Asymmetric Transfer Research Articles

Cryo-EM reveals the structural origins of asymmetric electron transfer in nitrogenase-like enzymes

Cryo-EM reveals the structural origins of asymmetric electron transfer in nitrogenase-like enzymes

Hierarchical K-means clustering method for accelerated Lorentzian estimation (KALE) in chemical exchange saturation transfer-magnetic resonance imaging quantification.

Quantification of in vivo chemical exchange saturation transfer (CEST) magnetic resonance signals is challenging due to contamination from coexisting effects, including the direct water effect and asymmetric magnetization transfer. Fitting-based analysis allows the calculation of multiple types of signals from the line shape of Z-spectra. However, the conventional voxelwise method has several drawbacks, including its long computation time and its susceptibility to image noise and Z-spectra oscillations, and it is difficult to determine the initial fitting parameters. Herein, we propose a K-means clustering method for accelerated Lorentzian estimation (KALE) in CEST quantification. Briefly, voxels in CEST images are clustered into K groups according to their Z-spectra characteristics. A 'groupwise' fitting process is then performed with preset initial values, yielding a set of fitted spectra and fitted parameters for each group. With the updated initial values, each group is further clustered into subgroups, and groupwise fitting is performed again. This hierarchical K-means clustering and parameter updating process continues until the pixel number or intensity error meets the termination criteria. Voxelwise fitting could be further conducted to improve the quantification images (termed voxel-K) by utilizing the previous groupwise KALE results as the initial values (termed group-K). Incorporated with Lorentzian difference (LD) quantification, KALE was first optimized and evaluated on 5 healthy human brain datasets at 3 Tesla. Compared with traditional voxel-by-voxel LD quantification, the computation times of group-K and voxel-K were significantly reduced by ~85% and ~70%, respectively (P<0.001). Furthermore, the group-K images exhibited better denoising performance than traditional LD and voxel-K. KALE was further validated on six ischemic rat brains acquired at 7 Tesla, with both LD_group-K and LD_voxel-K displaying almost identical contrast maps with traditional voxelwise maps. When incorporated with the five-pool Lorentzian fitting (LF), KALE exhibited an improved contrast-to-noise ratio (CNR) for amplitude maps of each pool [P=0.003, 0.015, 0.047, and 0.047 for amide, nuclear Overhauser effect (NOE), magnetic transfer (MT) and guanidine amine, respectively] and improved fitting goodness (P=0.033). KALE quantification provides comparable or even superior contrast maps to traditional voxelwise fitting, with significantly reduced computation time. The 'smart' and hierarchical voxel-clustering and parameter updating process of KALE may facilitate more preclinical and clinical CEST applications.

Asymmetric Transfer Hydrogenation of Heterocyclic Compounds in Continuous Flow Using an Immobilized Chiral Phosphoric Acid as the Catalyst

AbstractThis manuscript describes transfer hydrogenation of bicyclic nitrogen-containing heterocyclic compounds using the immobilized chiral phosphoric acid catalyst (R)-PS-AdTRIP in batch and continuous flow. A significant improvement in enantioselectivities is achieved in continuous flow with a fluidized bed reactor packed with (R)-PS-AdTRIP when the flow rate is increased from 0.2 mL/min to 2.0–2.5 mL/min. The optimized continuous flow conditions consistently provide 4–6% ee higher selectivity than transfer hydrogenation in batch with 2 mol% of (R)-PS-AdTRIP, and are used to generate multiple chiral products with the same fluidized bed reactor.

Iridium-Catalyzed Asymmetric Transfer Hydrogenation of 1-Aryl-1-alkylethenes with Ethanol.

Asymmetric transfer hydrogenation of 1-aryl-1-alkylethenes with ethanol was developed with a chiral (PCN)Ir complex as the precatalyst, featuring high enantioselectivities, good functional group tolerance, and operational simplicity. The method is further applied to formal intramolecular asymmetric transfer hydrogenation of alkenols without an external H-donor, producing a tertiary stereocenter and remote ketone group simultaneously. The utility of the catalytic system was highlighted by gram scale synthesis and the synthesis of the key precursor of (R)-xanthorrhizol.

Adaptive decentralized prescribed performance control for a class of large‐scale stochastic nonlinear systems subject to input saturation and full state constraints

SummaryThis paper focuses on an adaptive decentralized prescribed performance control problem for a class of large‐scale stochastic nonlinear systems with asymmetric input saturation and full state constraints. Firstly, the obstacle of input saturation is overcome by introducing the Gaussian error functions. Secondly, the transient performance of the system output is realized by introducing the asymmetric error transfer functions. Thirdly, the full state constraints are considered in the backstepping control process, and the boundary of state constraints is ensured by constructing barrier Lyapunov functions. Then, the multidimensional Taylor network is employed to approximate the unknown nonlinearity, and an adaptive decentralized controller is designed. Finally, it is shown that the proposed control strategy can ensure that the closed‐loop system is semi‐global ultimately uniformly bounded in probability, and the tracking error of the system can be kept within an adjustable neighborhood of the origin. Two simulation examples are provided to illustrate the feasibility of the proposed control strategy.

A Mixed Pseudo-spectral FFT-FE Method for Asymmetric Nonlinear Heat Transfer of a Functionally Graded Hollow Cylinder

A Mixed Pseudo-spectral FFT-FE Method for Asymmetric Nonlinear Heat Transfer of a Functionally Graded Hollow Cylinder

Asymmetric Transfer Hydrogenation of Naphthol and Phenol Derivatives with Cooperative Heterogeneous and Homogeneous Catalysis

Asymmetric Transfer Hydrogenation of Naphthol and Phenol Derivatives with Cooperative Heterogeneous and Homogeneous Catalysis

Stereoselective synthesis of S-norvaline and related amino acids through a common intermediate.

A divergent, enantioselective synthetic strategy is reported to produce the non-proteinogenic, biologically active natural amino acids norvaline, 5-hydroxy-4-oxo-L-norvaline, and ɣ-oxonorvaline. These were synthesized in good yields (45-75%) from the common starting material (S)-allylglycine obtained by asymmetric transfer allylation of glycine Schiff base using the Corey catalyst derived from cinchonidine in more than 97% enantiomeric excess.

Tuning catalyst performance in asymmetric carbene transfer toward CF3-cyclopropenes

In this issue of Chem Catalysis, Zhang, Ma, Marek, et al. report a highly enantioselective carbene-transfer reaction of 2,2,2-trifluorodiazoethane (CF3CHN2) and challenging disubstituted alkynes. Key to the success of this approach is the fine-tuning of chiral dirhodium(II) carboxylate paddlewheel complexes featuring a molecule of dioxane as achiral axial ligand.

New Trends in Asymmetric Phase Transfer Catalysis

AbstractPhase Transfer Catalysis (PTC) is a powerful tool to perform reactions in a practical fashion, both in laboratory and industrial scale. Significant cost savings and major process improvements can be achieved in reactions performed under PTC conditions. In the last few years remarkable results in stereoselective reactions were achieved using chiral, non‐racemic quaternary ammonium salts. Moreover, the use of bulky, chiral phosphate anions paired with achiral cations to generate lipophilic ion pairs allowed to design new avenues for the stereoselective construction of important building blocks. Hydrogen bond interactions were also shown to provide new pathways for asymmetric nucleophilic substitutions using insoluble reagents under PTC conditions. This Review will focus on recent advances in developing practical synthetic routes to construct molecules in a stereoselective fashion under PTC conditions.

Asymmetric negative transfer effects of working memory training.

Gathercole et al. (Journal of Memory and Language, 105, 19-42, 2019) presented a cognitive routine framework for explaining the underlying mechanisms of working memory (WM) training and transfer. This framework conceptualizes training-induced changes as the acquisition of novel cognitive routines similar to learning a new skill. We further infer that WM training might not always generate positive outcomes because previously acquired routines may affect subsequent task performance in various ways. Thus, the present study aimed to demonstrate the negative effects of WM training via two experiments. We conducted Experiment 1online using a two-phase training paradigm with only three training sessions per phase and replicated the key findings of Gathercole and Norris (in prep.) that training on a backward circle span task (a spatial task) transferred negatively to subsequent training on a backward letter span task (a verbal task). We conducted Experiment 2using a reversed task order design corresponding to Experiment 1. The results indicated that the transfer from backward letter training to backward circle training was not negative, but rather weakly positive, suggesting that the direction of the negative transfer effect is asymmetric. The present study therefore found that a negative transfer effect can indeed occur under certain WM training designs. The presence of this asymmetric effect indicates that backward circle and backward letter tasks require different optimal routines and that the locus of negative transfer might be the acquisition process of such optimal routines. Hence, the routines already established for backward circle might hinder the development of optimal routines for backward letter, but not vice versa.

Chiral Boro‐Phosphate Catalyzed Asymmetric Transfer Hydrogenation of 1‐Enal Substituted 2‐Naphthols: Access to Axially Chiral Styrene‐Type Allylalcohols

AbstractHerein, we present a chiral boro‐phosphate catalyzed atroposelective asymmetric transfer hydrogenation method, leading to a family of axially chiral styrene‐type allylalcohols. This dynamic kinetic resolution approach portrays simple procedure, mild conditions and good enantiocontrol (51–95% ee), thus providing an important alternative to assemble the challenging atropisomeric aryl‐acyclic alkene scaffolds.magnified image

Pincer Ru with a single stereogenic identity for highly efficient asymmetric transfer hydrogenation of ketones

Pincer Ru with a single stereogenic identity for highly efficient asymmetric transfer hydrogenation of ketones

Electrophysiological Evidence Reveals the Asymmetric Transfer from the Right to Left Hemisphere as Key to Reading Proficiency.

The present investigation aimed to explore the interhemispheric interactions that contribute to changes in reading proficiency by examining the processing of visual word recognition in relation to word familiarity. A lexical decision task was administered to 25 participants, and their electrophysiological activity was recorded. A behavioral analysis showed the faster and more accurate processing of highly familiar words compared to less familiar ones. An event-related potential analysis uncovered an asymmetric familiarity effect over the N100 and N400 components across the two hemispheres, indicating an asymmetrical word familiarity processing. Granger causality analyses demonstrated a stronger transfer of information from the right hemisphere (RH) to the left hemisphere (LH) during the N100 processing and a weaker transfer from the LH to the RH during the N400 processing for highly familiar word recognition. These findings suggest that the asymmetric coordination between the RH and LH occurs early in visual word recognition and highlight the importance of interhemispheric interactions in efficient visual word recognition and proficient reading.

Synthesis of cyclic amino acid baikiain via asymmetric phase transfer catalysis

AbstractThe synthesis of the cyclic, naturally occurring amino acid baikiain from glycine and cis‐1,4‐butenediol is reported. The key step of the synthesis is an enantioselective phase transfer catalyzed allylation of the benzophenone imine of glycine t‐butyl ester, catalyzed by a cinchonidine quaternary ammonium salt, which provided the allylated glycine in 82% yield and 80% e.e. Further deprotection and hydrolysis of the Schiff base followed by an intramolecular Mitsunobu ring closure reaction yielded the corresponding piperidine ring system in 77% yield, which by hydrolysis yielded baikiain hydrochloride in 36% overall yield. Hydrogenation of the double bond could provide access to (S)‐pipecolic acid, which is also an important chiral synthon.

수입가격을 고려한 고등어 가격의 유통단계별 인과성과 비대칭적 가격 전이 분석

수입가격을 고려한 고등어 가격의 유통단계별 인과성과 비대칭적 가격 전이 분석

Substrate-Directed Enantioselective Aziridination of Alkenyl Alcohols Controlled by a Chiral Cation.

Alkene aziridination is a highly versatile transformation for the construction of chiral nitrogen-containing compounds. Inspired by the success of analogous substrate-directed epoxidations, we report an enantioselective aziridination of alkenyl alcohols, which enables asymmetric nitrene transfer to alkenes with varied substitution patterns, including those not covered by the current protocols. We believe that our method is effective because it is substrate-directed, exploiting a network of attractive non-covalent interactions between the substrate, an achiral dianionic rhodium(II,II) tetracarboxylate dimer, and its two associated cinchona alkaloid-derived cations. It is these cations that provide a defined chiral pocket in which the aziridination can occur. In addition to a thorough evaluation of compatible alkene classes, we advance a practical mnemonic to predict reaction outcome and disclose a range of post-functionalization protocols that highlight the unique synthetic potential of the enantioenriched aziridine-alcohol products.

Enantioselective Synthesis of (R)-Tiagabine via Asymmetric Hydrogen Atom Transfer Protocol

AbstractAn enantioselective synthesis of tiagabine has been achieved utilizing an asymmetric hydrogen atom transfer protocol to construct its essential chiral tertiary carbon center. A cyclization reaction via double N-substitution is tactically orchestrated as the other key step to install the crucial alkaloid ring. Compared with the previous synthetic strategy, which used commercially available nicotinate as the starting material to ensure a short synthetic route, this strategy uses a readily modifiable and accessible alkyl-substituted acrylate as the starting material and thus provides a scenario for the facile synthesis of analogues and derivatives of tiagabine for further biological research.

Thermal rectification in thin film metalattice structures: A computational study

Thermal rectification is an asymmetric heat transfer process where directionally dependent transport occurs along a given axis. In this work, geometric parameters that govern thermal rectification in solids composed of various semiconducting materials were investigated utilizing metalattice data for seven materials with pore sizes ranging between 2 and 30 nm. Using numerical simulation, thermal rectification was calculated at different thermal biases in single material systems, including silicon, cubic boron nitride, and diamond, among others. The largest thermal rectification for each material was exhibited in bilayer sample stacks that were thermally matched (i.e., the thermal resistance of each layer in the stack is equal in either forward or reverse direction). Of the materials tested, diamond provided the highest thermal rectification for all cases, with its best case achieving a thermal rectification of 57.2%. This novel thermal functionality will find application in advanced applications for temperature regulation, including resonator systems where thermal effects may significantly alter and/or degrade performance.

(1S,2S)-2-[(S)-2,2,2-Tri-fluoro-1-hy-droxy-eth-yl]-1-tetra-lol.

The crystal structure of the title enanti-opure tetralol derivative {systematic name: (1S,2S)-2-[(S)-2,2,2-tri-fluoro-1-hy-droxy-eth-yl]-1,2,3,4-tetra-hydro-naph-thalen-1-ol}, C12H13F3O2, synthesized by asymmetric transfer hydrogenation, was elucidated by low-temperature single-crystal X-ray diffraction. The enanti-opure compound crystallizes in the Sohncke space group P212121 with one mol-ecule in the asymmetric unit and features intra-molecular as well as inter-molecular O-H⋯O hydrogen bonding. The absolute configuration was established from anomalous dispersion effects.