2,692 research outputs found
Part I: Asymmetric Synthesis of α-Allyl-α-Aryl α-Amino Acids Part II: Asymmetric Spirocyclization of Allenyl Ketones Part III: Chemoselective Activation of C(sp3)âh Bond Over C(sp2)âh Bond With Pd(II)
Part I. The first asymmetric synthesis of a-allyl-a-aryl a-amino acids by means of a three component coupling of a-iminoesters, Grignard reagents, and cinnamyl acetate is reported. Notably, the enolate from the tandem process provides a much higher level of reactivity and selectivity than the same enolate generated via direct deprotonation, presumably due to differences in the solvation/aggregation state.
A novel method for removal of a homoallylic amine protecting group delivers the free amine congeners. The a-allyl moiety offers a means to generate further valuable a-amino acid structures. Cross-metathesis of the tandem product provided allylic diversity not afforded in the parent reaction. Cyclic a-amino acid derivatives could be accessed by ring closing metathesis presenting a viable strategy to higher ring homologues of enantioenriched a-substituted proline. The 8-member proline analog was successfully converted to the pyrrolizidine natural product backbone.
Part II. The asymmetric spirocyclization of allenyl ketones is reported. High-throughput experimentation by means of a chiral Lewis acid library enabled the determination of a suitable catalyst system. Protecting group manipulation provides an orthogonal route to enantioenriched para-quinone and ortho-quinone spirocycles. This novel technology provides access to the spirocyclic core that is prevalent in many natural products.
Part III. Palladium has been identified as a suitable catalyst for the chemoselective activation of C(sp3)-H bond over C(sp2)-H bond of toluene and tolyl analogs. This technology has been combined with the C(sp3)-H activation of acidic C-H bonds to form new C-C bonds. High-throughput experimentation was used for identifying conditions that reduced toluene loading and engendered catalyst turnover via a suitable oxidant.
The parent reaction has been extended to include the Pd catalyzed alkylation of phenylglycine azlactones with ethylbenzene, 2-ethylnaphthalene, propylbenzene and butylbenzene. Mechanistic studies were initiated to determine whether the process occurs via free radicals or via Pd mediated C(sp3)-H activation. Our studies support a Pd mediated process in which the C(sp3)-H activation of the tolyl analog is the rate determining step. This finding represents a paradigm shift in our understanding of Pd and its selectivity for arene activation vs benzylic activation
Inequalities for Light Nuclei in the Wigner Symmetry Limit
Using effective field theory we derive inequalities for light nuclei in the
Wigner symmetry limit. This is the limit where isospin and spin degrees of
freedom can be interchanged. We prove that the energy of any three-nucleon
state is bounded below by the average energy of the lowest two-nucleon and
four-nucleon states. We show how this is modified by lowest-order terms
breaking Wigner symmetry and prove general energy convexity results for SU(N).
We also discuss the inclusion of Wigner-symmetric three and four-nucleon force
terms.Comment: 10 page
The realizability problem as a special case of the infinite-dimensional truncated moment problem
The realizability problem is a well-known problem in the analysis of complex systems, which can be modeled as an infinite-dimensional moment problem. More precisely, as a truncated K-moment problem where K is the space of all possible configurations of the components of the considered system. The power of this reformulation has been already exploited by Kuna, Lebowitz, and Speer [Ann. Appl. Probab. 21 (2011), pp. 1253-1281], where necessary and sufficient conditions of Haviland type have been obtained for several instances of the realizability problem. In this article we exploit this same reformulation to apply to the realizability problem the recent advances obtained by Curto, Ghasemi, Infusino, and Kuhlmann [J. Operator Theory 90 (2023), pp. 223-261] for the truncated moment problem for linear functionals on general unital commutative algebras. This provides alternative proofs and sometimes extensions of several results of Kuna, Lebowitz, and Speer [Ann. Appl. Probab. 21 (2011), pp. 1253-1281], allowing to finally embed them in the above-mentioned unified framework for the infinite-dimensional truncated moment problem
Convolutional Goppa Codes
We define Convolutional Goppa Codes over algebraic curves and construct their
corresponding dual codes. Examples over the projective line and over elliptic
curves are described, obtaining in particular some Maximum-Distance Separable
(MDS) convolutional codes.Comment: 8 pages, submitted to IEEE Trans. Inform. Theor
Isotope Spectroscopy
The measurement of isotopic ratios provides a privileged insight both into
nucleosynthesis and into the mechanisms operating in stellar envelopes, such as
gravitational settling. In this article, we give a few examples of how isotopic
ratios can be determined from high-resolution, high-quality stellar spectra. We
consider examples of the lightest elements, H and He, for which the isotopic
shifts are very large and easily measurable, and examples of heavier elements
for which the determination of isotopic ratios is more difficult. The presence
of 6Li in the stellar atmospheres causes a subtle extra depression in the red
wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the
highest quality. But even with the best spectra, the derived Li abundance
can only be as good as the synthetic spectra used for their interpretation. It
is now known that 3D non-LTE modelling of the lithium spectral line profiles is
necessary to account properly for the intrinsic line asymmetry, which is
produced by convective flows in the atmospheres of cool stars, and can mimic
the presence of 6Li. We also discuss briefly the case of the carbon isotopic
ratio in metal-poor stars, and provide a new determination of the nickel
isotopic ratios in the solar atmosphere.Comment: AIP Thinkshop 10 "High resolution optical spectroscopy", invited
talk, AN in pres
The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars
In 1992 we began a precision radial velocity (RV) survey for planets around
solar-like stars with the Coude Echelle Spectrograph and the Long Camera (CES
LC) at the 1.4 m telescope in La Silla (Chile). We have continued the survey
with the upgraded CES Very Long Camera (VLC) and HARPS, both at the 3.6 m
telescope, until 2007. The observations for 31 stars cover a time span of up to
15 years and the RV precision permit a search for Jupiter analogues. We perform
a joint analysis for variability, trends, periodicities, and Keplerian orbits
and compute detection limits. Moreover, the HARPS RVs are analysed for
correlations with activity indicators (CaII H&K and CCF shape). We achieve a
long-term RV precision of 15 m/s (CES+LC, 1992-1998), 9 m/s (CES+VLC,
1999-2006), and 2.8 m/s (HARPS, 2003-2009, including archive data), resp. This
enables us to confirm the known planets around Iota Hor, HR 506, and HR 3259. A
steady RV trend for Eps Ind A can be explained by a planetary companion. On the
other hand, we find previously reported trends to be smaller for Beta Hyi and
not present for Alp Men. The candidate planet Eps Eri b was not detected
despite our better precision. Also the planet announced for HR 4523 cannot be
confirmed. Long-term trends in several of our stars are compatible with known
stellar companions. We provide a spectroscopic orbital solution for the binary
HR 2400 and refined solutions for the planets around HR 506 and Iota Hor. For
some other stars the variations could be attributed to stellar activity. The
occurrence of two Jupiter-mass planets in our sample is in line with the
estimate of 10% for the frequency of giant planets with periods smaller than 10
yr around solar-like stars. We have not detected a Jupiter analogue, while the
detections limits for circular orbits indicate at 5 AU a sensitivity for
minimum mass of at least 1 M_Jup (2 M_Jup) for 13% (61%) of the stars.Comment: 63 pages, 24 figures (+33 online figures), 13 Tables, accepted for
publication in A&A (2012-11-13
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