BF(3)-OEt(2) in acetic anhydride. The introduction as well as the cleavage reaction proceeds through the formation of an intermediate episulfonium ion. The use of the new methodology in combination with traditional neighboring group participation by esters to introduce beta-glycosides makes it possible, for the first time, to synthesize a wide variety of oligosaccharides by routine procedures. The latter was demonstrated by the synthesis of the Galili trisaccharide, which has been identified as an epitope that can trigger acute rejections in xeno-transplantations, by the one-pot two-step glycosylation sequence.

Study of glycosylation with N-trichloroacetyl-D-glucosamine derivatives in the syntheses of the spacer-armed pentasaccharides sialyl lacto-N-neotetraose and sialyl lacto-N-tetraose, their fragments, and analogues

The syntheses of 2-aminoethyl glycosides of the pentasaccharides Neu5Ac-alpha(2-->3)-Gal-beta(1-->4)-GlcNAc-beta(1-->3)-Gal-beta(1-->4)-Glc and Neu5Ac-alpha(2-->3)-Gal-beta(1-->3)-GlcNAc-beta(1-->3)-Gal-beta(1-->4)-Glc, their asialo di-, tri-, and tetrasaccharide fragments, and analogues included a systematic study of glycosylation with variously protected mono- and disaccharide donors derived from N-trichloroacetyl-D-glucosamine of galactose, lactose, and lactosamine glycosyl acceptors bearing benzoyl protection around the OH group to be glycosylated. Despite the low reactivity of these acceptors, stereospecificity and good to excellent yields were obtained with NIS-TfOH-activated thioglycoside donors of such type, or with AgOTf-activated glycosyl bromides, while other promotors, as well as a trichloroacetimidate donor, were less effective, and a beta-acetate donor was inactive. In NIS-TfOH-promoted glycosylation with the thioglycosides, the use of TfOH in catalytic amount led to rapid formation of the corresponding oxazoline, but the quantity of TfOH necessary for further efficient coupling with an acceptor depended on the reactivity of the donor, varying from 0.07 equiv for a 3,6-di-O-benzylated monosaccharide derivative to 2.1 equiv for a peracetylated disaccharide one. In the glycosylation products, the N-trichloroacetyl group was easily converted into N-acetyl by alkaline hydrolysis followed by N-acetylation.