A glycosylation protocol based on activation of glycosyl 2-pyridyl sulfones with samarium triflate

[reaction--see text] Reaction of glycosyl 2-pridyl sulfones (e.g.,2) with alcohols and samarium(III) triflate affords glycosides in moderate to excellent yields. Benzylated sulfones can be activated in preference to their benzoylated counterparts, and the methodology has been used to prepare di- and trisaccharides containing both furanose and pyranose residues. Thioglycosides do not react under these conditions, and the sulfones are inert to the N-iodosuccinimide/silver triflate promoter system commonly used to activate thioglycosides. This selectivity allowed the efficient preparation of oligosaccharides via orthogonal glycosylation protocols.     

On-resin generation and reactions of orthogonal glycosyl donors

An orthogonal series of glycosyl donors has been generated in situ from thiophenylglycosides appended to a hydroxymethylpolystyrene resin through a succinate linker. The conditions used to generate these donors and their subsequent reactions with sugar acceptors will be described.     

Electrochemical synthesis of dendritic diarylcarbenium ion pools

Dendritic diarylcarbenium ion pools were synthesized by the low temperature electrochemical oxidation of the corresponding dendritic (diarylmethyl)trimethylsilanes, which were prepared by use of the iterative method consisting of electrochemical activation and Friedel-Crafts type coupling. Time-course NMR studies revealed that thermal stability of dendritic diarylcarbenium ions depends both on the generation of the dendritic structure and on the para-substituents of the terminal phenyl groups. Dendritic diarylcarbenium ions up to the third generation exhibited high reactivity as a carbon electrophile.     

Oxidative generation of diarylcarbenium ion pools

"Cation pools" of diarylcarbenium ions have been generated by the oxidative C-H bond dissociation of diarylmethanes using anodic oxidation. "Diarylcarbenium ion pools" thus generated react with various nucleophiles, such as allylsilanes, ketene silyl acetals, and aromatic compounds. The reductive homocoupling of the "diarylcarbenium ion pool" has been achieved. The dimer thus obtained also serves as a precursor of the "diarylcarbenium ion pool" via oxidative C-C bond dissociation. [reaction: see text]     

Glycal assembly by the in situ generation of glycosyl dithiocarbamates

Glycal assembly offers an expedient entry into β-linked oligosaccharides, but epoxyglycal donors can be capricious in their reactivities. Treatment with Et(2)NH and CS(2) enables their in situ conversion into glycosyl dithiocarbamates, which can be activated by copper triflate for coupling with complex or sterically congested acceptors. The coupling efficiency can be further enhanced by in situ benzoylation, as illustrated in an 11-step synthesis of a branched hexasaccharide from glucals in 28% isolated yield and just four chromatographic purifications.     

Electrochemical hydride generation of thallium

An electrochemical hydride generation (ECHG) technique was developed to improve the determination of thallium by atomic spectrometry. The technique is based on the catholyte variation system for production of thallium hydride. Using Pb-Sn alloy as cathode, a transient peak shaped signal was achieved and its height, the maximum absorbance value, was taken as an analytical parameter. Parameters that might affect the hydride generation efficiency were investigated and the analytical performance of the method under the optimized experimental conditions was assessed. The linear range was 1-250 ng mL⁻¹ for thallium and the relative standard deviation of the method was 4.2% (RSD, n = 7). The LOD for thallium was found to be 0.8 ng mL⁻¹, showing a significant improvement relative to conventional chemical hydride generation techniques. The proposed method was applied to the determination of thallium in unalloyed zinc standard reference material. This method offers high sensitivity, simplicity, rapidness, freeness from reagent and low acid consumption.     

Electrochemical generation of metastable rhenocene

Conclusions (⁵-C₅H₅)(³-C₅H₅)Re^– anions are formed in THF as a result of the irreversible two-electron electrochemical reduction of (Cp₂Re)₂ accompanied by cleavage of the Re-Re bond. The oxidation of these anions leads to the generation of monomeric rhenocene with E⁰=–1.06 V for the Cp₂Re^0/+ redox transition.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 483–485, February, 1987.     

Remarkably efficient activation of glycosyl trichloro- and (N-phenyl)trifluoroacetimidates with bismuth(III) triflate

Easily handled and nontoxic Bi(OTf)₃ is a powerful activator for trichloro- and (N-phenyl)trifluoroacetimidate glycosyl donors. This catalyst allows glycosidations to be performed at low temperatures in very short times. Rewarding yields were obtained from a wide range of donors of varying reactivity.     

Electrochemical caustic generation from sewage

Sewer corrosion caused by hydrogen sulfide generation represents a major issue in sewer management. One commonly used approach for mitigation is the periodic dosing of caustic to the sewer. This creates elevated pH levels that inactivate/kill sulfate reducing bacteria (SRB) present in the biofilms on the sewer pipe. In this study, we investigated the in-situ electrochemical generation of caustic from sewage. The average coulombic efficiency for caustic generation was 53 ± 8% (n = 10) at a current density of 10 mA/cm². The average required cell voltage input was 5.2 ± 0.7 V. Over time, this voltage increased due to a membrane fouling associated increase of the ohmic resistance of the cell. Switching the polarity of the electrodes every four hours was effective to control this fouling and thus retain a stable voltage requirement. This process constitutes a promising reagent-free method for sulfide abatement in sewers.     

A practical method for the stereoselective generation of beta-2-deoxy glycosyl phosphates

Beta-2-Deoxy sugar nucleotides are substrates used by a variety of glycosyltransferases (Gtfs). We have developed a chemical route to synthesize beta-2-deoxy sugar phosphates that starts from alpha-glycosyl chlorides. Our approach reliably provides access to a range of NDP beta-2-deoxy sugars essential for studying glycosyltransferases involved in the synthesis of biologically active natural products.     

Direct and indirect electrochemical generation of alkoxycarbenium ion pools from thioacetals

Thioacetals were found to be effective precursors to generate and accumulate alkoxycarbenium ions based on direct and indirect cation pool methods. Alkoxycarbenium ions thus generated reacted with carbon nucleophiles such as allylsilanes and enol silyl ethers to give C-C bond formation products in good yields.     

Synthesis and use of glycosyl phosphates as glycosyl donors

Differentially protected glycosyl phosphates prepared by a straightforward synthesis from glycal precursors are used as powerful glycosyl donors. Activation of beta-glycosyl phosphates by TMSOTf at -78 degrees C achieves the selective formation of beta-glycosidic linkages in excellent yields with complete stereoselectivity. Reaction with thiols results in the conversion of glycosyl phosphates into thioglycosides in nearly quantitative yield. An orthogonal coupling strategy using glycosyl phosphate donors and thioethyl glycoside acceptors allows for the rapid synthesis of a trisaccharide.     

Electrochemical generation of solvated electrons from nanostructured carbon

Voltammograms for electrodes fabricated of nanostructured carbon of various morphology (nanotube paper, columnar and filament structures) in hexamethylphosphoric triamide (HMPA) solution have been obtained and analysed. Intensive dark-blue coloration near cathode surface at potentials as low as E ∼ −1.2 V (s.c.e.) has been observed. An ESR (electron spin resonance) spectrum of this frozen dark-blue solution was recorded. This spectrum coincides with one of free electrons. Experimental proofs of the existence of electron emission into electrolytic solutions at moderate cathodic potentials are present for all electrodes. This effect is established to be connected with the presence of atomically sharp areas on the electrode surfaces.     

Electrochemical jet-cell for the in-situ generation of hydrogen peroxide

In this work, a proof of concept of a novel reactor design for the in-situ electrochemical production of H₂O₂ from oxygen reduction reaction is presented for the first time. The innovative design incorporates a venturi-based jet aerator to supply atmospheric oxygen without additional energy consumption to a 3D flow-through modified carbon felt (CF) cathode. Preliminary experiments confirmed that electro-generation of hydrogen peroxide is possible in a system as the one proposed. Comparison with a flow-by cell with a gas diffusion cathode under similar conditions revealed that current efficiency towards hydrogen peroxide accumulation is even higher (72 vs 65% at 1 h) than in the case of the conventional system. Jet aerator stands as a promising oxygen supply thanks to its excellent performance and both low investment cost and energy consumption. Considering all the above, the electrochemical jet cell stands as a rather promising design for the efficient hydrogen peroxide electro-generation.     

Indirect cation pool method. Rapid generation of alkoxycarbenium ion pools from thioacetals

A sequential one-pot indirect cation pool method has been developed. The method involves the electrochemical generation and accumulation of ArS(ArSSAr)+ at low temperature (step 1) and the follow-up reaction with a thioacetal to generate an alkoxycarbenium ion pool (step 2), which reacts with various carbon nucleophiles (step 3). Steps 2 and 3 are extremely fast. The electrogenerated ArS(ArSSAr)+ was well-characterized by 1H NMR and CSI-MS. The alkoxycarbenium ion pool generated by the present indirect method exhibited 1H and 13C NMR spectra and thermal stability similar to those of the alkoxycarbenium ion pool generated by the direct electrochemical method.     

Versatile application of trifluoromethyl triflate

Hydrolytically stable and easy to handle trifluoromethyl triflate was found to be a liquid reservoir of ‘masked’ difluorophosgene. Anhydrous F⁻ sources cleave the S-O bond in trifluoromethyl triflate yielding quantitatively the trifluoromethanolate salts, being useful trifluoromethoxy group carriers. Reaction of trifluoromethanolates with in situ generated from o-trimethylsilylphenyl triflate benzyne leads to (trifluoromethoxy)benzene and fluorobenzene (ratio 85:15). Whereas an addition of trifluoromethanethiolate anion across a triple bond of benzyne leads to [(trifluoromethyl)sulfanyl]benzene solely.     

Synthesis of glycosyl dipyrromethanes

Treatment of peracetylated sugar trichloroacetimidates with dipyrromethane in the presence of boron trifluoride diethyl etherate gave peracetylated glycosyl dipyrromethanes in good yields. Regio- and stereo-selectivity of the glycosylation reactions were established.     

Electrochemical generation of N-nitramine ions upon the reduction of N-nitramines

N. D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR, Moscow. Trnaslated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, p. 2653–2654, November, 1989. Original article submitted June 30, 1989.