Synthesis of a library of fucopyranosyl-galactopyranosides consisting of a complete set of anomeric configurations and linkage positions

A library composed of a complete set of fucopyranosyl-galactopyranosides was synthesized. A perbenzylated phenylthio fucopyranoside and a series of tri-O-benzyl-galactopyranosyl fluorides having single hydroxyl groups at the 2-, 3-, 4-, and 6-positions were used as the glycosyl donor and glycosyl acceptors, respectively. The chosen set of functionalities at the anomeric centers enabled rapid access to the oligosaccharides based on chemoselective activation. The first coupling reaction was achieved by the action of dimethyl(methylthio)sulfonium trifluoromethanesulfonate (DMTST). The resulting disaccharide fluoride was readily activated by hafnocene bistrifluoromethanesulfonate [Cp2Hf(OTf)2] and glycosidated with n-octanol.     

Synthesis of beta-C-glycopyranosyl-1,4-naphthoquinone derivatives and their cytotoxic activity

Beta-C-Glucosyl and beta-C-galactosyl-1,4-dimethoxynaphthalenes have been synthesized using a F3CCO2Ag/SnCl4 promoted Friedel-Crafts electrophilic substitution reaction. Both glycosyl acetates and methyl glycosides can be used as glycosyl donors. Further oxidation afforded the corresponding beta-C-glycosyl-1,4-naphthoquinones. The in vitro cytotoxic activity of these compounds was evaluated against the A375 cell line.     

Non-isosteric C-glycosyl analogues of natural nucleotide diphosphate sugars as glycosyltransferase inhibitors

A series of C-glycosyl ethylphosphonophosphate analogues of UDP-Glc, UDP-Gal, UDP-GlcNAc and GDP-Fuc were synthesized from the corresponding C-glycosyl ethylphosphonic acids. Analogues were obtained as alpha-anomers through either diastereoselective photo-induced radical addition of glycosyl bromides (D-Glc, D-Gal and L-Fuc) to diethyl vinylphosphonate, or a multi-step sequence (D-GlcNAc), with subsequent coupling with morpholidate-activated nucleotide monophosphates. The in vitro inhibitory activity of UDP-Gal, GDP-Fuc and UDP-GlcNAc analogues towards glycosyltransferases (beta-1,4-GalT, FUT3 and LgtA) was evaluated through a competition fluorescence assay and IC(50) values of 40 microM, 2 mM and 3.5 mM were obtained, respectively.     

Convenient syntheses of symmetrical and unsymmetrical glycosyl carbodiimides and N,N-bis(glycosyl)cyanamides

Reaction of glycosyl trimethylphosphinimides with carbon disulfide under mild conditions (room temperature, short reaction time) leads to symmetrical glycosyl carbodiimides. Addition of bis(trimethylsilyl)carbodiimide to peracetylated aldoses under the influence of SnCl(4) afforded N,N-bis(glycosyl)cyanamides for the first time. Readily accessible unsymmetrical N,N'-bis(glycosyl)thioureas can be desulfurated and transformed into the corresponding carbodiimides using HgO in CHCl(3)/water at room temperature.     

One-pot, high yield synthesis of alpha-ketols from Delta5-steroids

Alpha-hydroxy ketones (alpha-ketols) are present in many compounds with biological activity. Several methods are available for the preparation of alpha-ketols but only a few of them describe the synthesis of steroid alpha-ketols from olefins. In this work, a new system consisting of KMnO(4)/Fe(ClO(4))(3).nH(2)O was used in order to achieve the direct conversion of Delta(5)-steroids to their corresponding alpha-ketols, in high yields. Consideration of the probable reaction mechanism is provided. 2D homo- and heteronuclear correlation NMR spectroscopic techniques were used to assign (1)H and (13)C resonances of some synthesized compounds. This method has potential for the preparation of alpha-hydroxy ketones of biological interest.     

Glycosyl fluorides in enzymatic reactions

Glycosyl fluorides have considerable importance as substrates and inhibitors in enzymatic reactions. Their good combination of stability and reactivity has enabled their use as glycosyl donors with a variety of carbohydrate processing enzymes. Moreover, the installation of fluorine elsewhere on the carbohydrate scaffold commonly modifies the properties of the glycosyl fluoride such that the resultant compounds act as slow substrates or even inhibitors of enzyme action. This review covers the use of glycosyl fluorides as substrates for wild-type and mutant glycosidases and other enzymes that catalyze glycosyl transfer. The use of substituted glycosyl fluorides as inhibitors of enzymes that catalyze glycosyl transfer and as tools for investigation of their mechanism is discussed, including the labeling of active site residues. Synthetic applications in which glycosyl fluorides are used as glycosyl donors in enzymatic transglycosylation reactions for the synthesis of oligo- and polysaccharides are then covered, including the use of mutant glycosidases, the so-called glycosynthases, which are able to catalyze the formation of glycosides without competing hydrolysis. Finally, a short overview of the use of glycosyl fluorides as substrates and inhibitors of phosphorylases and phosphoglucomutase is given.     

Structural and functional characterization of seven spermicidal vanadium(IV) complexes: potentiation of activity by methyl substitution on the cyclopentadienyl rings.

In a systematic effort to identify and develop effective vanadocene(IV) complexes as a new class of contraceptive agents, the effect of methyl substitution in the cyclopentadienyl rings of Cpx2VCl2 on their spermicidal activity has been examined. The spermicidal activities of compounds Cpx2VCl2 [Cpx = Me5Cp (Cp*) (1), Cp (3), MeCp (Cp') (5)], as well as two of their corresponding vanadium(V) oxidation products Cp*V(O)Cl2 (2) and CpV(O)Cl2 (4), were examined by computer-assisted sperm analysis (CASA). These analyses have established that penta-substitution of the Cp ring by electron-donating methyl groups augments the spermicidal activity 10-fold. The corresponding V(V) oxo compounds, Cp*V(O)Cl2 (2) and CpV(O)Cl2 (4), tested under identical conditions did not show as effective spermicidal activity even though these complexes have a pseudo-tetrahedral geometry similar to the active vanadocene(IV) dichlorides. Two pseudo-octahedral V(IV) complexes with tris-pyrazolyl borate as ligand, (HBpz3)V(O)Cl.DMF (6) and (HB(3,5-Me2pz)3)V(O)Cl.DMF (7), were also found to exhibit potent spermicidal activity. Although some vanadium(IV) complexes may immobilize sperm due to the generation of .OH radicals, the lack of spermicidal activity of VOSO4 which generates .OH radicals, and the potent spermicidal activity of [Cp2V(acac)][O3SCF3] (8), and [Cp2V(DeDtc)][O3SCF3] (9) which do not generate .OH radicals, indicate that .OH radical mediated reactions may not be essential for the spermicidal activity of vanadium(IV) complexes.     

Synthesis of glycosyl amino acids by light-induced coupling of photoreactive amino acids with glycosylamines and 1-C-aminomethyl glycosides

The glycosylamines of O-acetyl-protected GlcNAc and chitobiose, as well as two partially unprotected 1-C-aminomethyl glucosides, were photochemically coupled with orthogonally protected N-aspartyl-5-bromo-7-nitroindoline derivatives. The reactions proceeded under neutral conditions by irradiation with near-UV light. The glycosyl asparagines with N- or C-glycosyl linkages were afforded in 60-85% yield on a 10-70 mg scale. Moreover, the ability of a highly photoreactive N-glutamyl-4-methoxy-7-nitroindoline derivative to acylate amino saccharides was tested. Upon irradiation in the presence of a dimeric 1-C-aminomethyl glycoside, or a glycosylamine, the corresponding glycosyl glutamines were obtained in 50% and 30% yield, respectively. Preparations of the photoreactive aspartates and the 1-C-aminomethyl glycosides are also described.     

Design and synthesis of new models for diiron biosites

In order to mimic dinuclear active sites of some non-heme diiron proteins, ten new polydentate and potentially dinucleating ligands have been synthesized. Each ligand contains a carboxylate moiety designed to bridge two metal atoms. These central carboxylate moieties are derived from substituted benzoic acids that in turn are linked to terminal nitrogen or oxygen donors by spacers so that framework-type polydentate ligands similar to the polypeptide frames in diiron metallobiosites are formed. Reaction of these ligands with Fe(ClO4)3 x 9H2O leads to ferric mu-oxo-mu-carboxylato iron complexes of the general formulas [Fe2O(L)2(H2O)2](ClO4)2 and [Fe2O(L)(BzO)](ClO4)2 (L = ligand), containing one or two immobilized bridging carboxylates, respectively. While X-ray crystallography shows that some of these complexes are dimers or network polymers in the solid state, electrospray ionization mass spectrometry (ESMS) and spectroscopic data (UV-Vis, NMR, Móssbauer) indicate that they dissociate to monomeric Fe2O units in dilute CH3CN solutions.     

Synthesis, characterization and DNA binding of ruthenium(II) complexes containing the atatp ligand

Acenaphtheno[1,2-b]-1,4,8,9-tetraazatriphenylene (atatp) and its complexes [Ru(L)2atatp](ClO4)2 x nH2O (L = 2,2'-bipyridine (bpy), n=2 (1); 1,10-phenanthroline (phen), n=2 (2); and 2,9-dimethyl-1,10-phenanthroline (dmp), n=1 (3)) have been synthesized and characterized by elemental analyses and 1H NMR. The spectral and electrochemical properties of these complexes are also examined. Complexes 1 and 2 display bright luminescence in acetonitrile but very weak luminescence in water solution. However, complex 3 is not luminescent in either solvent. The interaction of the complexes with calf thymus DNA (CT-DNA) has been studied by absorption, emission and viscosity measurements. The intrinsic binding constants of complexes 1 and 2 are 7.6 x 10(4) and 8.8 x 10(4) M(-1) respectively. The relatively low affinities of complexes 1 and 2 with DNA may arise from the atatp ligand, indicating that the size and shape of the intercalated ligand have a marked effect on the strength of interaction. Complexes 1 and 2 bind with CT-DNA in an intercalative mode but complex 3 in a non-intercalative one, showing that changing the ancillary ligand affects not only the binding magnitude, but also the binding mode of the interaction.     

Studies on the origin of stereoselectivity in the synthesis of 1,2-trans glycofuranosyl azides

The stereoselectivity of the 1,2-trans directed, Lewis acid-catalysed azidation of peracylated furanoses was found to depend on the reactivity of the azide donor (azide nucleophilicity) and the configuration at the anomeric centre relative to the neighbouring 2-O-acyl group. Reactions of 1,2-trans glycosyl esters with highly nucleophilic azide donors, generated from SnCl4 and Me3SiN3, were stereospecific. The results are interpreted in terms of the rapid reaction of the azide species with bicyclic 1,2-acyloxonium (1,2-O-alkyliumdiyl-D-glycofuranose) ions, which were the primarily formed reactive intermediates. When using 1,2-cis glycosyl esters as starting materials the selectivity was reduced (90-94% de); the same is true with 1,2-trans counterparts if less nucleophilic Me3SiN3 in combination with Me3SiOTf catalyst was used. This occurred due to the appearance of the more reactive but less selective oxocarbenium (glycofuranoxonium) ions either as primarily formed reactive intermediates in the former case or after equilibration with acyloxonium ions in the latter case. Protected 1,2-trans beta-D-glycofuranosyl azides with ribo, xylo and 3-deoxy-erythro-pento configurations were best prepared from the corresponding glycosyl esters using 0.05 equivalents of SnCl4, i.e., under anomerization-free conditions. Azidation of methyl glycofuranosides proceeds with inferior (80-90% de) and less predictable selectivity irrespective of the starting anomeric configuration.     

A study of n-pentenylorthoesters having manno, gluco and galacto configurations in regioselective glycosylations

Kochetkov's extensive investigations of glycosyl orthoester and their analogs as glycosyl donors revealed that the alkyl derivatives were plagued by competition between the departing alcohol and the incoming acceptor. n-Pentenyl orthoesters (NPOEs) obviate competition by sequestering the departing pentenyl alcohol as a 2-halomethyl tetrahydrofuran. Exquisitely regioselective glycosidations of diol acceptors can be carried out with NPOEs triggered specifically with Yb(OTf)(3)/NIS. However with Sc(OTf)(3), double glycosidation is the major reaction. manno, gluco and galacto NPOEs have been investigated. The latter two, which require a different experimental procedure for the manno counterpart, also give an excellent regioselectivity with Yb(OTf)(3), but the yields are much lower than with manno.     

氟代糖在糖苷酶研究中的应用

近年来,氟代糖应用于糖苷酶反应研究,显示出越来越重要的作用。氟代糖可以作为糖苷酶及其突变酶的水解底物研究酶学性质;氟代糖抑制剂可以标记糖苷酶催化中心,鉴定亲核体氨基酸。尤为重要的是,氟代糖可作为糖苷酶的糖基供体来合成糖类。糖苷酶突变后,可生成糖苷合成酶和硫代糖苷合成酶,可以用与正常底物构型相反的氟代糖作为糖基供体高效合成糖类,收率一般为60%~90%,有的可达100%。糖苷酶及其突变酶以氟代糖为底物高效合成糖类的研究,必将促进生物学、糖生物学和纳米生物材料的发展。     

An efficient synthesis of two monosulfated trisaccharides with the Galbeta1,3GlcNacbeta1,3Galbeta-O-allyl backbone

The GlcNAcbeta(1-->3) Gal linked disaccharide 7 was synthesized as key building blocks for the construction of target monosulfated trisaccharides 1 and 2 using oxazoline 3 as glycosyl donor promoted by BF3 x Et2O.     

Synthesis of inositol glycan cyclic phosphates

An efficient synthesis of tri-, tetra-, and pentasaccharide cyclic phosphates 1-5, structurally related to natural inositol phosphate glycans, is reported. The title compounds were assembled by PhSeOTf-promoted glycosylation of the known glucosamine precursor, t-butyldimethylsilyl 2-azido-3,6-di-O-benzyl-2-deoxy-beta-D-glucopyranoside (8) with protected 1-methylthio mono-, di-, and trimannosides 7a-c, and, after conversion into glycosyl fluorides, Cp2ZrCl2- AgOTf-promoted glycosylation of differentially protected optically pure 1D-myo-inositol 11. The syntheses were completed by installing the cyclic phosphate moieties with methylpyridinium dichlorophosphate and finally, removal of all protecting groups by dissolving-metal reduction.     

The hydrolysis of glycosyl fluorides by glycosidases. Determination of the anomeric configuration of the products of glycosidase action

The enzymic hydrolysis of glycosyl fluorides is conveniently followed by using a pH-stat. Reactions involving glucosyl or galactosyl fluorides can also be followed by using glucose oxidase or galactose oxidase respectively. The pH-stat allows the rapid assay of intestinal alpha-glucosidase in crude homogenates. Use of glycosyl fluorides as substrates for glycosidases facilitates the polarimetric or g.l.c. determination of the anomeric nature of the initial product of hydrolysis. Hydrolysis by fungal amyloglucosidase proceeds with inversion of configuration whereas that by yeast and rat intestinal alpha-glucosidase, coffee-bean alpha-galactosidase and almond emulsin beta-glucosidase proceeds with retention of configuration. beta-d-Glucopyranosyl azide was not a detectable substrate for almond emulsin beta-d-glucosidase.     

Synthesis of a tetrasaccharide phosphate from the linkage region of the arabinogalactan-peptidoglycan complex in the mycobacterial cell wall

The synthesis of dibenzyl 6-O-naphthylmethyl-2,3,5-tri-O-benzoyl-beta-D-galactofuranosyl-(1-->5)-2,3-di-O-benzoyl-6-O-benzyl-beta-D-galactofuranosyl-(1-->4)-3-O-benzyl-2-O-pivaloyl-alpha-l-rhamnopyranosyl-(1-->3)-2-acetamido-2-deoxy-4,6-di-O-benzoyl-alpha-D-glucopyranosyl phosphate (1), a protected form of the tetrasaccharide phosphate of the linkage region of the arabinogalactan-peptidoglycan complex in the mycobacterial cell wall, has been accomplished. Key steps include the coupling of four monosaccharide building blocks with complete stereoselectivity by glycosylations employing thioglycosides, 2'-carboxybenzyl glycosides, and glycosyl fluorides as glycosyl donors. The alpha-glycosyl phosphate linkage was also stereoselectively elaborated by reaction of a tetrasaccharide hemiacetal with tetrabenzyl pyrophosphate in the presence of a base.     

Novel glycosylation of the nitroxyl radicals with peracetylated glycosyl fluorides using a combination of BF(3) x OEt(2) and an amine base as promoters

Glycosylation of the nitroxyl radicals, 4-acetoxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-acetoxy-TEMPO) and 3-carbamoyl-2,2,5,5-tetramethylpyrollin-1-oxyl (3-carbamoyl-PROXYL) with peracetylglycosyl fluoride as the glycosyl donor, in the presence of boron trifluoride diethyl etherate (BF(3) x OEt(2)) and an amine base afforded the corresponding hydroxylamine-O-glycosides in 25-100% yields.     

Isatin-Schiff base copper(II) complexes and their influence on cellular viability

Some copper(II) complexes with isatin (isa) or imine ligands derived from isatin were prepared, characterized by analytical and spectroscopic techniques, and had their biological activity toward proliferation of two different cell types verified. These complexes exhibit keto-enolic equilibria in aqueous solution, very dependent of pH, although isolated in the solid state in one defined form, and this type of equilibrium was previously verified to be crucial for their catalytic activity in the oxidation of carbohydrates, through intermediary generation of reactive oxygen species. Herein, biological studies carried out with tumor cells of different origin such as human neuroblastoma (SH-SY5Y) and promonocytic (U937) cells showed that these compounds exert different toxicity. In particular, while compounds [Cu(isaen)(H(2)O)]ClO(4).2H(2)O 2, [Cu(isahist)(H(2)O)](ClO(4))(2)4 and [Cu(isa)(2)]ClO(4)6 are not toxic for both cell lines at the concentrations used in this study, compounds [Cu(isapn)](ClO(4))(2)1, [Cu(isaepy)(2)](ClO(4))(2).2H(2)O 3 and [Cu(isami)(H(2)O)]ClO(4)5 are cytotoxic, with the compound 3 being the most effective. In these compounds, isaen, isahist, isapn, isaepy and isami stand for imine ligands prepared by condensation of ethylenediamine (en), histamine (hist), 1,3-diaminopropane (pn), 2-aminoethylpyridine (epy), and 8-aminoquinoline (ami) with isatin (isa). Cells treated with these compounds were committed to the apoptotic program as evidenced by cytofluorimetric analyses of cell cycle. Moreover, the toxicity of compound 5 was equivalent for both cell lines while the compound 1 was almost not toxic at 24h for SH-SY5Y cells where only an arrest in G1 phase was observed. Compound 3 was more efficient in inducing cell death and also in this case a striking effect on U937 cells (apoptotic cells 68% compared with 11% of SH-SY5Y) was observed. Therefore, the results indicated that their activity seems to be cell type specific.     

Synthesis of the C1-phosphonate analog of UDP-GlcNAc

We describe the first synthesis of the C1-phosphonate analog of UDP-GlcNAc, based on a new preparation of the corresponding glycosyl phosphonate. This C-glycosyl analog is shown to be a very weak inhibitor (Ki>10 mM) of fungal chitin synthase, indicating that at least in this case the replacement of the anomeric oxygen with a methylene group is not an innocent substitution.