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A mild method for the synthesis of 2-deoxysugars from the coupling of glycals with a range of nucleophiles is described. The method employs 1 mol % of an air- and moisture-tolerant rhenium-oxo complex [ReOCl3(SMe2)(Ph3PO)] as a catalyst for the formation of O-, N-, and S-alpha-glycosides. The catalytic system tolerates a number of commonly employed protecting groups, including isopropylidene acetals, alkyl and silyl ethers, acetates, and benzoates. Furthermore, the high-oxidation-state complex selectively catalyzes the coupling with the glycal acceptor in preference to oxidation of the glycals, alcohols, and even thiols.
The dynamics of the modified canonical nonlinear programming circuit are studied and how to guarantee the stability of the network's solution. By considering the total cocontent function, the solution of the canonical nonlinear programming circuit is reconciled with the problem being modeled. In addition, it is shown how the circuit can be realized using a neural network, thereby extending the results of D.W. Tank and J.J. Hopefield (ibid., vol.CAS-33, p.533-41, May 1986) to the general nonlinear programming problem.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
High-Q structures are very interesting theoretically, and very important practically, for a variety of engineering applications in communication systems. We address the issue of designing a thin-film metal structure of reflectivity higher than the intrinsic reflectivity of the bulk metal itself. We study a finite array of planar conducting layers of arbitrary thickness periodically placed an arbitrary distance apart, and we arrive at an exact analytical formula for the reflection and transmission coefficients. These structures are equivalent to a one-dimensional metallic photonic bandgap (PBG) system. We apply our formulas to the microwave regime and fully explore the system's three-dimensional parameter space, consisting of the number of layers, their thickness, and their spacing. We find very significant enhancements of the radio frequency-Q, relative to the bulk metal, in narrow regions of the parameter space.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
PURPOSE Models have been proposed to predict “salient” points in an image, but little progress has been made in relating these points to human fixation patterns (e.g. Itti & Koch, 2000). Part of the difficulty is that eye movements depend on top-down cues such as object knowledge and task in addition to bottom-up stimulus properties. We had subjects learn to recognize novel object silhouettes. The information in the stimulus can be defined in terms of edge orientations. We hypothesize that subjects employ a strategy of sequential information maximization, and propose a dynamic model that computes the most informative fixation sequence. METHODS Novel objects were constructed by superimposing the silhouettes of two inverted objects from the Snodgrass and Vanderwart (1980) dataset. The experiment consisted of 4 learning phases and 4 recognition phases, conducted over 5 consecutive days. We tracked the position of the dominant eye as subjects viewed the stimulus monocularly. MODEL We use entropy to characterize the distribution of possible orientations at each edge point in the stimulus. A polar grid based on the human cortical magnification factor is centered at a possible fixation. The entropy in each bin is computed from groundtruth edge orientations and used to update orientation information at each edge point in the bin. The fixation that maximizes information gain about the stimulus is chosen as the next fixation. RESULTS Cumulative information gain curves show that subjects maintain a consistent strategy for both learning and recognition trials. The model outperforms the human observers in collecting information quickly, but it produces a sequence of fixations that resembles human behavior. CONCLUSION A simple model of sequential information maximization shows promise in describing human eye movements driven by low-level stimulus properties. Extensions of the model can be used to probe the limits of human sensitivity in a learning and recognition task.
Nearly equivalent CC distances in the five-membered ring of the dianion C4Me4Si2−, which could be isolated in crystalline form as a potassium salt in the presence of [18]crown-6 (structure on the right), suggest a high degree of delocalization. In contrast, 29Si and 13C NMR spectroscopic investigations of the silolyl anions C4Me4SiR− (R SiMe3, Si(SiMe3)C4Me4) show that delocalization in these systems is minimal.
ABSTRACT Plasma cortisol concentrations were determined every 20 min for 24 h, in a nonstressful environment, among 48 rigorously assessed, mostly outpatient, drug‐free adolescent subjects during an episode of major depression (MDD) and among 40 normal adolescent subjects. There were no significant differences in the 24‐h mean, peak, or nadir, or the time of the nocturnal rise, in plasma cortisol in the 2 groups. Analyses of different subgroups of MDD adolescents according to suicidality, severity of depression, separation anxiety, psychotic subtype, endogenicity, duration of episode, and sex also revealed no significant group differences. Only one adolescent (with MDD) was identified clearly as a hypersecretor of cortisol. These results indicate that abnormalities of spontaneous cortisol secretion are an unusual finding among adolescents with major depression when studied in a nonstressful environment.
We report the photochemical reaction of transition metal boryl complexes of the type Cp*M(CO)nB(OR)2 (M = Fe, Ru, W) with alkanes to form alkylboronate esters in yields as high as 85% for reaction of Cp*W(CO)3[Bcat(Me)2] {[Bcat(Me)2] = B-1, 2-O2C6H2-3, 5-Me2} with pentane. Synthesis of a series of Cp and Cp* catecholboryl complexes showed that sterically blocking or eliminating sp2 positions on the metal boryl complex was important for alkane functionalization to occur. The metal boryl complexes reacted exclusively at the terminal C−H position of alkanes. Functionalization of 2-methylbutane occurred preferentially at the least sterically hindered terminal position with a selectivity of 10:1. This selectivity data, in addition to kinetic isotope effects measured for reaction of metal boryls with a mixture of pentane and pentane-d12, argues against radical chemistry. Several experiments were conducted to probe for CO dissociation. An experiment employing added 13CO, one conducted under 2 atm pressure of CO, and one conducted in the presence of PMe3 indicated that the mechanism involves CO dissociation to form a 16-electron intermediate that reacts faster with alkane solvent than it recoordinates CO. The effect of boryl electronics on the functionalization of alkanes was studied by examining the reactions of ruthenium dialkylboryl, dithioboryl, and dialkoxyboryl complexes with pentane. The dialkoxyboryl complexes gave the highest yields of functionalized product. A comparison between reactions of the different boryl complexes in arene and alkane solvents showed that the electronic properties of the boryl group had a greater effect on the reaction of the unsaturated intermediate with alkane than they did on the generation of the intermediate.
ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTHigh Oxidation-State (Formally d0) Tungsten Silylene Complexes via Double Si−H Bond ActivationBenjamin V. Mork and T. Don TilleyView Author Information Department of Chemistry University of California, Berkeley Berkeley, California 94720-1460 Cite this: J. Am. Chem. Soc. 2001, 123, 39, 9702–9703Publication Date (Web):September 1, 2001Publication History Received3 July 2001Published online1 September 2001Published inissue 1 October 2001https://pubs.acs.org/doi/10.1021/ja0165436https://doi.org/10.1021/ja0165436rapid-communicationACS PublicationsCopyright © 2001 American Chemical SocietyRequest reuse permissionsArticle Views876Altmetric-Citations70LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (2)»Supporting Information Supporting Information SUBJECTS:Anions,Cations,Ligands,Metals,Transition metals Get e-Alerts
