Multi-Coil MRI Reconstruction Challenge-Assessing Brain MRI Reconstruction Models and Their Generalizability to Varying Coil Configurations.

Deep-learning-based brain magnetic resonance imaging (MRI) reconstruction methods have the potential to accelerate the MRI acquisition process. Nevertheless, the scientific community lacks appropriate benchmarks to assess the MRI reconstruction quality of high-resolution brain images, and evaluate how these proposed algorithms will behave in the presence of small, but expected data distribution shifts. The multi-coil MRI (MC-MRI) reconstruction challenge provides a benchmark that aims at addressing these issues, using a large dataset of high-resolution, three-dimensional, T1-weighted MRI scans. The challenge has two primary goals: (1) to compare different MRI reconstruction models on this dataset and (2) to assess the generalizability of these models to data acquired with a different number of receiver coils. In this paper, we describe the challenge experimental design and summarize the results of a set of baseline and state-of-the-art brain MRI reconstruction models. We provide relevant comparative information on the current MRI reconstruction state-of-the-art and highlight the challenges of obtaining generalizable models that are required prior to broader clinical adoption. The MC-MRI benchmark data, evaluation code, and current challenge leaderboard are publicly available. They provide an objective performance assessment for future developments in the field of brain MRI reconstruction.

An air-stable cationic iridium hydride as a highly active and general catalyst for the isomerization of terminal epoxides.

We describe the use of an air-stable iridium hydride catalyst for the isomerization of terminal epoxides into aldehydes with perfect regioselectivity. The system operates at low loadings of catalyst (0.5 mol%), is highly practical, scalable, and tolerates functional groups that would not be compatible with Lewis acids typically used in stoichiometric amounts. Evidence for a rare hydride mechanism are provided.

Isomerization of terminal epoxides by a [Pd-H] catalyst: a combined experimental and theoretical mechanistic study.

An unusual palladium hydride complex has been shown to be a competent catalyst in the isomerization of a variety of terminal and internal epoxides. The reaction displayed broad scope and synthetic utility. Experimental and theoretical evidence are provided for an unprecedented hydride mechanism characterized by two distinct enantio-determining steps. These results hold promise for the development of an enantioselective variant of the reaction.

Asymmetric synthesis of α-chiral allylic silanes by enantioconvergent γ-selective copper(I)-catalyzed allylic silylation.

Gamma way: Regio- and enantioselective allylic substitution with a silicon nucleophile generated by copper(I)-catalyzed Si-B bond activation provides direct access to α-chiral allylic silanes from linear acceptors. The enantioconvergent catalysis employing McQuade's six-membered N-heterocyclic-carbene-copper(I) catalyst is applicable to aryl- and alkyl-substituted allylic phosphates (see scheme).

Copper-catalyzed 1,2-addition of nucleophilic silicon to aldehydes: mechanistic insight and catalytic systems.

Activation of the Si-B inter-element bond with copper(I) alkoxides produces copper-based silicon nucleophiles that react readily with aldehydes to yield α-silyl alcohols (that is, α-hydroxysilanes) after hydrolysis. Two independent protocols were developed, one employing a well-defined NHC-CuOtBu complex and one using the simple CuCN-NaOMe combination without added ligand. The mechanism of the aldehyde addition was investigated in detail by stoichiometric and catalytic experiments as well as NMR spectroscopic measurements. The primary reaction product of the addition of the Si-B reagent and the aldehyde (a boric acid ester of the α-silyl alcohol) and also the "dead-end" intermediate, formed in the competing [1,2]-Brook rearrangement, were characterized crystallographically. Based on these data, a reasonable catalytic cycle is proposed. The NHC-CuOtBu catalytic setup performs nicely at elevated temperature. A more reactive catalytic system is generated from CuCN-NaOMe, showing fast turnover at a significantly lower temperature. Both aromatic and aliphatic aldehydes are transformed into the corresponding α-silyl alcohols in good to very good yields under these mild reaction conditions.

Copper(I)-catalyzed regioselective propargylic substitution involving Si-B bond activation.

The silicon nucleophile generated by copper(I)-catalyzed Si-B bond activation allows several γ-selective propargylic substitutions. The regioselectivity (γ:α ratio) is strongly dependent on the propargylic leaving group. Chloride is superior to oxygen leaving groups in linear substrates (γ:α > 99:1), and it is only the phosphate group that also shows promising regiocontrol (γ:α = 90:10). That leaving group produces superb γ-selectivity (γ:α > 99:1) in α-branched propargylic systems, and enantioenriched substrates react with excellent central-to-axial chirality transfer.

Enantioselective formal hydration of α,ß-unsaturated acceptors: asymmetric conjugate addition of silicon and boron nucleophiles.

The direct enantioselective 1,4-addition of water to α,ß-unsaturated acceptors is an open challenge in asymmetric catalysis. Enantioselective conjugate addition of either silicon or boron nucleophiles and subsequent enantiospecific oxidative degradation of the carbon-element bond represents, however, an attractive detour. A single extra step thereby enables an indirect enantiocontrolled construction of (in a broader sense) aldols from α,ß-unsaturated carbonyl and carboxyl compounds. While that strategy had been obvious for a long time, it was recent stunning progress in transition metal-catalysed activation of interelement linkages that brought about the solution to that long-standing problem. A concise introduction of existing protocols for stereoselective 1,4-addition of oxygen nucleophiles is followed by a comprehensive summary of the recent rapid advances in transition metal-catalysed (and metal-free) asymmetric conjugate transfer of nucleophilic silicon and boron onto α,ß-unsaturated acceptors.

Activation of the Si-B linkage: copper-catalyzed addition of nucleophilic silicon to imines.

Activation of the Si-B bond through copper-catalyzed transmetalation quickly developed into a practical method to generate Cu-Si reagents. These silicon nucleophiles cleanly add to aldehyde-derived imine electrophiles to form α-silylated amines in protic media, and no carbon-to-nitrogen Brook-type rearrangement of the intermediate anion is observed. Aside from electron-withdrawing groups at the imine nitrogen atom, for example, SO(2)Tol and P(O)Ph(2), previously delicate nitrogen substituents such as phenyl or benzhydryl are tolerated. The same protocol also allows the unprecedented addition to representative ketone-derived imines.

Stereochemical surprises in the Lewis acid-mediated allylation of isatins.

The BF(3)·OEt(2)-mediated allylation of isatin with an α-chiral allylic stannane is diastereo- and enantioselective. Conversely, allylation of any substituted isatin employing the identical protocol is not diastereoselective at all and only enantioselective for the major diastereomer having syn relative configuration. The anti isomer is, however, formed in almost racemic form. Both absolute and relative configurations are unambiguously secured by X-ray analysis of major isomers, and the stereochemical assignment of the other 3-substituted 3-hydroxy oxindoles is based on similar NMR spectroscopic characteristics. The remarkable observations are rationalized by an acyclic transition state model.

Microstructure change in poly(ethersulfone) films by swift heavy ions.

PES membrane of thickness 25 microm was irradiated by Cl(9+) ions of energy 100 MeV at IUAC, New Delhi. Microstructure changes due to exposure to high-energy ions were investigated by Fourier transform infrared (FTIR) and ultraviolet/visible (UV/vis) absorption spectroscopies, X-ray diffraction technique and by dynamic mechanical analysis (DMA). A significant loss of crystallinity is observed by the XRD data. Particle size or grain size calculated using Scherrer formula indicates measurable change in particle size of irradiated samples. The polymer chain scissions and structure degradations are expected to occur for irradiated samples. Optical properties of the films were changed due to irradiation that could be clearly seen in the absorption spectra. FTIR does not show the remarkable change in the irradiated samples, but there is some change in the surface roughness observed by AFM.

Expedient access to branched allylic silanes by copper-catalysed allylic substitution of linear allylic halides.

An unprecedented copper-catalysed allylic transposition enables the regioselective synthesis of branched allylic silanes from linear allylic halides through direct C-Si bond formation.