Surfactant-mediated thioglycosylation of 1-hydroxy sugars in water

Cascade Synthesis in Water: Michael Addition/Hemiketalization/Retro-Claisen Fragmentation Catalyzed by CatAnionic Vesicular Nanoreactor from Dithiocarbamate

N,N-didodecylammonium N,N-didodecyldithiocarbamate (AmDTC-C12C12) underwent self-assembly to form a CatAnionic vesicular nanoreactor in water. AmDTC-C12C12 can be readily prepared by condensation between N,N-didodecylamine and carbon disulfide. Previously, the cascade Michael addition/hemiketalization/retro-Claisen fragmentation was reported, but it required petroleum-based organic solvents as reaction media. Herein, the application of AmDTC-C12C12 in aqueous cascade synthesis is investigated. Initially, we explored the catalytic activity of AmDTC-C12C12 (10 mol %) in the synthesis of 4H-chromene through a double-cascade Michael addition/hemiketalization. The reaction occurred in water at room temperature using 2-hydroxy-trans-β-nitrostyrene as Michael acceptor and acetylacetone as Michael donor, yielding 2-chromanol intermediates. Subsequent acidic dehydration of 2-chromanols produced 4H-chromenes with moderate yields (34-60 %) and phenyl acetates of γ-nitro ketone as co-products (13-31 %), deriving from retro-Claisen fragmentation. Surprisingly, using Michael donors with aromatic moieties on the 1,3-dicarbonyls resulted in spontaneous triple-cascade Michael addition/hemiketalization/retro-Claisen fragmentation in water without the need for acidic dehydration. The γ-nitro ketones were obtained as sole products, with no detection of 4H-chromenes, in moderate to high yields (24-84 %) for symmetrical 1,3-dicarbonyl containing two aromatic groups. Unsymmetrical 1,3-dicarbonyl bearing aromatic/aliphatic or aromatic/aromatic groups afforded γ-nitro ketones in favorable yields (73-97 %).

Why is thiol unexpectedly less reactive but more selective than alcohol in phenanthroline-catalyzed 1,2-cis O- and S-furanosylations?

The lack of catalytic stereoselective approaches for producing 1,2-cis S-furanosides emphasizes the critical need for further research in this area. Herein, we present a stereoselective S-furanosylation method, utilizing a 4,7-dipiperidine-substituted phenanthroline catalyst. This developed protocol fills a gap in the field, enabling the coupling of cysteine residues and thiols with furanosyl bromide electrophiles. The process allows for stereoselective access to 1,2-cis S-furanosides. Through computational and experimental investigations, thiol is found to be less reactive than alcohol but exhibits greater stereoselectivity. The 1,2-cis stereoselectivity of O-products depends on the nature of the electrophile, while S-products are obtained with excellent 1,2-cis stereoselectivity, irrespective of the furanose structure. The displaced bromide ion from the glycosyl electrophile influences the reaction's reactivity and stereoselectivity. Alcohol-OH forms a stronger hydrogen bond with bromide ion than thiol-SH, contributing to the difference in their reactivity. The energy difference between forming S-furanoside and O-furanoside transition states is 3.7 kcal mol-1, supporting the increased reactivity of alcohol over thiol. The difference in transition state energies between the major and minor S-product is greater than that for the major and minor O-product. This is consistent with experimental data showing how thiol is more stereoselective than alcohol. The catalyst and reaction conditions utilized for the generation of 1,2-cis O-furanosides in our prior studies are found to be unsuitable for the synthesis of 1,2-cis S-furanosides. In the present study, a highly reactive phenanthroline catalyst and specific reaction conditions have been developed to achieve stereoselective S-linked product formation.

Complement receptor 3-dependent engagement by Candida glabrata β-glucan modulates dendritic cells to induce regulatory T-cell expansion

Candida glabrata is an important pathogen causing invasive infection associated with a high mortality rate. One mechanism that causes the failure of Candida eradication is an increase in regulatory T cells (Treg), which play a major role in immune suppression and promoting Candida pathogenicity. To date, how C. glabrata induces a Treg response remains unclear. Dendritic cells (DCs) recognition of fungi provides the fundamental signal determining the fate of the T-cell response. This study investigated the interplay between C. glabrata and DCs and its effect on Treg induction. We found that C. glabrata β-glucan was a major component that interacted with DCs and consequently mediated the Treg response. Blocking the binding of C. glabrata β-glucan to dectin-1 and complement receptor 3 (CR3) showed that CR3 activation in DCs was crucial for the induction of Treg. Furthermore, a ligand-receptor binding assay showed the preferential binding of C. glabrata β-glucan to CR3. Our data suggest that C. glabrata β-glucan potentially mediates the Treg response, probably through CR3-dependent activation in DCs. This study contributes new insights into immune modulation by C. glabrata that may lead to a better design of novel immunotherapeutic strategies for invasive C. glabrata infection.

Glycosylation of n-pentenyl glycosides using bromodiethylsulfonium salt as an activator: interception of the glycosyl intermediate by chloride ion transfer

Utilization of n-pentenyl glycosides (NPGs) in modern carbohydrate synthesis may be hindered by their sluggish activation, which results from reversible halogenation and cyclization processes. Bromodiethylsulfonium bromopentachloroantimonate (BDSB) has been previously shown to be a powerful brominating agent for the cation-π polyene cyclization of less reactive and electron-poor polyenes. This study demonstrates the activation of NPGs using BDSB as a powerful brominating agent. BDSB effectively activates the terminal olefins of NPGs and the reaction proceeds through 5-exo-tet cyclization, offering a rapid and mild approach for glycosylation with a wide range of glycosyl donors, including n-pentenyl mannoside, n-pentenyl galactoside, and n-pentenyl glucoside. The success of this approach derives from the chloride ion transfer from the nonnucleophilic SbCl5Br anion to the glycosyl intermediate, which disrupts the equilibrium and produces a glycosyl chloride intermediate that is smoothly converted to 22 coupling products, with yields ranging from moderate to excellent (49-100%). The β-selective glycosylation is accomplished when employing NPGs equipped with a neighboring participating group. The practicality of the BDSB-activated glycosylation is demonstrated by a gram-scale synthesis. This study showcases BDSB as a potent activator for NPG glycosylation through the interception of a glycosyl intermediate that diminishes the equilibration during halogenation and 5-exo-tet cyclization.

One-Pot Synthesis of Glycosyl Chlorides from Thioglycosides Mediated by Bromodiethylsulfonium Salt as A Mild Oxidant

Conventional synthesis of glycosyl chlorides from thioglycosides relies on the two sequential oxidation and chlorination. A one-pot synthesis of glycosyl chlorides is warranted as an alternative method. Herein, we report the one-pot synthesis of glycosyl chlorides from thioglycoside precursors. The transformation was mediated at low temperature by bromodiethylsulfonium bromopentachloroantimonate (BDSB) as a mild oxidant and n-Bu4NCl as an additive. The armed thioglycosides afforded the corresponding α-glycosyl chlorides in moderate to good yields under the optimized conditions. Low conversions and yields were obtained when the less reactive disarmed thioglycosides were used. Unexpectedly, BDSB-mediated oxidation of thioglycosides without the addition of n-Bu4NCl also afforded the α-glycosyl chlorides in moderate yields. We suggest a mechanism involving the transfer of chloride ions from the non-nucleophilic bromopentachloroantimonate (SbCl5Br) anion to the oxocarbenium ion.

Surfactant-Assisted Ozonolysis of Alkenes in Water: Mitigation of Frothing Using Coolade as a Low-Foaming Surfactant

Aqueous-phase ozonolysis in the atmosphere is an important process during cloud and fog formation. Water in the atmosphere acts as both a reaction medium and a reductant during the ozonolysis. Inspired by the atmospheric aqueous-phase ozonolysis, we herein report the ozonolysis of alkenes in water assisted by surfactants. Several types of surfactants, including anionic, cationic, and nonionic surfactants, were investigated. Although most surfactants enhanced the solubility of alkenes in water, they also generated excessive foaming during the ozone bubbling, which led to the loss of products. Mitigation of the frothing was accomplished by using Coolade as a nonionic and low-foaming surfactant. Coolade-assisted ozonolysis of alkenes in water provided the desired carbonyl products in good yields and comparable to those achieved in organic solvents. During the ozonolysis reaction, water molecules trapped within the polyethylene glycol region of Coolade were proposed to intercept the Criegee intermediate to provide a hydroxy hydroperoxide intermediate. Decomposition of the hydroxy hydroperoxide led to formation of the carbonyl product without the need for a reductant typically required for the conventional ozonolysis using organic solvents. This study presents Coolade as an effective surfactant to improve the solubility of alkenes while mitigating frothing during the ozonolysis in water.