Fluoride Donors Research Articles

Revealing Disorder, Sorption Locations and a Sorption-Induced Single Crystal-Single Crystal Transformation in a Rare-Earth fcu-Type Metal-Organic Framework.

Rare-earth metal-organic frameworks (RE-MOFs) formed in the presence of fluoride donors are a group of complex and applicable MOFs. Determining structural complexity is crucial in applying such MOFs and has been achieved to uncover framework disorders in the important fcu framework topology MOF, Y-ndc-fcu-MOF (1). 1 is found to contain F- groups disordered over the μ3-face-capping sites in its secondary building unit (SBU) and framework distortions upon sorption of different guest molecules. The favored location of the guests is within the octahedral cage of 1 where they interact with the Y3+ centers. The size, shape, and interactions of the different guests lead to subtle distortions within the SBU and adoption of specific orientations of the naphthalene group of the 1,4-naphthalenedicarboxylate framework linkers. The sorption of DMF(l)/H2O(l) lowers the symmetry from cubic Fm3̅m (for MeOH(l), N2(g), CO2(gorl)) to cubic Pa3̅ (for DMF(l)/H2O(l)) symmetry with retention of the fcu topology, and conversion between the Pa3̅ and Fm3̅m structures is induced by solvent exchange. Such disorder and sorption locations and transformation are important considerations during the optimization and application of MOFs for sorption-based technologies.

B3H3Be ← F-: A Case of Extremely Strong Dative Bond.

Chemical bonding has attracted chemists since its inception. Dative bonding between a donor and acceptor moiety is also an important phenomenon, which results in stabilization of many chemical compounds. Herein, we show that an extremely strong dative bond is possible between a fluoride ion and a beryllium center which is a part of a half-sandwich complex, B3H3Be. Quantum chemical calculations have shown the possibility of formation of a half-sandwich complex of Be with a B3H32- ring. Calculations reveal that the complex is stable toward dissociation. The half-sandwich complex features a very low-lying lowest unoccupied molecular orbital (LUMO) concentrated on the Be atom, thereby indicating a Lewis acidic character of the complex. This lower-lying LUMO at the beryllium center is responsible for forming an extremely strong dative bond with the fluoride donor.

Stereoselective synthesis of α-3-deoxy-ᴅ-manno-oct-2-ulosonic acid (α-Kdo) derivatives using a C3-p-tolylthio-substituted Kdo fluoride donor

Stereoselective synthesis of α-3-deoxy-ᴅ-manno-oct-2-ulosonic acid (α-Kdo) derivatives using a C3-p-tolylthio-substituted Kdo fluoride donor

Deoxyfluorination of 1°, 2°, and 3° Alcohols by Nonbasic O-H Activation and Lewis Acid-Catalyzed Fluoride Shuttling.

A method for deoxyfluorination of aliphatic primary, secondary, and tertiary alcohols is reported, employing a nontrigonal phosphorus triamide for base-free alcohol activation in conjunction with an organic soluble fluoride donor and a triarylborane fluoride shuttling catalyst. Mechanistic experiments are consistent with a reaction that proceeds by the collapse of an oxyphosphonium fluoroborate ion pair with fluoride transfer. The substrate scope complements existing deoxyfluorination methods and enables the preparation of homochiral secondary and tertiary alkylfluorides by stereoinversion of the substrate alcohol.

A Strategic Synthesis of Fluoroethers via Ring-Opening Fluorinative Beckmann Fragmentation.

An SN1-type fluorination method for monofluoroethers is developed. The key to this reaction is fluorinative C-C bond cleavage that is driven by oxygen-assisted Beckmann fragmentation. To enable this transformation, cyclic α-aryloxyoximes derived from 3-coumaranone and 1-indanones were investigated as substrates, using N,N-diethylaminosulfur trifluoride (DAST) as a dual-role reagent of an oxime activator and fluoride donor. This method features the synthesis of an underdeveloped chemical motif with simple and mild operating conditions.

EFluorination Using Cheap and Readily Available Tetrafluoroborate Salts.

A practical electrochemical method for the rapid, safer, and mild synthesis of tertiary hindered alkyl fluorides from carboxylic acids has been developed without the need for hydrofluoric acid salts or non-glass reactors. In this anodic fluorination, collidinium tetrafluoroborate acts as both the supporting electrolyte and fluoride donor. A wide range of functional groups has been shown to be compatible, and the possibility of scale-up using flow electrochemistry has also been demonstrated.

Synthesis of a conjugable hexasaccharide corresponding to the capsular polysaccharide of Campylobacter jejuni strain BH0142

The first assembly of a conjugation-ready hexasaccharide from the capsular glycan of C. jejuni. strain BH0142 has been accomplished. The synthesis features the efficient preparation of 6-deoxy-d-ido-heptopyranosyl fluoride donors proceeding from allyl α-d-C-glucopyranoside by a C1-to-C5 switch strategy with radical dehydroxymethylative fluorination as a key step, stereocontrolled construction of 1,2-trans-α-d-ido-heptopyranosidic bonds and of 1,2-cis-α-d-galactopyranosidic linkages. The obtained target oligosaccharide sets a solid foundation for making structurally-defined multivalent glycoconjugate vaccine candidates against C. jejuni. infections.

Glycosyl Exchange of Unactivated Glycosidic Bonds: Suppressing or Embracing Side Reactivity in Catalytic Glycosylations.

While developing boron-catalyzed glycosylations using glycosyl fluoride donors and trialkylsilyl ether acceptors, competing pathways involving productive glycosylation or glycosyl exchange were observed. Experimental and computational mechanistic studies suggest a novel mode of reactivity where a dioxolenium ion is a key intermediate that promotes both pathways through addition to either a silyl ether or to the acetal of an existing glycosidic linkage. Modifications in catalyst structure enable either pathway to be favored, and with this understanding, improved multicomponent iterative couplings and glycosyl exchange processes were demonstrated.

Structural basis for transglycosylation in glycoside hydrolase family GH116 glycosynthases

Glycosynthases are glycoside hydrolase mutants that can synthesize oligosaccharides or glycosides from an inverted donor without hydrolysis of the products. Although glycosynthases have been characterized from a variety of glycoside hydrolase (GH) families, family GH116 glycosynthases have yet to be reported. We produced the Thermoanaerobacterium xylanolyticum TxGH116 nucleophile mutants E441D, E441G, E441Q and E441S and compared their glycosynthase activities to the previously generated E441A mutant. The TxGH116 E441G and E441S mutants exhibited highest glycosynthase activity to transfer glucose from α-fluoroglucoside (α-GlcF) to cellobiose acceptor, while E441D had low but significant activity as well. The E441G, E441S and E441A variants showed broad specificity for α-glycosyl fluoride donors and p-nitrophenyl glycoside acceptors. The structure of the TxGH116 E441A mutant with α-GlcF provided the donor substrate complex, while soaking of the TxGH116 E441G mutant with α-GlcF resulted in cellooligosaccharides extending from the +1 subsite out of the active site, with glycerol in the −1 subsite. Soaking of E441A or E441G with cellobiose or cellotriose gave similar acceptor substrate complexes with the nonreducing glucosyl residue in the +1 subsite. Combining structures with the ligands from the TxGH116 E441A with α-GlcF crystals with that of E441A or E441G with cellobiose provides a plausible structure of the catalytic ternary complex, which places the nonreducing glucosyl residue O4 2.5 Å from the anomeric carbon of α-GlcF, thereby explaining its apparent preference for production of β-1,4-linked oligosaccharides. This functional and structural characterization provides the background for development of GH116 glycosynthases for synthesis of oligosaccharides and glycosides of interest.

Rhamnogalacturonan II: Chemical Synthesis of a Substructure Including α-2,3-Linked Kdo*.

The synthesis of a fully deprotected Kdo-containing rhamnogalacturonan II pentasaccharide is described. The strategy relies on the preparation of a suitably protected homogalacturonan tetrasaccharide backbone, through a post-glycosylation oxidation approach, and its stereoselective glycosylation with a Kdo fluoride donor.

Protecting-group-free S-glycosylation towards thioglycosides and thioglycopeptides in water.

A facile and green S-glycosylation method has been developed featuring protecting-group-free and proceeding-in-water like enzymatic synthesis. Glycosylation of fluoride donors with thiol sugar acceptors using Ca(OH)2 as a promoter afforded various thioglycosides in good yields with exclusive stereoselectivity. This method also enabled the successful production of S-linked oligosaccharides and S-linked glycopeptides.

Ferrocenium complex aided O-glycosylation of glycosyl halides.

A new strategy for the activation of glycosyl halide donors to be utilized in glycosylation reactions is presented, utilizing the ferrocenium (Fc) complexes [FcB(OH)2]SbF6 and FcBF4 as promoters. The scope of the new system has been investigated using glycosyl chloride and glycosyl fluoride donors in combination with common glycosyl acceptors, such as protected glucose. The corresponding glycosylation products were formed in 95 to 10% isolated yields with α/β ratios ranging from 1/1 to β only (2 to 14 h reaction time at room temperature, 40 to 100% ferrocenium promoter load).

Fluoride Migration Catalysis Enables Simple, Stereoselective, and Iterative Glycosylation.

Challenges in the assembly of glycosidic bonds in oligosaccharides and glycoconjugates pose a bottleneck in enabling the remarkable promise of advances in the glycosciences. Here, we report a strategy that applies unique features of highly electrophilic boron catalysts, such as tris(pentafluorophenyl)borane, in addressing a number of the current limitations of methods in glycoside synthesis. This approach utilizes glycosyl fluoride donors and silyl ether acceptors while tolerating the Lewis basic environment found in carbohydrates. The method can be carried out at room temperature using air- and moisture-stable forms of the catalyst, with loadings as low as 0.5 mol %. These characteristics enable a wide array of glycosylation patterns to be accessed, including all C1-C2 stereochemical relationships in the glucose, mannose, and rhamnose series. This method allows one-pot, iterative glycosylations to generate oligosaccharides directly from monosaccharide building blocks. These advances enable the rapid and experimentally straightforward preparation of complex oligosaccharide units from simple building blocks.

Rapid phenolic O-glycosylation of small molecules and complex unprotected peptides in aqueous solvent.

Glycosylated natural products and synthetic glycopeptides represent a significant and growing source of biochemical probes and therapeutic agents. However, methods that enable the aqueous glycosylation of endogenous amino acid functionality in peptides without the use of protecting groups are scarce. Here, we report a transformation that facilitates the efficient aqueous O-glycosylation of phenolic functionality in a wide range of small molecules, unprotected tyrosine, and tyrosine residues embedded within a range of complex, fully unprotected peptides. The transformation, which uses glycosyl fluoride donors and is promoted by Ca(OH)2, proceeds rapidly at room temperature in water, with good yields and selective formation of unique anomeric products depending on the stereochemistry of the glycosyl donor. High functional group tolerance is observed, and the phenol glycosylation occurs selectively in the presence of virtually all side chains of the proteinogenic amino acids with the singular exception of Cys. This method offers a highly selective, efficient, and operationally simple approach for the protecting-group-free synthesis of O-aryl glycosides and Tyr-O-glycosylated peptides in water.

Indium(III) Iodide-Catalyzed Stereoselective Synthesis of β-Glucopyranosides by Using a Glucosyl Fluoride Donor with 2-O-Benzoyl-3,4,6-Tri-O-Benzyl Protection

We have developed a novel protocol for glucosylation by adopting a glucosyl fluoride donor with 2-O-benzoyl-3,4,6-tri-O-benzyl protection. The protocol is useful for the ready assembly of β-linked functional glycoconjugates, and the reaction accommodates a broad range of substrates. Conveniently, water-tolerant and commercially available InI3 is used as a catalyst, and no other additional reagent is required. The method involves an interesting process for glucosyl fluoride activation and, in particular, permits the stereoselective construction of partially benzylated glucopyranosides carrying a selectively removable 2-O-benzoyl group, which hold great potential as glycosyl receptors for building further 1,2-glycosidic linkages.

Production and upgrading of lactic acid: An overview and future prospects

The synthesis of glycosidic structures by catalysis via glycosynthases has gained much interest due to the potential high product yields and specificity of the enzymes. Nevertheless, the characterisation and implementation of new glycosynthases is greatly hampered by the lack of high-throughput methods for reaction analysis and screening of potential glycosynthase variants. Fluoride detection, via silyl ether chemosensors, has recently shown high potential for the identification of glycosynthase mutants in a high-throughput manner, though limited by the low maximal detection concentration. In the present paper, we describe a new version of a glycosynthase activity assay using a silyl ether of p-nitrophenol, allowing fast reliable detection of fluoride even at concentrations of 4 mM and higher. This improvement of detection allows not only screening and identification but also kinetic characterisation of glycosynthases and synthetic reactions in a fast microtiter plate format. The applicability of the assay was successfully demonstrated by the biochemical characterisation of the mesophilic β-glucosynthase of Abg-E358S (Rhizobium radiobacter) and psychrotolerant β-glucosynthase BglU-E377A (Micrococcus antarcticus). The limitation of hyperthermophilic glycosidases as potential glycosynthases, when using glycosyl fluoride donors, was also illustrated by the example of the putative β-galactosidase GalPf from Pyrococcus furiosus.

Lysosomal enzymes in Parkinson's disease

The synthesis of glycosidic structures by catalysis via glycosynthases has gained much interest due to the potential high product yields and specificity of the enzymes. Nevertheless, the characterisation and implementation of new glycosynthases is greatly hampered by the lack of high-throughput methods for reaction analysis and screening of potential glycosynthase variants. Fluoride detection, via silyl ether chemosensors, has recently shown high potential for the identification of glycosynthase mutants in a high-throughput manner, though limited by the low maximal detection concentration. In the present paper, we describe a new version of a glycosynthase activity assay using a silyl ether of p-nitrophenol, allowing fast reliable detection of fluoride even at concentrations of 4 mM and higher. This improvement of detection allows not only screening and identification but also kinetic characterisation of glycosynthases and synthetic reactions in a fast microtiter plate format. The applicability of the assay was successfully demonstrated by the biochemical characterisation of the mesophilic β-glucosynthase of Abg-E358S (Rhizobium radiobacter) and psychrotolerant β-glucosynthase BglU-E377A (Micrococcus antarcticus). The limitation of hyperthermophilic glycosidases as potential glycosynthases, when using glycosyl fluoride donors, was also illustrated by the example of the putative β-galactosidase GalPf from Pyrococcus furiosus.

Development of a colourimetric assay for glycosynthases

The synthesis of glycosidic structures by catalysis via glycosynthases has gained much interest due to the potential high product yields and specificity of the enzymes. Nevertheless, the characterisation and implementation of new glycosynthases is greatly hampered by the lack of high-throughput methods for reaction analysis and screening of potential glycosynthase variants. Fluoride detection, via silyl ether chemosensors, has recently shown high potential for the identification of glycosynthase mutants in a high-throughput manner, though limited by the low maximal detection concentration. In the present paper, we describe a new version of a glycosynthase activity assay using a silyl ether of p-nitrophenol, allowing fast reliable detection of fluoride even at concentrations of 4mM and higher. This improvement of detection allows not only screening and identification but also kinetic characterisation of glycosynthases and synthetic reactions in a fast microtiter plate format. The applicability of the assay was successfully demonstrated by the biochemical characterisation of the mesophilic β-glucosynthase of Abg-E358S (Rhizobium radiobacter) and psychrotolerant β-glucosynthase BglU-E377A (Micrococcus antarcticus). The limitation of hyperthermophilic glycosidases as potential glycosynthases, when using glycosyl fluoride donors, was also illustrated by the example of the putative β-galactosidase GalPf from Pyrococcus furiosus.

Crystal structure of the enzyme-product complex reveals sugar ring distortion during catalysis by family 63 inverting α-glycosidase.

Glycoside hydrolases are divided into two groups, known as inverting and retaining enzymes, based on their hydrolytic mechanisms. Glycoside hydrolase family 63 (GH63) is composed of inverting α-glycosidases, which act mainly on α-glucosides. We previously found that Escherichia coli GH63 enzyme, YgjK, can hydrolyze 2-O-α-d-glucosyl-d-galactose. Two constructed glycosynthase mutants, D324N and E727A, which catalyze the transfer of a β-glucosyl fluoride donor to galactose, lactose, and melibiose. Here, we determined the crystal structures of D324N and E727A soaked with a mixture of glucose and lactose at 1.8- and 2.1-Å resolutions, respectively. Because glucose and lactose molecules are found at the active sites in both structures, it is possible that these structures mimic the enzyme-product complex of YgjK. A glucose molecule found at subsite -1 in both structures adopts an unusual 1S3 skew-boat conformation. Comparison between these structures and the previously determined enzyme-substrate complex structure reveals that the glucose pyranose ring might be distorted immediately after nucleophilic attack by a water molecule. These structures represent the first enzyme-product complex for the GH63 family, as well as the structurally-related glycosidases, and it may provide insight into the catalytic mechanism of these enzymes.

Rational design of a thermostable glycoside hydrolase from family 3 introduces β-glycosynthase activity.

The thermostable β-glucosidase from Thermotoga neapolitana, TnBgl3B, is a monomeric three-domain representative from glycoside hydrolase family 3. By using chemical reactivation with exogenous nucleophiles in previous studies with TnBg13B, the catalytic nucleophile (D242) and corresponding acid/base residue (E458) were determined. Identifying these residues led to the attempt of converting TnBgl3B into a β-glucosynthase, where three nucleophilic variants were created (TnBgl3B_D242G, TnBgl3B_D242A, TnBgl3B_D242S) and all of them failed to exhibit glucosynthase activity. A deeper analysis of the TnBgl3B active site led to the generation of three additional variants, each of which received a single-point mutation. Two of these variants were altered at the -1 subsite (Y210F, W243F) and the third received a substitution near the binding site's aglycone region (N248R). Kinetic evaluation of these three variants revealed that W243F substitution reduced hydrolytic turnover while maintaining KM This key W243F mutation was then introduced into the original nucleophile variants and the resulting double mutants were successfully converted into β-glucosynthases that were assayed using two separate biosynthetic methods. The first reaction used an α-glucosyl fluoride donor with a 4-nitrophenyl-β-d-glucopyranoside (4NPGlc) acceptor, and the second used 4NPGlc as both the donor and acceptor in the presence of the exogenous nucleophile formate. The primary specificity observed was a β-1,3-linked disaccharide product, while a secondary β-1,4-linked disaccharide product was observed with increased incubation times. Additional analysis revealed that substituting quercetin-3-glycoside for the second reaction's acceptor molecule resulted in the successful production of quercetin-3,4'-diglycosides with yields up to 40%.