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Boron carboxylate catalysis of homoallylboration.
Boron tris(trifluoroacetate) is identified as the first effective catalyst for the homoallyl- and homocrotylboration of aldehydes by cyclopropylcarbinylboronates. NMR spectroscopic studies and theoretical calculations of key intermediates and transition states both suggest that a ligand-exchange mechanism, akin to our previously reported PhBCl2-promoted homoallylations, is operative. Our experimental and theoretical results also suggest that the catalytic activity of boron tris(trifluoroacetate) might originate from more facile catalytic turnover of the trifluoroacetate ligands (in agreement with DFT calculations) or from a lower propensity for formation of off-pathway reservoir intermediates (as observed by (1)H NMR). This work shows that carboxylates are viable catalytic ligands for homoallyl- and homocrotylations of carbonyl compounds and opens the door to the development of catalytic asymmetric versions of this transformation
Measurement of the and Total B Solar Neutrino Fluxes with the Sudbury Neutrino Observatory Phase I Data Set
This article provides the complete description of results from the Phase I
data set of the Sudbury Neutrino Observatory (SNO). The Phase I data set is
based on a 0.65 kt-year exposure of heavy water to the solar B neutrino
flux. Included here are details of the SNO physics and detector model,
evaluations of systematic uncertainties, and estimates of backgrounds. Also
discussed are SNO's approach to statistical extraction of the signals from the
three neutrino reactions (charged current, neutral current, and elastic
scattering) and the results of a search for a day-night asymmetry in the
flux. Under the assumption that the B spectrum is undistorted, the
measurements from this phase yield a solar flux of cm s, and a non- component
cm s. The sum of these components provides a
total flux in excellent agreement with the predictions of Standard Solar
Models. The day-night asymmetry in the flux is found to be , when the asymmetry in
the total flux is constrained to be zero.Comment: Complete (archival) version of SNO Phase I results. 78 pages, 46
figures, 34 table
Robust estimation of bacterial cell count from optical density
Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data
Total Synthesis of Spirotenuipesines A and B
Spirotenuipesines A and B, isolated from the entomopathogenic fungus <i>Paecilomyces tenuipes</i> by Oshima and co-workers, have been synthesized. The synthesis features the highly stereoselective construction of two vicinal all-carbon quaternary centers (C<sub>5</sub> and C<sub>6</sub>) via an intramolecular cyclopropanation/radical initiated fragmentation sequence and a diastereoselective intermolecular Diels−Alder reaction between α-methylenelactone dienophile <b>20</b> and synergistic diene <b>6a</b>. Installation of the C<sub>9</sub> tertiary alcohol occurred via nucleophilic methylation. An RCM reaction to produce a tetrasubstituted double bond in the presence of free allylic alcohol and homoallylic oxygenated functional group is also described. This route shortened the synthesis of <b>11</b> from 9 steps to 3 steps. We have further developed a strategy to gain access to optically active spirotenuipesines A and B through the synthesis of enantioenriched <b>10</b> from commercially available <i>R</i>-(−)-epichlorohydrin
Enantioselective <i>syn</i> and <i>anti</i> Homocrotylation of Aldehydes: Application to the Formal Synthesis of Spongidepsin
Whereas
crotylboration has been a useful method for synthesis of
stereochemically complex products, we have shown that <i>homo</i>crotylboration can be achieved with <i>cyclopropanated</i> crotylation reagents, and that the stereoselectivity of the reaction
can be predicted by analogous models. This paper presents a full account
of this work, including the first examples of asymmetric <i>anti</i> homocrotylation. The scope of this reaction is demonstrated with
highly enantioselective homocrotylation of both aliphatic and aromatic
aldehydes, as well as double diastereoselection studies. An application
of the synthesis of the marine natural product spongidepsin is presented,
as well as streamlined syntheses of homocrotylation reagents