425 research outputs found

    LEGAL EDUCATION IN GERMANY

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    <div><p>Predicting the subcellular localization of proteins conquers the major drawbacks of high-throughput localization experiments that are costly and time-consuming. However, current subcellular localization predictors are limited in scope and accuracy. In particular, most predictors perform well on certain locations or with certain data sets while poorly on others. Here, we present PSI, a novel high accuracy web server for plant subcellular localization prediction. PSI derives the wisdom of multiple specialized predictors via a joint-approach of group decision making strategy and machine learning methods to give an integrated best result. The overall accuracy obtained (up to 93.4%) was higher than best individual (CELLO) by ∼10.7%. The precision of each predicable subcellular location (more than 80%) far exceeds that of the individual predictors. It can also deal with multi-localization proteins. PSI is expected to be a powerful tool in protein location engineering as well as in plant sciences, while the strategy employed could be applied to other integrative problems. A user-friendly web server, PSI, has been developed for free access at <a href="http://bis.zju.edu.cn/psi/" target="_blank">http://bis.zju.edu.cn/psi/</a>.</p></div

    Enantioselective Synthesis of (+)-Crocacin C. An Example of a Highly Challenging Mismatched Double Asymmetric δ-Stannylcrotylboration Reaction

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    A concise, enantioselective synthesis of (+)-crocacin C is described, featuring a highly diastereoselective mismatched double asymmetric δ-stannylcrotylboration of the stereochemically demanding chiral aldehyde <b>9</b> with the bifunctional crotylborane reagent (<i>S</i>)-<i>E</i>-<b>10</b>. The total synthesis of (+)-crocacin C was accomplished in seven steps (longest linear sequence) starting from commercially available precursors

    Crotylboron-Based Synthesis of the Polypropionate Units of Chaxamycins A/D, Salinisporamycin, and Rifamycin S

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    Syntheses of the C(15)-C(27) fragments of chaxamycins A/D, rifamycin S, and the C(12)-C(24) fragment of salinisporamycin have been accomplished in 10 steps from commercially available starting materials. Three crotylboron reagents were utilized to construct the seven contiguous stereocenters in these fragments with excellent stereoselectivity

    Enantioselective Synthesis of (−)-Basiliskamide A

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    Basiliskamide A is an antifungal polyketide natural product isolated by Andersen and co-workers from a <i>Bacillus laterosporus</i> isolate, PNG-276. A nine-step enantioselective synthesis of (−)-basiliskamide A is reported, starting from commercially available β-hydroxy ester 7. The synthesis features a highly diastereoselective mismatched double asymmetric δ-stannylallylboration reaction of aldehyde 5 with the bifunctional allylborane reagent 4

    Highly Stereoselective Synthesis of <i>anti</i>,<i>anti</i>-Dipropionate Stereotriads: A Solution to the Long-Standing Problem of Challenging Mismatched Double Asymmetric Crotylboration Reactions

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    The stereocontrolled synthesis of the β-branched <i>anti</i>,<i>anti</i>-dipropionate stereotriad <b>4</b> via aldol or crotylmetal chemistry represents a historical challenge to the organic synthesis community. Here we describe a general solution to the long-standing problem associated with the synthesis of <b>4</b> by utilizing mismatched double asymmetric crotylboration reactions of enantioenriched α-methyl substituted aldehydes with the chiral, nonracemic crotylborane reagent (<i>S</i>)-(<i>E</i>)-<b>22</b> (or its enantiomer). This method not only provides direct access to <i>anti</i>,<i>anti</i>-dipropionate stereotriads <b>24</b> [a synthetic equivalent of <b>4</b>] with very good (5–8:1) if not excellent (≥15:1) diastereoselectivity from β-branched chiral aldehydes with ≤50:1 intrinsic diastereofacial selectivity preferences but also provides a vinylstannane unit in the products that is properly functionalized for use in subsequent C–C bond-forming events. We anticipate that this method will be widely applicable and will lead to substantial simplification of strategies for synthesis of polyketide natural products

    Enantioselective Synthesis of (<i>Z</i>)- and (<i>E</i>)‑2-Methyl-1,5-<i>anti</i>-Pentenediols via an Allene Hydroboration–Double-Allylboration Reaction Sequence

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    Kinetically controlled hydroboration of allenylboronate <b>5</b> followed by double allylboration with the resulting allylborane (<i>Z</i>)-<b>7</b> gave (<i>Z</i>)-2-methyl-1,5-<i>anti</i>-pentenediols <b>6</b> in good yield and high enantioselectivity in the presence of 10% BF<sub>3</sub>·OEt<sub>2</sub> as the catalyst in the second allylboration step. Under thermodynamically controlled isomerization conditions, (<i>Z</i>)-<b>7</b> can readily isomerize to (<i>E</i>)-<b>7</b>. Double allylboration of representative aldehydes with allylborane (<i>E</i>)-<b>7</b> gave (<i>E</i>)-2-methyl-1,5-<i>anti</i>-pentenediols <b>4</b> in good yield and high enantioselectivity without requiring use of the BF<sub>3</sub>·OEt<sub>2</sub> catalyst. Thus, 2-methyl-1,5-<i>anti</i>-pentenediols with either olefin geometry can be synthesized from the same allenylboronate precursor <b>5</b>. Furthermore, 1,5-pentenediols <b>4</b> and <b>6</b> can be easily converted to 1,3,5-triols with excellent diastereoselectivity in one step

    Anion Exchange on Cationic Surfactant Micelles, and a Speciation Model for Estimating Anion Removal on Micelles during Ultrafiltration of Water

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    Surfactant micelles combined with ultrafiltration can partially, or sometimes nearly completely, separate various ionic and nonionic pollutants from water. To this end, the selectivity of aqueous micelles composed of either cetyltrimethylammonium (CTA<sup>+</sup>) bromide or cetylpyridinium (CP<sup>+</sup>) chloride toward many environmentally relevant anions (IO<sub>3</sub><sup>–</sup>, F<sup>–</sup>, Cl<sup>–</sup>, HCO<sub>3</sub><sup>–</sup>, NO<sub>2</sub><sup>–</sup>, Br<sup>–</sup>, NO<sub>3</sub><sup>–</sup>, H<sub>2</sub>PO<sub>4</sub><sup>–</sup>, HPO<sub>4</sub><sup>2–</sup>, SO<sub>4</sub><sup>2–</sup>, and CrO<sub>4</sub><sup>2–</sup>) was investigated. Selectivity coefficients of CTA<sup>+</sup> micelles (with respect to Br<sup>–</sup>) and CP<sup>+</sup> micelle (with respect to Cl<sup>–</sup>) for these anions were evaluated using a simple thermodynamic ion exchange model. The sequence of anion affinity for the CTA<sup>+</sup> micelles and for the CP<sup>+</sup> micelles were the same, with decreasing affinity occurring in the order of: CrO<sub>4</sub><sup>2–</sup> > SO<sub>4</sub><sup>2–</sup> > HPO<sub>4</sub><sup>2–</sup> > NO<sub>3</sub><sup>–</sup> > Br<sup>–</sup> > NO<sub>2</sub><sup>–</sup> > Cl<sup>–</sup> > HCO<sub>3</sub><sup>–</sup> > H<sub>2</sub>PO<sub>4</sub><sup>–</sup> ≈ F<sup>–</sup>. From the associated component mass balance and ion exchange (i.e., mass action) equations, an overall speciation model was developed to predict the distribution of all anions between the aqueous and micellar pseudophase for complex ionic mixtures. Experimental results of both artificial and real surface waters were in good agreement to model predictions. Further, the results indicated that micelles combined with ultrafiltration may be a potential technology for nutrient and other pollutant removal from natural or effluent waters

    Enantioselective Synthesis of (−)-Basiliskamide A

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    Basiliskamide A is an antifungal polyketide natural product isolated by Andersen and co-workers from a <i>Bacillus laterosporus</i> isolate, PNG-276. A nine-step enantioselective synthesis of (−)-basiliskamide A is reported, starting from commercially available β-hydroxy ester 7. The synthesis features a highly diastereoselective mismatched double asymmetric δ-stannylallylboration reaction of aldehyde 5 with the bifunctional allylborane reagent 4

    Iridium-Catalyzed Enantioselective Allylic Substitution of Enol Silanes from Vinylogous Esters and Amides

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    The enol silanes of vinylogous esters and amides are classic dienes for Diels–Alder reactions. Here, we report their reactivity as nucleophiles in Ir-catalyzed, enantioselective allylic substitution reactions. A variety of allylic carbonates react with these nucleophiles to give allylated products in good yields with high enantioselectivities and excellent branched-to-linear ratios. These reactions occur with KF or alkoxide as the additive, but mechanistic studies suggest that these additives do not activate the enol silanes. Instead, they serve as bases to promote the cyclometalation to generate the active Ir catalyst. The carbonate anion, which was generated from the oxidative addition of the allylic carbonate, likely activates the enol silanes to trigger their activity as nucleophiles for reactions with the allyliridium electrophile. The synthetic utility of this method was illustrated by the synthesis of the <i>anti</i>-muscarinic drug, fesoterodine

    Total Synthesis of (−)-Tirandamycin C

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    Tirandamycin C is a newly isolated member of the tetramic acid family natural products. We described herein the first enantioselective synthesis of natural (−)-tirandamycin C, the postulated biosynthetic precursor of other members of this family. The highly stereoselective (>15:1) mismatched double asymmetric γ-stannylcrotylboration reaction of aldehyde <b>8</b> with crotylborane reagent (<i>R</i>)-<i>E</i>-<b>9</b> was utilized to access the key <i>anti,anti</i>-stereotriad <b>18</b>
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