8,156 research outputs found
Reactivity of (Bicyclo[5.1.0]octadienyl)iron(1+) Cations: Application to the Synthesis of cis-2-(2’-carboxycyclopropyl)glycines
The addition of carbon and heteroatom nucleophiles to (bicyclo[5.1.0]octadienyl)Fe(CO)2L+ cations 5 or 8 (L = CO, PPh3) generally proceeds via attack at the dienyl terminus on the face of the ligand opposite to iron to generate 6-substituted (bicyclo[5.1.0]octa-2,4-diene)iron complexes (11 or 13). In certain cases, these products are unstable with respect to elimination of a proton and the nucleophilic substituent to afford (cyclooctatetraene)Fe(CO)2L (4 or 7). Decomplexation of 13f, arising from addition of phthalimide to 8, gave N-(bicyclo[5.1.0]octa-3,5-dien-2-yl)phthalimide (19). Oxidative cleavage of 19 (RuCl3/NaIO4) followed by esterification gave the cyclopropane diester 22, which upon hydrolysis gave cis-2-(2‘-carboxycyclopropyl)glycine (CCG-III, 18) (eight steps from 4, 43% overall yield). This methodology was also utilized for preparation of stereospecifically deuterated CCG-III (d-18) and optically enriched (−)-18. Deprotonation of 22 resulted in cyclopropane ring opening to afford the benzoindolizidine (23)
Fast gradient descent for drifting least squares regression, with application to bandits
Online learning algorithms require to often recompute least squares
regression estimates of parameters. We study improving the computational
complexity of such algorithms by using stochastic gradient descent (SGD) type
schemes in place of classic regression solvers. We show that SGD schemes
efficiently track the true solutions of the regression problems, even in the
presence of a drift. This finding coupled with an improvement in
complexity, where is the dimension of the data, make them attractive for
implementation in the big data settings. In the case when strong convexity in
the regression problem is guaranteed, we provide bounds on the error both in
expectation and high probability (the latter is often needed to provide
theoretical guarantees for higher level algorithms), despite the drifting least
squares solution. As an example of this case we prove that the regret
performance of an SGD version of the PEGE linear bandit algorithm
[Rusmevichientong and Tsitsiklis 2010] is worse that that of PEGE itself only
by a factor of . When strong convexity of the regression problem
cannot be guaranteed, we investigate using an adaptive regularisation. We make
an empirical study of an adaptively regularised, SGD version of LinUCB [Li et
al. 2010] in a news article recommendation application, which uses the large
scale news recommendation dataset from Yahoo! front page. These experiments
show a large gain in computational complexity, with a consistently low tracking
error and click-through-rate (CTR) performance that is close
Crystal structure of \u3cem\u3ecis\u3c/em\u3e-2-(2-carboxycyclopropyl)-glycine (CCG-III) monohydrate
The title compound, C6H9NO4·H2O [systematic name: (αR,1R,2S)-rel-α-amino-2-carboxycyclopropaneacetic acid monohydrate], crystallizes with two organic molecules and two water molecules in the asymmetric unit. The space group is P21 and the organic molecules are enantiomers, thus this is an example of a `false conglomerate\u27 with two molecules of opposite handedness in the asymmetric unit (r.m.s. overlay fit = 0.056 Å for one molecule and its inverted partner). Each molecule exists as a zwitterion, with proton transfer from the amino acid carboxylic acid group to the amine group. In the crystal, the components are linked by N-H···O and O-H···O hydrogen bonds, generating (100) sheets. Conformationally restricted glutamate analogs are of interest due to their selective activation of different glutamate receptors, and the naturally occurring (+)-CCG-III is an inhibitor of glutamate uptake and the key geometrical parameters are discussed
Comparison of Wechsler Memory Scale–Fourth Edition (WMS–IV) and Third Edition (WMS–III) dimensional structures: Improved ability to evaluate auditory and visual constructs
Dimensional structures underlying the Wechsler Memory Scale–Fourth Edition (WMS–IV) and Wechsler Memory Scale–Third Edition (WMS–III) were compared to determine whether the revised measure has a more coherent and clinically relevant factor structure. Principal component analyses were conducted in normative samples reported in the respective technical manuals. Empirically supported procedures guided retention of dimensions. An invariant two-dimensional WMS–IV structure reflecting constructs of auditory learning/memory and visual attention/memory (C1 = .97; C2 = .96) is more theoretically coherent than the replicable, heterogeneous WMS–III dimension (C1 = .97). This research suggests that the WMS–IV may have greater utility in identifying lateralized memory dysfunction
The correlation space of Gaussian latent tree models and model selection without fitting
We provide a complete description of possible covariance matrices consistent
with a Gaussian latent tree model for any tree. We then present techniques for
utilising these constraints to assess whether observed data is compatible with
that Gaussian latent tree model. Our method does not require us first to fit
such a tree. We demonstrate the usefulness of the inverse-Wishart distribution
for performing preliminary assessments of tree-compatibility using
semialgebraic constraints. Using results from Drton et al. (2008) we then
provide the appropriate moments required for test statistics for assessing
adherence to these equality constraints. These are shown to be effective even
for small sample sizes and can be easily adjusted to test either the entire
model or only certain macrostructures hypothesized within the tree. We
illustrate our exploratory tetrad analysis using a linguistic application and
our confirmatory tetrad analysis using a biological application.Comment: 15 page
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