Role of fluoride in accelerating the reactions of dialkylstannylene acetals.

The most common method for achieving the regioselective monoalkylation of diols involves formation of dialkylstannylene acetals as intermediates. Reactions of dialkylstannylene acetals with alkyl halides are slow, but rates are enhanced by addition of fluoride or other nucleophiles. The mechanism of the fluoride-accelerated alkylation of dialkylstannylene acetals was studied at several levels of theory in the gas phase, in N,N-dimethylformamide (DMF) solution, and in DMF solution in the presence of tetramethylammonium ions. The reactive species were adducts involving addition of fluoride to tin. Under the conditions that most closely simulated experiment, reactions from fluoridated monomers and monofluoridated dimers were calculated to have similar activation energies. In the transition states in the rate-determining steps for the two pathways, carbon-oxygen bond formation was between 60 and 75% complete while tin-oxygen bond cleavage was much less advanced, between 6 and 16% complete. A test of Sn-O bond dissociation indicated that the "Sn-O bond cleavage first" mechanism is not a minimum energy pathway.

Direct synthesis of maradolipids and other trehalose 6-monoesters and 6,6'-diesters.

It was shown that reaction of trehalose with 1 equiv of a fatty acid in pyridine promoted by 1 equiv of the uronium-based coupling agent 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU) at room temperature gives a good yield of the primary ester accompanied by small amounts of the diprimary ester using hexanoic, palmitic, and oleic acids as examples. Reactions using 2 equiv of the fatty acids gave the symmetrical diesters. The monoesters were reacted with different fatty acids to give nonsymmetric 6,6'-diesters in very good yields. Compounds synthesized include the most abundant component of the very complex maradolipid mixture, 6-O-(13-methyltetradecanoyl)-6'-O-oleoyltrehalose, and a component potentially present in this mixture, 6-O-(12-methyltetradecanoyl)-6'-O-oleoyltrehalose, a derivative of an ante fatty acid. The C5-C6 rotameric populations of 6-O-monoesters, symmetrical 6,6'-diesters, and 2,6,6'-triesters of fatty acids were calculated from the values of the H5-H6R and H5-H6S coupling constants and found to be similar to those found for glucose. The rotameric populations of the monosubstituted glucose residues in the 2,6,6'-triesters was altered considerably to favor the gt rotamer, presumably because of attraction between the 2- and 6'-fatty acid chains.

How do nucleophiles accelerate the reactions of dialkylstannylene acetals? The effects of adding fluoride to dialkoxydi-n-butylstannanes.

Dialkylstannylene acetals are organotin intermediates widely used to facilitate regioselective monofunctionalization of diols or polyols by electrophiles. Alkylation is both the slowest and the most useful reaction of these intermediates, and this reaction is markedly accelerated by the addition of nucleophiles to the reaction media, usually cesium fluoride in dimethylformamide (DMF) or tetrabutylammonium iodide or bromide in toluene. The regioselectivity may be influenced by aggregation of the dialkylstannylene acetals into dimers and higher oligomers, and by the addition of these nucleophiles. The stabilities and the geometries of the species potentially involved in these processes were examined by using theoretical chemistry methods with di-n-butyldialkoxytin derivatives as examples and fluoride as the nucleophile. Geometry optimizations were performed at B3LYP/6-31G(d,p) level, and single point energies obtained at the MP2/6-311G(2d,p) level with diffuse functions added for fluorine. The LANL2DZdp basis set with diffuse and polarization functions and its effective core potential were used to describe tin. The addition of fluoride to monomeric di-n-butyldialkoxytin derivatives to give fluoridated monomers is predicted to be strongly exothermic, by 187 to 209 kJ/mol, depending on the alkoxyl group. The fluoridated monomers are calculated to react with monomers exothermically to give monofluoridated dimers, except for the di-t-butoxy derivative. Dimer formation on average is about 20 kJ mol(-1) more exothermic than for the nonfluoridated monomers alone. Monofluoridated monomers strongly prefer to exist as monomers because the difluoridated dimers are estimated to be 209 to 278 kJ mol(-1) less stable at 298 K.

Hydrolyzable tannins (chebulagic acid and punicalagin) target viral glycoprotein-glycosaminoglycan interactions to inhibit herpes simplex virus 1 entry and cell-to-cell spread.

Herpes simplex virus 1 (HSV-1) is a common human pathogen that causes lifelong latent infection of sensory neurons. Non-nucleoside inhibitors that can limit HSV-1 recurrence are particularly useful in treating immunocompromised individuals or cases of emerging acyclovir-resistant strains of herpesvirus. We report that chebulagic acid (CHLA) and punicalagin (PUG), two hydrolyzable tannins isolated from the dried fruits of Terminalia chebula Retz. (Combretaceae), inhibit HSV-1 entry at noncytotoxic doses in A549 human lung cells. Experiments revealed that both tannins targeted and inactivated HSV-1 viral particles and could prevent binding, penetration, and cell-to-cell spread, as well as secondary infection. The antiviral effect from either of the tannins was not associated with induction of type I interferon-mediated responses, nor was pretreatment of the host cell protective against HSV-1. Their inhibitory activities targeted HSV-1 glycoproteins since both natural compounds were able to block polykaryocyte formation mediated by expression of recombinant viral glycoproteins involved in attachment and membrane fusion. Our results indicated that CHLA and PUG blocked interactions between cell surface glycosaminoglycans and HSV-1 glycoproteins. Furthermore, the antiviral activities from the two tannins were significantly diminished in mutant cell lines unable to produce heparan sulfate and chondroitin sulfate and could be rescued upon reconstitution of heparan sulfate biosynthesis. We suggest that the hydrolyzable tannins CHLA and PUG may be useful as competitors for glycosaminoglycans in the management of HSV-1 infections and that they may help reduce the risk for development of viral drug resistance during therapy with nucleoside analogues.

Synthesis and removal of trichloroethylidene derivatives of carbohydrates.

Two hexoses and two pentoses have been reacted with two equivalents of trichloroacetaldehyde using boron trifluoride etherate as promoter in dioxane at reflux. The major products are exo-1,2-O-trichloroethylidenealdofuranoses in every case in varying yields, identical to the products previously obtained in lower yields using sulfuric acid or hydrochloric acid as promoter in trichloroacetaldehyde as solvent. Careful fractionation of the remainders of the product mixtures showed that the other isomer in the acetal ring was also produced in most cases but small amounts of the two 1,2-O-trichloroethylidenealdopyranose isomers were also produced. In addition, very small amounts of other products were identified that were derived from boron trifluoride etherate and the solvent. A mechanism involving rate-determining acetal ring closure was proposed to explain the formation of furanose derivatives, rather than pyranose derivatives. Methods for removal of the trichloroethylidene acetals were investigated for substituted derivatives and it was found that a process involving reduction with tributyltin hydride was effective.

2-Propynyl 2,3,4,6-tetra-O-acetyl-α-D-mannopyranoside.

The 2-propynyl group in the title compound, C(17)H(22)O(10), adopts an exoanomeric conformation, with the acetylenic group gauche with respect to position C1. Comparison of (13)C NMR chemical shifts from solution and the solid state suggest that the acetylenic group also adopts a conformation anti to C1 in solution. The pyranose ring adopts a (4)C(1) conformation. Of the three secondary O-acetyl groups, that on position O4, flanked by two equatorial groups, adopts a syn conformation, in agreement with recent generalizations [González-Outeiriño, Nasser & Anderson (2005). J. Org. Chem. 70, 2486-2493]. The acetyl group on position O3 adopts a gauche conformation, also in agreement with the recent generalizations, but that on position O2 adopts a syn conformation, not in agreement with the recent generalizations.

Synthesis of surfactants based on pentaerythritol. I. Cationic and zwitterionic gemini surfactants.

Simple strategies for the synthesis of five series of cationic gemini surfactants and one series of zwitterionic gemini surfactants from pentaerythritol have been developed. Two lipophilic groups were introduced onto pentaerythritol by alkylation of the known compound, O-benzylidenepentaerythritol, with 1-bromooctane, 1-bromodecane, 1-bromododecane, and 1-bromotetradecane. Hydrogenolysis of the benzylidene acetals gave diols which were converted into three different series of trimethylammonium derivatives. The diiodides derived from the diols could be displaced by dimethylamine, even though they are adjacent to a quaternary carbon atom. Alkylation with methyl iodide gave the first series. The iodides were easily displaced by cyanide ion and the resulting dinitriles were hydrolyzed, converted to N,N-dimethylamides, and reduced to give a second series. Oxidation of the diols to dialdehydes under Swern conditions followed by Horner-Wadsworth-Emmons reactions with diethyl N,N-dimethylcarbamoylmethylphosphonate followed by two-stage reduction gave a third series. The dialkoxides derived from the four di-O-alkylpentaerthritol diols were reacted with 2-dimethylaminoethyl chloride and 3-dimethylaminopropyl chloride in neighboring-group assisted double Williamson ether syntheses to give precursors to two more series. As expected because the neighboring group participation occurs through a four-membered-ring intermediate, considerably more vigorous conditions were required for the reactions with 3-dimethylaminopropyl chloride. 2-Diethylaminoethyl bromide was found to be less reactive than 2-dimethylaminoethyl chloride. The products were alkylated with methyl iodide and, in some cases, other alkyl halides to give cationic gemini surfactants. Alkylation of one series with ethyl bromoacetate followed by anion exchange resin catalyzed ester hydrolysis gave zwitterionic gemini surfactants. The members of all series have superior surfactant properties.

First isolation and structural determination of cyclic beta-(1-->2)-glucans from an alga, Chlorella pyrenoidosa.

The aqueous extract of the edible green microalgae Chlorella pyrenoidosa is of interest because of its immunostimulatory activity. Some components in the extract have been identified previously, namely a unique type of arabinogalactan and a galactofuran. Further fractionation of this extract was accomplished by treating the aqueous solution of the fraction precipitated by addition of 1.5vol of 95% ethanol with cetyltrimethylammonium bromide. The residue obtained by concentration of the supernatant was fractionated further by anion-exchange chromatography and size-exclusion chromatography on Sephadex G-100. Two fractions from the latter column were retained, of which one was a starch-like alpha-(1-->4)-linked d-glucan with some alpha-(1-->6) branches, and the other contained a starch plus a mixture of beta-(1-->2)-d-glucans. ESI mass spectrometry was used to show that the mixture contained both cyclic and linear beta-(1-->2)-d-glucans in a cyclic:linear ratio of 64:36, based on intensities of mass spectral peaks. For the cyclic beta-(1-->2)-d-glucans, ring sizes ranged from 18 to 35 monosaccharides with the ring containing 21 glucose units (54% of the cyclic glucans) being greater than three times more abundant than the next most abundant component, the ring containing 22 glucose units (15%). No rings containing 20 glucose units were present. This is the first observation of cyclic beta-(1-->2)-d-glucans in algae, as far as we are aware. For the linear beta-(1-->2)-d-glucans, the component containing 20 glucoses was most abundant (35% of the linear glucans), while the component containing 21 glucose units was the next most abundant (17%). These relatively low-molecular-weight glucans had low immunostimulatory activity.

Assembly and inhibitory activity of monovalent mannosides terminated with aromatic methyl esters: The effect of naphthyl groups.

A series of monovalent α-D-mannoside ligands terminated with aromatic methyl esters have been synthesized in excellent yields using the Cu(I) catalyzed azide-alkyne 1,3-dipolar cycloaddition ("click chemistry"). These mannosides were designed to have a unique aglycone moiety (tail) that combines a triazole ring attached to aromatic methyl esters via a six carbon alkyl chain. The mannose unit of these ligands was linked at the ortho, meta, and para positions of substituted methyl benzoates and 1-, 3-, and 6-substituted methyl 2-napthaoates. In hemagglutination assays, ligands (32A-38A) showed better inhibitory activities than the standard inhibitor, methyl α-D-mannopyranoside. Overall, the naphthyl-based mannoside ligand (37A) showed the best activity and therefore merits further development.

Semi-synthesis of a 3-O-sulfated red seaweed galactan-derived disaccharide alditol.

Beta-D-Galp3-SO3-(1-->4)-3,6-anhydro-L-GalOH (agarobiitol 3(2)-sulfate, 4) was semi-synthetically prepared as follows: production of agarobiitol (1) from agarose by partial reductive hydrolysis, protection of the primary hydroxyl groups of 1 with trityl groups to produce the 1(1),6(2)-di-O-tritylated derivative (2), regioselective dibutylstannylene-mediated sulfation of 2 to give the 3(2)-O-sulfated-1(1),6(2)-di-O-tritylated compound (3), and detritylation of compound 3 to give the final product (4). This semi-synthetic route allowed the preparation of a red seaweed galactan-derived disaccharide alditol with sulfate group located at C-3 of the galactopyranosidic ring. Because red seaweed galactans are glycosidically linked at C-3 of the beta-D-Galp unit, a sulfated derivative with this structure could not be obtained by partial reductive hydrolysis of sulfated red seaweed galactans.

Regioselective synthesis of long-chain ethers and their sulfates derived from methyl beta-D-galactopyranoside and derivatives via dibutylstannylene acetal intermediates.

A number of different conditions were investigated for the alkylation of the dibutylstannylene acetals of methyl beta-d-galactopyranoside with long-chain primary alkyl bromides, decyl, dodecyl, and tetradecyl bromide. The best yields of the major products, the 3-O-alkyl ethers, were obtained by reaction of the alkyl bromide with the monodibutylstannylene acetal in DMF in the presence of cesium fluoride for extended periods of time at moderate temperatures (65 degrees C). These products were always accompanied by minor amounts of the 3,6-di-O-alkyl derivative. Performing the reaction with excess alkyl halide on the bis(dibutylstannylene) acetal resulted in more of the 3,6-di-O-alkyl derivative, particularly for the shorter alkyl bromides, but this product was never predominant. Sulfation of the dibutylstannylene acetal of methyl 3-O-tetradecyl-beta-D-galactopyranoside resulted in the 6-sulfate in 96% yield.

Isolation, characterization and structural determination of a unique type of arabinogalactan from an immunostimulatory extract of Chlorella pyrenoidosa.

An arabinogalactan was isolated from a hot water extract of freeze-dried cells of the green microalga, Chlorella pyrenoidosa. This hot water extract is a proprietary immunomodulator, with the trademark Respondintrade mark (ONC-107). The arabinogalactan was recovered from the ethanol-soluble fraction of the supernatant resulting from a process that involved controlled ethanol precipitation followed by size exclusion chromatography on Sephadex G-100, then Cetavlon precipitation. Sugar analyses, GC-MS data for (S)-2-octyl glycosides, and 1D and 2D NMR experiments established unambiguously that the repeating unit was -->2)-alpha-L-Araf-(1-->3)-[alpha-L-Araf-(1-->4)]-beta-D-Galp-(1-->. This structure does not fit into any of the known classes of arabinogalactans. SEC/MALS experiments gave a molecular mass for the arabinogalactan isolated as 47 +/- 4 kDa but the original structure was probably larger.

Unique tetrameric and hexameric mannoside clusters prepared by click chemistry.

The synthesis of novel tetrameric and hexameric mannoside clusters bearing 1,2,3-trizole linkages via Cu(I)-catalyzed azide-alkyne cycloaddition reaction ("click chemistry") is described. An attractive feature of these multiarmed mannoside clusters as potential inhibitors of uropathogenic Escherichia coli is the use of an aglycone whose length is designed to fit in the tyrosine gate. The acetylated mannosides were deprotected and the corresponding de-O-acetylated mannosides were found to exhibit good water solubility.

Syntheses of monohydroxy benzyl ethers of polyols: tri-O-benzylpentaerythritol and other highly benzylated derivatives of symmetrical polyols.

Symmetrical polyols can be converted into benzyl ethers with one free hydroxyl group in good yield by reaction of the monodibutylstannylene acetal with excess benzyl bromide in the presence of tetrabutylammonium bromide and diisopropylethylamine in xylene. The reaction pathway involves initial benzylation of the dibutylstannylene acetal to give benzyl and bromodibutylstannyl ethers; if a hydroxyl group remains unsubstituted, the latter ether ring closes and reacts further.

The micellar and surface properties of a unique type of two-headed surfactant--pentaerythritol based di-cationic surfactants.

The surface properties of some families of cationic two-headed surfactants based on a pentaerythritol backbone are described. The compounds have the following general structure (1), where R' are head groups and R are linear alkyl groups ranging from octyl to tetradecyl. The syntheses of these compounds has been published in detail previously. Critical micelle concentrations (cmc values) of these two-headed surfactants have been determined and compared to conventional ionic surfactants and gemini surfactants of similar structure. In addition, the surface activity of these two-headed surfactants, expressed as the C20 value and the surface tension at the cmc, have been determined. Transmission electron microscopy has been used to examine the morphology of the aggregates formed by these amphiphiles. In general, when compared to conventional ionic and two-headed surfactants, these new two-headed surfactants exhibit a remarkable efficiency in the tendency to self-assemble and are significantly more surface active than their conventional counterparts [structure: see text].

Facile synthesis of a library of Lyme disease glycolipid antigens.

A library of one of the two Lyme disease antigens, BbGL1, has been synthesized in four steps from D-galactose using BF(3)-promoted glycosylation of the peracetate to introduce the cholesteryl ß-glycoside and TBTU-promoted esterification to add a range of fatty acids regioselectively at O-6 of D-galactose in good yield.

Efficient and controllably selective preparation of esters using uronium-based coupling agents.

Carboxylic acid esters can be prepared in excellent yields at room temperature from an acid and either a phenol or an aliphatic alcohol using the peptide coupling reagents, TBTU, TATU, or COMU, in the presence of organic bases. Reactions using TBTU and TATU are faster but do not occur with tertiary alcohols. Selectivity between reaction with primary or secondary alcohols in diols and polyols can be achieved with choice of base and coupling agent.

Isolation of phosphorylated polysaccharides from algae: the immunostimulatory principle of Chlorella pyrenoidosa.

The major immunostimulatory principle in the hot aqueous extract of Chlorella pyrenoidosa has been isolated by a sequence of ethanol precipitation, precipitation with a cationic surfactant (CTAB), size exclusion chromatography, and anion exchange chromatography. A series of phosphorylated polysaccharides were obtained having different molecular masses but with similar structures. The higher molecular mass fractions showed considerable activity in the stimulation of mouse peritoneal macrophages to synthesize nitric oxide. The structure of the major polysaccharide was established by sugar analysis, configurational analysis, and 1D and 2D NMR experiments at 500 and 800 MHz on the parent polysaccharide, the de-O-acetylated polysaccharide, and on the components obtained after hydrolysis of the phosphate diesters. It had a beta-D-Galp-(1-->3)-beta-D-Galp-(1-->3)-backbone with half of the Galp units substituted at O-6 by terminal beta-D-Glcp units. The remaining Galp units were substituted on O-6 by about equal amounts of alpha-D-Manp-1-phosphate and 3-O-Me-alpha-Manp-1-phosphate diesters. The substituents were not located in a regularly alternating fashion on the backbone. The O-acetyl groups were largely located on O-2 and O-4 of Galp and 35% of the Galp residues were O-acetylated. This is the second observation of a phosphorylated polysaccharide in an alga and the first where it is present to a significant extent.

ESI-MS differential fragmentation of positional isomers of sulfated oligosaccharides derived from carrageenans and agarans.

We have prepared a number of isomeric red seaweed galactan-derivative sulfated oligosaccharides to determine whether there were diagnostic differences among the isomeric mass spectra obtained using ESI CID MS/MS (triple quadrupole instrument). Fragmentation of the single or multicharged molecular ions from di-, tetra-, and hexasaccharides indicated that the relative positioning of the sulfate groups and type of monosaccharide unit affect the rate of cleavage of the glycosidic bonds. We also performed a comparative [M-Na](-) fragmentation study of positional isomers of sulfated disaccharides that present all four monosulfation possibilities on the galactopyranosidic ring. In this case, negative-ion ESI CID MS/MS approach gave diagnostic product ions from cross-ring cleavages along with the same main B(1) ion (from sulfated Galp), at m/z 241, for all isomers. The isomeric disaccharides were also submitted to increased spray energy conditions inducing in-source fragmentation; preformed B(1) ions were then fragmented to give similar product ions as those found in [M-Na](-) analysis. Evaluation of the relative abundances mainly for cross-ring fragment ions at m/z 138, 139, 151, 153 allowed clear distinction among the members of the disaccharide series. The different ratios for m/z 151/153 ions were consistent with the predominance of m/z 153 being related to the cases when the bond involved in the cleavage process links a sulfated carbon. A quadrupole ion trap instrument (MS(n) analysis) was also utilized to compare the results obtained with the triple quadrupole instrument.

Semisynthesis of long-chain alkyl ether derivatives of sulfated oligosaccharides via dibutylstannylene acetal intermediates.

Long-chain alkyl ether derivatives of sulfated oligosaccharides were semisynthesized as follows: two naturally occurring red seaweed galactans (neutral agarose and kappa-carrageenan) were submitted to partial reductive hydrolysis to give neutral and sulfated oligosaccharide alditols. The neutral disaccharide alditol (1) and its trityl ether (5) were sulfated and/or alkylated through formation of their dibutylstannylene or (bis)dibutylstannylene acetals. In these reactions, the dibutylstannylene acetals of the terminal 1,2-diols in the alditol units were more reactive than those formed on the cis-diols of the galactopyranosidic units. This property allowed the regioselective monoalkylation of a neutral tetrasaccharide alditol (2), which contained eleven free hydroxyl groups, the highest selectivity ever observed with dibutylstannylene acetals. An alkylated/sulfated derivative (11) was also obtained through the regioselective alkylation of a naturally sulfated disaccharide alditol (10, a kappa-carrageenan derivative).