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In control: Gold(I)-catalyzed intramolecular addition of silyl enol ethers to alkynes and allenes allows for the diastereoselective synthesis of cyclopentenes with control of the position of the double bond. The utility of these reactions is demonstrated by an efficient total synthesis of (+)-lycopladine A that takes advantage of the orthogonal reactivity of AuI and Pd0 towards unsaturated iodides (see scheme).
Quantitation of photosystem II (PSII) activity in spinach chloroplasts is presented. Rates of PSII electron‐transport were estimated from the concentration of PSII reaction‐centers (Chl/PSII = 380:1 when measured spectrophotometrically in the ultraviolet [ΔA 320 ] and green [ΔA 540–550 ] regions of the spectrum) and from the rate of light utilization by PSII under limiting excitation conditions. Rates of PSII electron‐transport were measured under the same light‐limiting conditions using 2,5‐dimethylbenzoquinone or 2,5‐dichlorobenzoquinone as the PSII artificial electron acceptors. Evaluation is presented on the limitations imposed in the measurement of PSII electron flow to artificial quinones in chloroplasts. Limitations include the static quenching of excitation energy in the pigment bed by added quinones, the fraction of PSII centers (PSII β ) with low affinity to native and added quinones, and the loss of reducing equivalents to molecular oxygen. Such artifacts lowered the yield of steady‐state electron transport in isolated chloroplasts and caused underestimation of PSII electron‐transport capacity. The limitations described could explain the low PSII concentration estimates in higher plant chloroplasts (Chl/PSII = 600 ± 50) resulting from proton flash yield and/or oxygen flash‐yield measurements. It is implied that quantitation of PSII by repetitive flash‐yield methods requires assessment of the slow turnover of electrons by PSII β and, in the presence of added quinones, assessment of the PSII quantum yield.
This paper considers the support of real-time applications in an Integrated Services Packet Network (ISPN). We first review the characteristics of real-time applications. We observe that, contrary to the popular view that real-time applications necessarily require a fixed delay bound, some real-time applications are more flexible and can adapt to current network conditions. We then propose an ISPN architecture that supports two distinct kinds of real-time service: guaranteed service, which is the traditional form of real-time service discussed in most of the literature and involves pre-computed worst-case delay bounds, and predicted service which uses the measure performance of the network in computing delay bounds. We then propose a packet scheduling mechanism that can support both of these real-time services as well as accommodate datagram traffic. We also discuss two other aspects of an overall ISPN architecture: the service interface and the admission control criteria.
There is widespread agreement on the need for architectural change in the Internet, but very few believe that current ISPs will ever effect such changes. In this paper we ask what makes an architecture evolvable, by which we mean capable of gradual change led by the incumbent providers. This involves both technical and economic issues, since ISPs have to be able, and incented, to offer new architectures. Our study suggests that, with very minor modifications, the current Internet architecture could be evolvable.
Crop loss due to soil salinization is an increasing threat to agriculture worldwide. This review provides an overview of cellular and physiological mechanisms in plant responses to salt. We place cellular responses in a time- and tissue-dependent context ...Read More
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A study on the iridium-catalyzed C-H borylation of heteroarenes is reported. Several heteroarenes containing multiple heteroatoms were found to be amenable to C-H borylation catalyzed by the combination of an iridium(I) precursor and tetramethylphenanthroline. The investigations of the scope of the reaction led to the development of powerful rules for predicting the regioselectivity of borylation, foremost of which is that borylation occurs distal to nitrogen atoms. One-pot functionalizations are reported of the heteroaryl boronate esters formed in situ, demonstrating the usefulness of the reported methodology for the synthesis of complex heteroaryl structures. Application of this methodology to the synthesis and late-stage functionalization of biologically active compounds is also demonstrated. Mechanistic studies show that basic heteroarenes can bind to the catalyst and alter the resting state from the olefin-bound complex observed during arene borylation to a species containing a bound heteroarene, leading to catalyst deactivation. Studies on the origins of the observed regioselectivity show that borylation occurs distal to N-H bonds due to rapid N-H borylation, creating an unfavorable steric environment for borylation adjacent to these bonds. Computational studies and mechanistic studies show that the lack of observable borylation of C-H bonds adjacent to basic nitrogen is not the result of coordination to a bulky Lewis acid prior to C-H activation, but the combination of a higher-energy pathway for the borylation of these bonds relative to other C-H bonds and the instability of the products formed from borylation adjacent to basic nitrogen.
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Algorithms in computational geometry often use the real-RAM model of computation. In particular, this model assumes that exact real numbers can be stored in and retrieved from memory in constant $O$ (1) time, and that field operations (+, -, *, /) and certain other operations (like square root, sine and cosine) are also "exact," and can be applied in constant time.
Peg-in-hole insertion is not only a longstanding problem in robotics but the most common automated mechanical assembly task. In this paper we present a high precision, self-calibrating peg-in-hole insertion strategy using several very simple, inexpensive, and accurate optical sensors. The self-calibrating feature allows us to achieve successful dead-reckoning insertions with tolerances of 25 microns without any accurate initial position information for the robot, pegs, or holes. The program we implemented works for any cylindrical peg, and the sensing steps do not depend on the peg diameter, which the program does not know. The key to the strategy is the use of a fixed sensor to localize both a mobile sensor and the peg, while the mobile sensor localizes the hole. Our strategy is extremely fast, localizing pegs as they are in route to their insertion location without pausing. The result is that insertion times are dominated by the transport time between pick and place operations.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSynthesis of poly(p-hydroxy-α-methylstyrene) by cationic polymerization and chemical modificationHiroshi Ito, C. Grant Willson, Jean M. J. Frechet, M. Jean Farrall, and Eva EichlerCite this: Macromolecules 1983, 16, 4, 510–517Publication Date (Print):April 1, 1983Publication History Published online1 May 2002Published inissue 1 April 1983https://pubs.acs.org/doi/10.1021/ma00238a004https://doi.org/10.1021/ma00238a004research-articleACS PublicationsRequest reuse permissionsArticle Views698Altmetric-Citations74LEARN 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-Alertsclose Get e-Alerts
