Stereochemical Structures Research Articles

Anionic ring-opening polymerization of 2-oxabicyclo[2.1.1]hexan-3-one: manipulating topology and conformation for circular polymer design.

Polyesters featuring a linear topology and in-chain 1,3-cyclobutane rings, synthesized via ring-opening polymerization (ROP) of 2-oxabicyclo[2.1.1]hexan-3-one (4R-BL, R = Bu, Ph) through a coordination-insertion mechanism, display excellent thermal and hydrolytic stability, making them promising candidates for sustainable circular materials. However, achieving diverse topological and stereochemical structures remains challenging. Herein, we demonstrate precise control over linear and cyclic topologies of these polyesters and the conformation of in-chain cyclobutane rings through anionic ROP of 4R-BLwith appropriate catalysts or initiators. Usingtert-butoxide (tBuOK) as the catalyst, low loading (0.05-0.1 mol%) produces high-molar-mass cyclic polyester P(4R-BL) (up to 571 kg/mol), whereas high loading (2 mol%) promotes transesterification and isomerization, ultimately yielding cyclic oligomers. Remarkably, the tetramer(4Ph-BL)4undergoes conformationalturnoverof the puckered cyclobutane rings and can be repolymerized into polymerP(4Ph-BL). This establishes a "monomer ⇄ polymer ⇄ tetramer" dual closed-loop life cycle, enhancing the potential for a circular material economy.

Mutual Relationships of Nanoconfined Hexoses: Impacts on Hydrodynamic Radius and Anomeric Ratios.

Although all hexose sugars share the same chemical formula, C6H12O6, subtle differences in their stereochemical structures lead to their various biological roles. Due to their prominent role in metabolism, hexose sugars are commonly found in nanoconfined environments. The complexity of authentic nanoconfined biological environments makes it challenging to study how confinement affects their behavior. Here, we present a study using a common model system, AOT reverse micelles, to study hexose sugars in nanoconfinement. We examine how reverse micelles affect the hexoses, how the hexoses affect reverse micelle formation, and the differences between specific hexoses: glucose, mannose, and galactose. We find that addition of glucose, mannose or galactose to reverse micelles that already contain water leaves their size smaller or nearly unchanged. Introducing aqueous hexose solution yields reverse micelles smaller than those prepared with the same volume of water. We use 1H NMR to show how the nanoconfined environment impacts hexose sugars' anomeric ratios. Nanoconfined mannose and galactose display smaller changes in their anomeric ratios compared to glucose. These conclusions may provide insights about the biological roles of each hexose when studied under a more authentic nanoconfined system.

Chirality differences of three borneols contribute to controlling Escherichia coli biofilm formation by chemotaxis mediating movement

SummaryEscherichia coli (E. coli) biofilm is the major cause of its high lethality in foodborne illness. Controlling E. coli biofilm formation is a critical way to reduce foodborne diseases. Here, chiral borneol isomers (L‐borneol, D‐borneol, and isoborneol) were identified as E. coli chemorepellents, which reduced reversible adhesion at the initial stage of biofilm formation. Borneols directly inhibited biofilm production and their inhibitory capacities were associated with the chiral stereochemical structures (L‐borneol > D‐borneol > isoborneol). A transcriptomic study revealed the inhibitory mechanism of borneols was attributed to the bacterial motility intention and motility energy system. The most effective isomer, L‐borneol, reduced bacterial energy metabolism and increased E. coli chemotaxis motility intention through the up‐regulation of genes associated with flagellar assembly and bacterial chemotaxis pathways. This work demonstrates that borneols are biofilm inhibitors and supports the potential of borneols as a stereochemical antimicrobial strategy to reduce foodborne pathogenic bacterial contamination.

Novel Bicyclic P,S-Heterocycles via Stereoselective hetero-Diels-Alder Reactions of Thiochalcones with 1-Phenyl-4H-phosphinin-4-one 1-Oxide.

Thiochalcones undergo cycloaddition reactions in THF solution at 60 °C with the synthetically unexplored 1-phenyl-4H-phosphinin-4-one 1-oxide in a highly regio- and stereoselective manner, yielding hitherto unknown bicyclic P,S-heterocycles containing fused thiopyran and phosphinine rings. The stereochemical structures of two of the obtained (4+2)-cycloadducts were unambiguously assigned by means of the X-ray single-crystal analysis. Based on these assignments, a concerted mechanism of the hetero-Diels-Alder reaction via the preferred endo approach of the heterodiene from the less hindered P=O side of the phosphininone molecule is postulated to explain the established rac-(4RS,8SR,9SR,10SR)-configured (4+2)-cycloadducts isolated as major products.

Stereochemical and Computational NMR Survey of 1,2,3-Triazoles: in Search of the Original Tauto-Conformers.

The conformational analysis of nine functionalized 1,2,3-triazoles was carried out by the correlation of calculated and experimental high-level nuclear magnetic resonance (NMR) chemical shifts. In solution, the studied triazoles are in exchange dynamic equilibrium caused by their prototropic tautomerism of the NH-proton. The experimentally unresolved NMR signals were assigned for most of the compounds. A more thorough survey was conducted for 4-t-butyl-1,2,3-triazole-5-carbaldehyde oxime. The analysis performed within the framework of the DP4+ formalism completely confirmed the hypothesis of the predominance of the 2H-tautomer. Thus, the methodology for estimating stereochemical structures in the absence of some experimental data allowed the most stable conformations for dynamic systems with different tautomeric ratios to be reliably identified.

9,9′-epoxylignans from Syringa pinnatifolia: A typical case of stereochemical assignment by a quantum chemical calculation with MAEΔΔδ parameter

In the current study, twenty-two stereochemical 9,9′-epoxylignans including 19 undescribed ones were isolated from the ethanol extract of Syringa pinnatifolia in our continuing effort to understand the overall chemical spectrum of this species. These isolates were structurally elucidated by extensive spectroscopic data analysis, X-ray diffraction, modified Mosher's method, and quantum chemical calculations. Meanwhile, the utilization of 13C NMR calculation and the MAEΔΔδ parameter facilitated the stereochemical assignment of groups of lignan stereoisomers. The 13C NMR data were corrected by the averaged errors at each corresponding carbon position in groups of lignan stereoisomers, which improved the theoretic 13C NMR calculation. The finding of the stereochemical structures of 9,9′-epoxylignans is significant. It is helpful to determine the absolute configurations of molecules with the similar core. In addition, these lignans exhibited potential cardioprotective activities on H9c2 cardiomyocytes in vitro and presented significant antioxidant effect.

Synthesis and self-assembly of dendritic-linear block copolymers containing poly(mandelic acid) with discrete molecular weights and stereochemical structures.

In this work, we present the synthesis of uniform PMAs, where the number of repeat units and their stereochemical arrangement are precisely defined. Utilizing an iterative convergent approach with orthogonally protected dimandelic acid building blocks, we achieved high molecular weight PMAs with the desired number of repeat units, extending up to 144 mandelic acids. Additionally, stereochemically defined poly(l-mandelic acid)s with up to 32 repeat units were successfully synthesized. These uniform PMAs were subsequently coupled with uniform branched poly(ethylene glycol) blocks to create uniform dendritic-linear block copolymers. The self-assembly of these block copolymers in solution was systematically investigated. In solution self-assembly, the synthesized block copolymers showed multiple phases from cylinder to inverse cubic as the molecular weight of PMA increased. In the case of solvent diffusion-evaporation-mediated self-assembly, the block copolymers underwent a phase transition as the rate of water addition decreased.

Synthesis, Characterization and Thermal Analysis of Cu (II), Co (II), Ru(III) and Rh(III) Complexes of a New Acidic Ligand

Hippuric acid and 3-amino phenol were used to make the 4-(2-Amino-4-hydroxy-phenylazo)-benzoylamino-acetic acid diazonium salt, a new Azo molecule that is a derivative of the (4-Amino-benzoylamino)-acetic acid diazonium salt. We found out what the ligand's chemical structures were by using information from 1HNMR, FTIR, CHN, UV-Vis, LC-mass spectroscopy, and thermal analyses. To make metal complexes of the azo ligand with Co(II), Cu(II), Ru(III), and Rh(III) ions, extra amounts of each azo ligand were mixed with metal chloride salts in a 2:2 mole ratio. The stereochemical structures and geometries of the metal complexes that were studied were guessed based on the fact that the ligand exhibited tetradentate bonding behavior when combined with the metalions. The azodye ligand is coordinated with the metal ions (Co(II), Cu(II),Ru(III), and Rh(III)) through (NNOO) the N atoms of azo and amine groups and the oxygen of carboxylic and phenolic hydroxyl groups. According to analytical results, the Ru(III) and Rh(III) complexes have binuclear octahedral geometry, whereas the Cu(II) and Co(II) complexes have binuclear distorted octahedral and binuclear tetrahedral geometry, respectively. The results indicated that the following formulas for ligand complexes should be used: [Ru2Cl2(H2O)2(LI)2], [Rh2Cl2(H2O)2(LI)2], [Co2(LI)2], and [Cu2(LI)2]. The thermal analysis conducted by TG, DTG, and DTA demonstrated partial breakdown at temperatures between 820°C and 850°C.

On the Efficiency of the Density Functional Theory (DFT)-Based Computational Protocol for 1H and 13C Nuclear Magnetic Resonance (NMR) Chemical Shifts of Natural Products: Studying the Accuracy of the pecS-n (n = 1, 2) Basis Sets.

The basis set issue has always been one of the most important factors of accuracy in the quantum chemical calculations of NMR chemical shifts. In a previous paper, we developed new pecS-n (n = 1, 2) basis sets purposed for the calculations of the NMR chemical shifts of the nuclei of the most popular NMR-active isotopes of 1-2 row elements and successfully approbated these on the DFT calculations of chemical shifts in a limited series of small molecules. In this paper, we demonstrate the performance of the pecS-n (n = 1, 2) basis sets on the calculations of as much as 713 1H and 767 13C chemical shifts of 23 biologically active natural products with complicated stereochemical structures, carried out using the GIAO-DFT(PBE0) approach. We also proposed new alternative contraction schemes for our basis sets characterized by less contraction depth of the p-shell. New contraction coefficients have been optimized with the property-energy consistent (PEC) method. The accuracies of the pecS-n (n = 1, 2) basis sets of both the original and newly contracted forms were assessed on massive benchmark calculations of proton and carbon chemical shifts of a vast variety of natural products. It was found that less contracted pecS-n (n = 1, 2) basis sets provide no noticeable improvement in accuracy. These calculations represent the most austere test of our basis sets as applied to routine calculations of the NMR chemical shifts of real-life compounds.

New Olimycins from a Cold-Seep-Derived Streptomyces olivaceus.

Cold-seeps are areas of the ocean floor in which hydrogen sulfide and methane are released into the open water. The cold-seep microbes are an emerging source of novel bioactive natural products. Four new ansa-ring opened linear ansamycin analogues, named olimycins E-H (1-4) were isolated from the cold-seep-derived Streptomyces olivaceus OUCLQ19-3. The planar and stereochemical structures of the isolated compounds were elucidated based on extensive MS and NMR spectroscopic analyses together with ECD calculations.

Two pairs of phloroglucinol enantiomers from Hypericum wightianum and their stereochemical structures

The chemical constituents in the ethanol extract of Hypericum wightianum(Hypericaceae) were purified by column chromatography and identified via magnetic resonance imaging(NMR), high-resolution mass spectrum, and circular dichroism. A total of 22 compounds were identified, including eight polyprenylated phloroglucinols(1-8), three chromones(9-11), and three terpenoids(14-16) and so on. Among them, compounds 16 and 17 were first reported in the genus Hypericum, and compounds 1-11, 14, 15, and 19 were first isolated from H. wightianum. Compounds 1-4 were previously reported as two pairs of enantiomers. This study reported the chiral resolutions and absolute configurations of compounds 1-4 for the first time.

Variable Effects of Twenty Sugars and Sugar Alcohols on the Retrogradation of Wheat Starch Gels.

Starch retrogradation is desirable for some food textures and nutritional traits but detrimental to sensory and storage qualities of other foods. The objective of this study was to determine the impact of sweetener structure and concentration on the retrogradation of wheat starch gels. The effects of 20 sweeteners selected based on common food usage and stereochemical structures of interest, and ranging in concentration from 10 to 50%w/w, on the retrogradation of wheat starch gels were monitored spectrophotometrically over time. The sweeteners were sucrose, xylose, ribose, glucose, galactose, fructose, mannose, mannitol, L-sorbose, xylitol, tagatose, allulose, maltose, lactose, isomaltulose, isomalt, sorbitol, maltitol, and raffinose. Retrogradation rates and amounts were compared by Avrami equation rate constants (k = 0.1–0.7) and absorbance values measured on day 28 (Abs = 0.1–1.0), respectively. Both sweetener concentration and type significantly affected retrogradation. Gels made with sugar alcohols and high sweetener concentrations (≈≥40%) tended to retrograde more and faster, whereas gels made with sugars and low sweetener concentrations tended to have lower retrogradation rates and amounts. Sweeteners with more equatorial and exocyclic hydroxyl groups (e.g., glucose and maltitol) and those with larger molar volumes (e.g., isomaltulose and raffinose) tended to increase the rate and amount of retrogradation, particularly at higher concentrations. The impact of sweeteners on retrogradation was a balance of factors that promoted retrogradation (intermolecular interactions and residual short-range molecular order) and inhibiting behaviors (interference at crystallization sites), which are influenced by sweetener concentration and structure. Understanding which sweeteners at which concentrations can be used to promote or inhibit retrogradation is useful for product formulation strategies.

Stereoselective Self-Assembly of Complex Chiral Radial [5]Catenanes Using Half-Sandwich Rhodium/Iridium Building Blocks.

Herein, we have successfully achieved the stereoselective synthesis of two chiral radial [5]catenanes in a single step through the self-assembly of bidentate ligands containing l-alanine residues and binuclear half-sandwich organometallic rhodium(III)/iridium(III) clips. Remarkably, these two chiral radial [5]catenanes exhibit complex stereochemical structures as revealed by single-crystal X-ray diffraction. The eight binuclear units and eight bidentate ligands in their solid-state structures all exhibit a single planar chirality, and the interlocking between molecular macrocycles exhibits a single co-conformational mechanical helical chirality. This indicates that the introduction of the point chirality in the ligands enables the efficient stereoselective construction of mechanically interlocked molecules. Furthermore, by using ligands containing d-alanine residues, radial [5]catenanes with the opposite planar chirality and opposite co-conformational mechanical helical chirality have also been obtained.

Assigning the stereochemical structures of aurantinin A and B with the assistance of biosynthetic investigations

The stereochemistry of aurantinin was determined by spectroscopic and computational analysis with the assistance of biosynthetic studies. The latter method provided critical evidence for the assignment of the configuration of the 3-ketosugar moiety.

Simple and Versatile Scheme for the Stereochemical Identification of Natural Products and Diverse Organic Compounds with Multiple Asymmetric Centers.

A simple and versatile scheme of stereochemical identification of the stereochemically rich natural products and organic compounds with multiple asymmetric centers is proposed based on a detailed parsing of calculated 1H and 13C NMR chemical shifts in combination with their DP4+ analysis, as exemplified for three natural products: sungucine, physalin D, and anabsinthin. Performed benchmark calculations of the considered diastereomers provided very good agreement with their known experimental stereochemical structures.

Multi-Layer π-Stacked Molecules as Efficient Thermally Activated Delayed Fluorescence Emitters.

Multi-layer π-stacked emitters based on spatially confined donor/acceptor/donor (D/A/D) patterns have been developed to achieve high-efficiency thermally activated delayed fluorescence (TADF). In this case, dual donor moieties and a single acceptor moiety are introduced to form two three-dimensional (3D) emitters, DM-BD1 and DM-BD2, which rely on spatial charge transfer (CT). Owing to the enforced face-to-face D/A/D pattern, effective CT interactions are realized, which lead to high photoluminescence quantum yields (PLQYs) of 94.2 % and 92.8 % for the two molecules, respectively. The resulting emitters exhibit small singlet-triplet energy splitting (ΔEST ) and fast reverse intersystem crossing (RISC) processes. Maximum external quantum efficiencies (EQEs) of 28.0 % and 26.6 % were realized for devices based on DM-BD1 and DM-BD2, respectively, which are higher than those of their D/A-type analogues.

Cadinane- and drimane-type sesquiterpenoids produced by Paecilomyces sp. TE-540, an endophyte from Nicotiana tabacum L., are acetylcholinesterase inhibitors

Sesquiterpenoids with diverse skeleton types are regarded as potential lead compounds in pharmacological and other applications. Herein, we report the discovery of two new cadinane-type sesquiterpenoids, paecilacadinol A (1) and B (2); two new drimane-type sesquiterpenoids, ustusol D (3) and ustusol E (4); and six known analogs (5–10) from the endophytic fungus Paecilomyces sp. TE-540, enriching the structural diversity of naturally occurring sesquiterpenoids. Their planar structures were determined on the basis of detailed interpretation of 1D and 2D NMR spectroscopy and HRESIMS data, while their stereochemical structures were established by X-ray crystallographic analyses for 1 and 3–8 and theoretical calculations for 2. Notably, compounds 1 and 2 represent novel examples of cadinane-type sesquiterpenoids with ether bonds formed by intramolecular dehydration. Compounds 5 and 6 showed moderate activities against acetylcholinesterase (AChE), with IC50 values of 43.02 ± 6.01 and 35.97 ± 2.12 μM, respectively. Docking analysis predicted that 5 bound well in the catalytic pocket of AChE via hydrophobic interactions with Trp84, Gly117, Ser122, and Tyr121 residues, while 6 was located with Asp72 and Ser122 residues.

Phytochemical profile and insecticidal activity of Drimia pancration (Asparagaceae) against adults of Stegobium paniceum (Anobiidae)

Chemical and spectroscopic investigation of the bulbs of Drimia pancration resulted in the isolation of one known flavonol (1), never isolated from this plant species, and of three previously described steroidal saponins (2–4), but whose configuration at their stereogenic centres was not clearly determined. By mean of 1H NMR, 13C NMR, nuclear overhauser effects (NOE) and two-dimensional NMR spectra the full stereochemical structures of compounds 2–4 were proved and all the 1H and 13C signals were assigned. Furthermore, the methanol and butanol extracts of D. pancration were tested against adults of Stegobium paniceum beetles. Despite the non-significant results regarding the repellent activity and contact toxicity, promising results were obtained from the feeding tests.

Two folate-derived analogues from an aqueous decoction of Uncaria rhynchophylla.

Two new folate-derived analogues, named uncarophyllofolic acids A (1) and B (2), respectively, were isolated from the Uncaria rhynchophylla hook bearing stem (Gouteng in Chinese). The distinct stereochemical structures of 1 and 2 were determined by spectroscopic data analysis in combination with acidic hydrolysis and Marfey's derivatization, along with comparison of their specific rotation and Cotton effect (CE) data with those of the biogenetically related known derivatives as well as theoretical calculations of electronic circular dichroism (ECD) spectra. A plausible biosynthetic pathway of 1 and 2, associating to folate metabolism and the previously reported orychophragines A-C from Orychophragmus violaceus, is discussed.

Teaching structural diversity of hexoses to graduate and postgraduate students: Methods to correlate stereochemistry.

Carbohydrates, or in strictly chemical terms, polyhydroxy-aldehydes or ketones represent enormous structural diversity in terms of the arrangement of atoms in space, resulting in hundreds of stereoisomers. Although the chemical properties of most stereoisomers may not be very different, their metabolic fate and utilization in biological systems is significantly different and known to influence the overall carbohydrate metabolism. This dictates the importance of understanding the structures of various stereochemical structures for a biochemist. At graduate level teaching, it is really a challenging task to correlate, connect, and recall all the structures. Conventional teaching methods and standard textbook illustrations seldom enable the learners and instructors to sustain the information for a prolonged period. We, in this article, present a unique approach and introduce sigma notation rules for the correlation of stereochemistry of carbohydrates in order to enhance the understanding of stereochemical structures. © 2019 International Union of Biochemistry and Molecular Biology, 48(1):8-20, 2020.