Simple protocols for NMR analysis of the enantiomeric purity of chiral diols

Biocatalytic Ether Lipid Synthesis by an Archaeal Glycerolprenylase

Although ethers are common in secondary natural products, they are an underrepresented functional group in primary metabolism. As such, there are comparably few enzymes capable of constructing ether bonds in a general fashion. However, such enzymes are highly sought after for synthetic applications as they typically operate with higher regioselectivity and under milder conditions than traditional organochemical approaches. To expand the repertoire of well characterized ether synthases, we herein report on a promiscuous archaeal prenyltransferase from the scarcely researched family of geranylgeranylglyceryl phosphate synthases (GGGPSs or G3PSs). We show that the ultrastable Archaeoglobus fulgidus G3PS makes various (E)- and (Z)-configured prenyl glycerol ethers from the corresponding pyrophosphates while exerting perfect control over the configuration at the glycerol unit. Based on experimental and computational data, we propose a mechanism for this enzyme which involves an intermediary prenyl carbocation equivalent. As such, this study provides the fundamental understanding and methods to introduce G3PSs into the biocatalytic alkylation toolbox.

Tailoring the Performance of 19F-Labeled Probes via Dynamic Covalent Chemistry for Improved Chiral Discrimination

This work presents a novel strategy for postmodifying probes using dynamic covalent chemistry. Leveraging reversible interactions between boronic acid and diols, we obtained a panel of 19F-labeled probes with distinct resolving abilities. This approach enables rapid identification of probes with satisfactory performance, streamlining synthesis, and enhancing efficiency in chiral analysis. Our findings demonstrate an exceptional ability to differentiate compounds with distal chirality and challenging aliphatic amines. The postmodified probes also exhibit accuracy and reliability in determining enantiomeric excess, promising advancements in enantio-analysis techniques and chiral discrimination.

Enantioselective Surface-Enhanced Raman Scattering by Chiral Au Nanocrystals with Finely Modulated Chiral Fields and Internal Standards

The chiral discrimination of enantiomers is crucial for drug screening and agricultural production. Surface-enhanced Raman scattering (SERS) is proposed for discriminating enantiomers benefiting from chiral plasmonic materials. However, the mechanism of enantioselective SERS is unclear, and fluctuating SERS intensities may result in errors. Herein, this work demonstrates a reliable SERS substrate using chiral Au nanocrystals with finely modulated chiral fields and internal standards. Chiral electromagnetic fields are enhanced after modulation, which is conducive to increasing the difference in the enantiomeric SERS intensity, as evidenced by the experimental and simulation results. Furthermore, the SERS stability is improved by the corrective effect of the internal standards, and the relative standard deviation is significantly reduced. Using finely modulated chiral fields and internal standards, L- and D-phenylalanine exhibit a stable six times difference in SERS ratio. Theoretical simulations reveal that linearly polarized light can also excite the chiral fields of chiral Au nanocrystals, indicating non-chiral far-field light is converted into chiral near-field sources by chiral Au nanocrystals. Thus, the mechanism of enantioselective SERS can be elucidated by the scattering difference of chiral molecules in chiral near fields. This study will pave the way for the development of enantioselective SERS and related chiroptical technologies.

Differentiation and identification of enantiomers by nuclear magnetic resonance spectroscopy with support of quantum mechanical computations

We report nuclear magnetic resonance studies of two chiral building blocks of solifenacin, phenyltetrahydroisoquinoline and quinuclidinol, in which chiral solvating agents, Mosher's acid, and Pirkle's alcohol were used for spectral discrimination between enantiomers of solifenacin constituents. Based on the constraints following from measurements of the nuclear Overhauser effect, structures of phenyltetrahydroisoquinoline and Pirkle's alcohol solvates were found. Next, shifts of nuclear magnetic resonance signals of phenyltetrahydroisoquinoline due to the application of Pirkle's alcohol were computed using density functional theory methods. The computed carbon-13 shifts reproduce those determined experimentally, allowing us to attribute the absolute configuration to phenyltetrahydroisoquinoline enantiomers without the need for the use of empirical rules.

Synthesis of a Complex Brasilicardin Analogue Utilizing a Cobalt-Catalyzed MHAT-Induced Radical Bicyclization Reaction

We designed and executed an expedient synthesis of a complex analogue of the potent immunosuppressive natural product brasilicardin A. Our successful synthesis featured application of our recently developed MHAT-initiated radical bicyclization, which delivered the targeted, complex analogue in 17 steps in the longest linear sequence. Unfortunately, this analogue showed no observable immunosuppressive activity, which speaks to the importance of the structural and stereochemical elements of the natural core scaffold.

A54145 Factor D Is Not Less Susceptible to Inhibition by Lung Surfactant than Daptomycin

A54145 factor D (A5D) is a cyclic lipopeptide antibiotic that shares several structural and mechanistic features with the clinically important antibiotic daptomycin, such as their requirement for calcium and phosphatidylglycerol (PG) for activity. Studies by others have suggested that daptomycin's activity is strongly inhibited by lung surfactant while A5D's activity is not. This finding has inspired efforts, albeit unsuccessful, to develop an A5D analogue that is highly active in the presence of lung surfactant and can be used for treating community acquired pneumonia (CAP). Here we demonstrate that A5D, like daptomycin, has a strong preference for the 1,2-diacyl-sn-glycero-3-phospho-1'-sn-glycerol stereoisomer (2R,2'S configuration) of PG. This PG stereoisomer was determined to be the only stereoisomer of PG in lung surfactant. Both antibiotics are completely antagonized by approximately 1-2 mol equiv of 2R,2'S-PG. Studies performed in the presence of lung surfactant revealed that the antagonism of these peptides by surfactant is mainly due to their interaction with PG and that A5D is not significantly less susceptible to inhibition by lung surfactant than daptomycin.

Chiral FexCuySe nanoparticles as peroxidase mimics for colorimetric detection of 3, 4-dihydroxy-phenylalanine enantiomers

L-3,4-dihydroxy-phenylalanine (L-dopa) is the most widely used drug in Parkinson's disease treatment. However, development of cost-effective and high-throughput sensors to accurate enantioselective discrimination of L-dopa and D-dopa remains challenging to date. Herein, on the basis of the peroxidase-mimic activity of chiral Fe_xCu_ySe nanoparticles, we demonstrated a novel colorimetric sensor for determination of chiral dopa. The surface chiral ligand, L/D-histidine (L/D-His), endowed the nanozymes with enantioselectivity in catalyzing the oxidation of dopa enantiomers. According to the values ofk_cat/K_m, the efficiency of L-His modified nanoparticles (L-Fe_xCu_ySe NPs) towards L-dopa was 1.56 times higher than that of D-dopa. While, D-His can facilely reverse the preference of the nanozyme to D-dopa. On the basis of high catalytic activity and enantioselectivity of L-Fe_xCu_ySe NPs in oxidation of L-dopa, the L-Fe_xCu_ySe NPs-based system can be utilized for detection of L-dopa. The linear ranges for L-dopa determination were 5μM-0.125 mM and 0.125 mM-1 mM with a detection limit of 1.02μM. Critically, the developed sensor has been successfully applied in the quality control of clinical used L-dopa tablets. Our work sheds light on developing simple and sensitive chiral nanomaterials-based sensors for drug analysis.

NMR analysis of the enantiomeric purity of chiral diols by a new chiral boron agent

A new boric agent with bridged structure, boric acid D, was first synthesized and used as an excellent chiral derivative agent for highly efficient enantiodiscrimination of various diols.

A chiral phosphazane reagent strategy for the determination of enantiomeric excess of amines

Methods for measuring enantiomeric excess (ee) of organic molecules by NMR spectroscopy provide rapid analysis using a standard technique that is readily available. Commonly this is accomplished by chiral derivatisation of the detector molecule (producing a chiral derivatisation agent, CDA), which is reacted with the mixture of enantiomers under investigation. However, these CDAs have almost exclusively been based on carbon frameworks, which are generally costly and/or difficult to prepare. In this work, a methodology based on the readily prepared inorganic cyclodiphosph(iii)azane CDA ClP(μ-N^tBu)₂POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be highly effective in the measurement of ee’s of chiral amines, involving in situ reaction of the chiral amines (R*NH₂) with the P–Cl bond of the CDA followed by quaternization of the phosphorus framework with methyl iodide. This results in sharp ³¹P NMR signals with distinct chemical shift differences between the diastereomers that are formed, which can be used to obtain the ee directly by integration. Spectroscopic, X-ray structural and DFT studies suggest that the NMR chemical shift differences between diastereomers is steric in origin, with the sharpness of these signals resulting from conformational locking of the bornyl group relative to the P₂N₂ ring induced by the presence of the P(v)-bonded amino group (R*NH). This study showcases cheap inorganic phosphazane CDAs as simple alternatives to organic variants for the rapid determination of ee.

The simple inorganic cyclodiphosph(iii)azane chiral derivatisation agent ClP(μ-^tBuN)₂POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be effective in the measurement of ee’s of chiral amines using ³¹P NMR spectroscopy.

The Chiral Target of Daptomycin Is the 2R,2'S Stereoisomer of Phosphatidylglycerol

Daptomycin (dap) is an important antibiotic that interacts with the bacterial membrane lipid phosphatidylglycerol (PG) in a calcium-dependent manner. The enantiomer of dap (ent-dap) was synthesized and was found to be 85-fold less active than dap against B. subtilis, indicating that dap interacts with a chiral target as part of its mechanism of action. Using liposomes containing enantiopure PG, we demonstrate that the binding of dap to PG, the structural transition that occurs upon dap binding to PG, and the subsequent oligomerization of dap, depends upon the configuration of PG, and that dap prefers the 1,2-diacyl-sn-glycero-3-phospho-1'-sn-glycerol stereoisomer (2R,2'S configuration). Ent-dap has a lower affinity for 2R,2'S liposomes than dap and cannot oligomerize to the same extent as dap, which accounts for why ent-dap is less active than dap. To our knowledge, this is the first example whereby the activity of an antibiotic depends upon the configuration of a lipid head group.

A Three-Component Derivatization Protocol for Determining the Enantiopurity of Sulfinamides by 1H and 19F NMR Spectroscopy

A practically simple three-component chiral derivatization protocol has been developed to determine the enantiopurity of eight S-chiral sulfinamides by ¹H and ¹⁹F NMR spectroscopic analysis, based on their treatment with a 2-formylphenylboronic acid template and enantiopure pinanediol to afford a mixture of diastereomeric sulfiniminoboronate esters whose diastereomeric ratio is an accurate reflection of the enantiopurity of the parent sulfinamide.

Chiroptical sensing of unprotected amino acids, hydroxy acids, amino alcohols, amines and carboxylic acids with metal salts

Optical chirality sensing of unprotected amino acids, hydroxy acids, amino alcohols, amines and carboxylic acids based on a practical mix-and-measure protocol with readily available copper, iron, palladium, manganese, cerium or rhodium salts is demonstrated. The generation of strong cotton effects allows quantitative ee analysis of small sample amounts with high speed. In contrast to previously reported assays the use of chromophoric reporter ligands and the control of metal coordination kinetics and redox chemistry are not necessary which greatly simplifies the sensing procedure with the benefit of reduced waste production and cost.

The Bull-James assembly as a chiral auxiliary and shift reagent in kinetic resolution of alkyne amines by the CuAAC reaction

The Bull-James boronic acid assembly is used simultaneously as a chiral auxiliary for kinetic resolution and as a chiral shift reagent for in situ enantiomeric excess (ee) determination by ¹H NMR spectroscopy. Chiral terminal alkyne-containing amines, and their corresponding chiral triazoles formed via CuAAC, were probed in situ. Selectivity factors of up to s = 4 were imparted and measured, accurate to within ±3% when compared to chiral GC.

High-Throughput Assay for Enantiomeric Excess Determination in 1,2- and 1,3-Diols and Direct Asymmetric Reaction Screening

A simple and efficient method for determination of the yield, enantiomeric/diasteriomeric excess (ee/de), and absolute configuration of crude chiral diols without the need of work-up and product isolation in a high throughput setting is described. This approach utilizes a self-assembled iminoboronate ester formed as a product by dynamic covalent self-assembly of a chiral diol with an enantiopure fluorescent amine such as tryptophan methyl ester or tryptophanol and 2-formylphenylboronic acid. The resulting diastereomeric boronates display different photophysical properties and allow for fluorescence-based ee determination of molecules containing a 1,2- or 1,3-diol moiety. This method has been utilized for the screening of ee in a number of chiral diols including atorvastatin, a statin used for the treatment of hypercholesterolemia. Noyori asymmetric hydrogenation of benzil was performed in a highly parallel fashion with errors <1 % ee confirming the feasibility of the systematic examination of crude products from the parallel asymmetric synthesis in real time and in a high-throughput screening (HTS) fashion.

A non-chiral lithium aluminate reagent for the determination of enantiomeric excess of chiral alcohols

Here we illustrate a new method for the rapid determination of ee's of chiral alcohols using the thermally-stable, non-chiral lithium aluminate reagent [EtAl(6-Me-2-py)₃Li] (1).

Self and directed assembly: people and molecules

Self-assembly and directed-assembly are two very important aspects of supramolecular chemistry. As a young postgraduate student working in Canada with Tom Fyles my introduction to Supramolecular Chemistry was through the self-assembly of phospholipid membranes to form vesicles for which we were developing unimolecular and self-assembling transporter molecules. The next stage of my development as a scientist was in Japan with Seiji Shinkai where in a “Eureka” moment, the boronic acid templating unit (directed-assembly) of Wulff was combined with photoinduced electron transfer systems pioneered by De Silva. The result was a turn-on fluorescence sensor for saccharides; this simple result has continued to fuel my research to the present day. Throughout my career as well as assembling molecules, I have enjoyed bringing together researchers in order to develop collaborative networks. This is where molecules meet people resulting in assemblies worth more than the individual “molecule” or “researcher”. My role in developing networks with Japan was rewarded by the award of a Daiwa-Adrian Prize in 2013 and I was recently rewarded for developing networks with China with an Inaugural CASE Prize in 2015.

Improved δ-valerolactam templates for the assembly of Aβ-miniamyloids by boronic ester formation

Numerically defined oligomers of amyloidogenic peptides were obtained by boronic ester formation with synthetic polyol templates.

Salen-based enantiomeric polymers for enantioselective recognition

In a simple way, the spatial arrangement of the building blocks in a main chain polymer determines its recognition properties.

A simple and rapid approach for testing enantiopurity of hydroxy acids and their derivatives using 1H NMR spectroscopy

A rapid and the simple chiral derivatizing protocol involving the coupling of 2-formylphenylboronic acid and an optically pure [1,1-binaphthalene]-2,2-diamine is introduced for the accurate determination of the enantiopurity of hydroxy acids and their derivatives.

Sugar-boronate ester scaffold tethered pyridyl-imine palladium(ii) complexes: synthesis and their in vitro anticancer evaluation

The anticancer activity of sugar-boronate ester containing palladium(ii) complexes is reported.

RNA nucleosides as chiral sensing agents in NMR spectroscopy

The study reports chiral sensing properties of RNA nucleosides. A three component derivitazation protocol has been adopted to differentiate chiral amines. All RNA nucleosides exhibit chiral sensing property.

Hierarchical supramolecules and organization using boronic acid building blocks

Current progress on hierarchical supramolecules using boronic acids has been highlighted in this feature article. The feasibility of the structure-directing ability is fully discussed from the standpoint of the generation of new smart materials.

Exploiting the reversible covalent bonding of boronic acids: recognition, sensing, and assembly

Boronic acids can interact with Lewis bases to generate boronate anions, and they can also bind with diol units to form cyclic boronate esters. Boronic acid based receptor designs originated when Lorand and Edwards used the pH drop observed upon the addition of saccharides to boronic acids to determine their association constants. The inherent acidity of the boronic acid is enhanced when 1,2-, 1,3-, or 1,4-diols react with boronic acids to form cyclic boronic esters (5, 6, or 7 membered rings) in aqueous media, and these interactions form the cornerstone of diol-based receptors used in the construction of sensors and separation systems. In addition, the recognition of saccharides through boronic acid complex (or boronic ester) formation often relies on an interaction between a Lewis acidic boronic acid and a Lewis base (proximal tertiary amine or anion). These properties of boronic acids have led to them being exploited in sensing and separation systems for anions (Lewis bases) and saccharides (diols). The fast and stable bond formation between boronic acids and diols to form boronate esters can serve as the basis for forming reversible molecular assemblies. In spite of the stability of the boronate esters' covalent B-O bonds, their formation is reversible under certain conditions or under the action of certain external stimuli. The reversibility of boronate ester formation and Lewis acid-base interactions has also resulted in the development and use of boronic acids within multicomponent systems. The dynamic covalent functionality of boronic acids with structure-directing potential has led researchers to develop a variety of self-organizing systems including macrocycles, cages, capsules, and polymers. This Account gives an overview of research published about boronic acids over the last 5 years. We hope that this Account will inspire others to continue the work on boronic acids and reversible covalent chemistry.