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Search Results: 1 - 10 of 10461 matches for " Jonathan Clayden "
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Looking forward to volume six
Jonathan Clayden
Beilstein Journal of Organic Chemistry , 2010, DOI: 10.3762/bjoc.6.1
Beilstein Journal of Organic Chemistry
Clayden Jonathan
Beilstein Journal of Organic Chemistry , 2005, DOI: 10.1186/1860-5397-1-1
Carbamate-directed benzylic lithiation for the diastereo- and enantioselective synthesis of diaryl ether atropisomers
Abigail Page,Jonathan Clayden
Beilstein Journal of Organic Chemistry , 2011, DOI: 10.3762/bjoc.7.156
Abstract: Diaryl ethers carrying carbamoyloxymethyl groups may be desymmetrised enantio- and diastereoselectively by the use of the sec-BuLi–( )-sparteine complex in diethyl ether. Enantioselective deprotonation of one of the two benzylic positions leads to atropisomeric products with ca. 80:20 e.r.; an electrophilic quench typically provides functionalised atropisomeric diastereoisomers in up to 97:3 d.r.
Asymmetric synthesis of tertiary thiols and thioethers
Jonathan Clayden,Paul MacLellan
Beilstein Journal of Organic Chemistry , 2011, DOI: 10.3762/bjoc.7.68
Abstract: Enantiomerically pure tertiary thiols provide a major synthetic challenge, and despite the importance of chiral sulfur-containing compounds in biological and medicinal chemistry, surprisingly few effective methods are suitable for the asymmetric synthesis of tertiary thiols. This review details the most practical of the methods available.
Asymmetric synthesis of biaryl atropisomers by dynamic resolution on condensation of biaryl aldehydes with ( )-ephedrine or a proline-derived diamine
Ann Bracegirdle,Jonathan Clayden,Lai Wah Lai
Beilstein Journal of Organic Chemistry , 2008, DOI: 10.3762/bjoc.4.47
Abstract: Atropisomeric biaryl aldehydes undergo diastereoselective condensation with ( )-ephedrine and with a proline-derived diamine, with selectivity highly dependent on solvent, temperature and reaction conditions. Levels of thermodynamic control up to 5:1 may be obtained by heating the diamine with the aldehyde in a sealed tube. Alternatively, crystallisation-induced dynamic transformation allows isolation of a single diastereoisomer in up to 85% yield. Hydrolysis and reduction of the major diastereoisomeric product of the reaction yields atropisomeric biaryls in >99:1 enantiomeric ratios.
Fused bicyclic piperidines and dihydropyridines by dearomatising cyclisation of the enolates of nicotinyl-substituted esters and ketones
Heloise Brice,Jonathan Clayden,Stuart D. Hamilton
Beilstein Journal of Organic Chemistry , 2010, DOI: 10.3762/bjoc.6.22
Abstract: The silyl enol ether derivatives of ketones or esters tethered by a hydrocarbon or ether linkage to the 3-position of a pyridine ring undergo dearomatising nucleophilic attack on the ring once it is activated (as an acylpyridinium species) by the addition of methyl chloroformate. The bicyclic dihydropyridine products are in some cases unstable, but may be isolated after hydrogenation as fused bicyclic piperidines.
Carbolithiation of N-alkenyl ureas and N-alkenyl carbamates
Julien Lefranc,Alberto Minassi,Jonathan Clayden
Beilstein Journal of Organic Chemistry , 2013, DOI: 10.3762/bjoc.9.70
Abstract: N-Alkenyl ureas and N-alkenyl carbamates, like other N-acyl enamines, are typically nucleophilic at their β-carbon. However, by incorporating an α-aryl substituent, we show that they will also undergo attack at the β-carbon by organolithium nucleophiles, leading to the products of carbolithiation. The carbolithiation of E and Z N-alkenyl ureas is diastereospecific, and N-tert-butoxycarbonyl N-alkenyl carbamates give carbolithiation products that may be deprotected in situ to provide a new connective route to hindered amines.
Principal Networks
Jonathan D. Clayden, Michael Dayan, Chris A. Clark
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0060997
Abstract: Graph representations of brain connectivity have attracted a lot of recent interest, but existing methods for dividing such graphs into connected subnetworks have a number of limitations in the context of neuroimaging. This is an important problem because most cognitive functions would be expected to involve some but not all brain regions. In this paper we outline a simple approach for decomposing graphs, which may be based on any measure of interregional association, into coherent “principal networks”. The technique is based on an eigendecomposition of the association matrix, and is closely related to principal components analysis. We demonstrate the technique using cortical thickness and diffusion tractography data, showing that the subnetworks which emerge are stable, meaningful and reproducible. Graph-theoretic measures of network cost and efficiency may be calculated separately for each principal network. Unlike some other approaches, all available connectivity information is taken into account, and vertices may appear in none or several of the subnetworks. Subject-by-subject “scores” for each principal network may also be obtained, under certain circumstances, and related to demographic or cognitive variables of interest.
Relating resting-state fMRI and EEG whole-brain connectomes across frequency bands
Fani Deligianni,Maria Centeno,David W. Carmichael,Jonathan D. Clayden
Frontiers in Neuroscience , 2014, DOI: 10.3389/fnins.2014.00258
Abstract: Whole brain functional connectomes hold promise for understanding human brain activity across a range of cognitive, developmental and pathological states. So called resting-state (rs) functional MRI studies have contributed to the brain being considered at a macroscopic scale as a set of interacting regions. Interactions are defined as correlation-based signal measurements driven by blood oxygenation level dependent (BOLD) contrast. Understanding the neurophysiological basis of these measurements is important in conveying useful information about brain function. Local coupling between BOLD fMRI and neurophysiological measurements is relatively well defined, with evidence that gamma (range) frequency EEG signals are the closest correlate of BOLD fMRI changes during cognitive processing. However, it is less clear how whole-brain network interactions relate during rest where lower frequency signals have been suggested to play a key role. Simultaneous EEG-fMRI offers the opportunity to observe brain network dynamics with high spatio-temporal resolution. We utilize these measurements to compare the connectomes derived from rs-fMRI and EEG band limited power (BLP). Merging this multi-modal information requires the development of an appropriate statistical framework. We relate the covariance matrices of the Hilbert envelope of the source localized EEG signal across bands to the covariance matrices derived from rs-fMRI with the means of statistical prediction based on sparse Canonical Correlation Analysis (sCCA). Subsequently, we identify the most prominent connections that contribute to this relationship. We compare whole-brain functional connectomes based on their geodesic distance to reliably estimate the performance of the prediction. The performance of predicting fMRI from EEG connectomes is considerably better than predicting EEG from fMRI across all bands, whereas the connectomes derived in low frequency EEG bands resemble best rs-fMRI connectivity.
On the control of secondary carbanion structure utilising ligand effects during directed metallation
Andrew E. H. Wheatley,Jonathan Clayden,Ian H. Hillier,Alison Campbell Smith
Beilstein Journal of Organic Chemistry , 2012, DOI: 10.3762/bjoc.8.5
Abstract: N,N-Diisopropyl-2-propylbenzamide 6-H undergoes lateral deprotonation by t-BuLi in the presence of the Lewis base PMDTA (N,N,N′,N″,N″-pentamethyldiethylenetriamine) to give a benzyllithium 6-Lil·PMDTA that incorporates a trigonal planar secondary carbanion. In the solid state, the amide directing group and the PMDTA additive work together to abstract the metal ion from the deprotonated α-C of the propyl group (4.107(4) ). A short distance of 1.376(3) is observed between the deprotonated carbon centre and a planar aromatic system that shows a pattern of bond lengths which contrasts with that reported for related tertiary carbanion systems. Analogous benzylic deprotonation is seen if 6-H is treated with t-BuLi in the presence of diglyme to give 6-Lil·DGME. X-ray crystallography now shows that the metal ion more closely approaches the tertiary carbanion (2.418(6) ) but that the planarity of the deprotonated carbon centre and the bonding pattern in the organic anion seen in the PMDTA complex are retained. DFT analysis corroborates both the short distance between aromatic ring and carbanion centre and the unperturbed nature of aromaticity in 6-Lil·L (L = Lewis base). The observation of two structure-types for the carbanion in solution is explained theoretically and by NMR spectroscopy in terms of cis and trans isomerism imparted by partial double bond character in the arene–(α-C) bond.
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