12,273 research outputs found
Asymmetric Electrophilic alpha-Amidoalkylation, VII1): Generation, Crystal Structure, and Trapping Reactions of a Chiral 6,7-Dimethoxy-1,2,3,4-tetrahydroisoquinoline Derived N-Acyliminium Ion
The camphanic acid amide 4 has efficiently been oxidized with triphenylcarbenium tetrafluoroborate (3) to yield the chiral N-acyliminium ion 1. Trapping reactions of 1 with the silyl nucleophiles 7a-c and 10a-f proceeded with stereoselective bond formation, affording the diastereomers (R)-8/(S)-9a-c and (R)-11/(S)-12a-f, respectively, with diastereoselectivities of up to 93.9/6.1.
The amido ketones (R)-8/(S)-9a-c were employed in the synthesis of the secondary amines (R)-16a-c, (S)-16a and for the preparation of (-)-homolaudanosine (R)-18.
By X-ray crystallography the conformation of 1 in the crystal lattice was established and the preferred conformation of 1 in solution was elucidated by NOE experiments. Finally, the addition reaction of 7a to the iminium ion 21 derived from menthyl carbamate 20 was investigated, which reaction, however, proceeded only with insignificant asymmetric induction
Comparison of two notions of subharmonicity on non-archimedean curves
We show that a continuous function on the analytification of a smooth proper
algebraic curve over a non-archimedean field is subharmonic in the sense of
Thuillier if and only if it is psh, i.e. subharmonic in the sense of
Chambert-Loir and Ducros. This equivalence implies that the property psh for
continuous functions is stable under pullback with respect to morphisms of
curves. Furthermore, we prove an analogue of the monotone regularization
theorem on the analytification of the projective line and Mumford curves using
this equivalence.Comment: v3: To appear in Mathematische Zeitschrift. 32 page
Topology-guided sampling of nonhomogeneous random processes
Topological measurements are increasingly being accepted as an important tool
for quantifying complex structures. In many applications, these structures can
be expressed as nodal domains of real-valued functions and are obtained only
through experimental observation or numerical simulations. In both cases, the
data on which the topological measurements are based are derived via some form
of finite sampling or discretization. In this paper, we present a probabilistic
approach to quantifying the number of components of generalized nodal domains
of nonhomogeneous random processes on the real line via finite discretizations,
that is, we consider excursion sets of a random process relative to a
nonconstant deterministic threshold function. Our results furnish explicit
probabilistic a priori bounds for the suitability of certain discretization
sizes and also provide information for the choice of location of the sampling
points in order to minimize the error probability. We illustrate our results
for a variety of random processes, demonstrate how they can be used to sample
the classical nodal domains of deterministic functions perturbed by additive
noise and discuss their relation to the density of zeros.Comment: Published in at http://dx.doi.org/10.1214/09-AAP652 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Computer-assisted proof of heteroclinic connections in the one-dimensional Ohta-Kawasaki model
We present a computer-assisted proof of heteroclinic connections in the
one-dimensional Ohta-Kawasaki model of diblock copolymers. The model is a
fourth-order parabolic partial differential equation subject to homogeneous
Neumann boundary conditions, which contains as a special case the celebrated
Cahn-Hilliard equation. While the attractor structure of the latter model is
completely understood for one-dimensional domains, the diblock copolymer
extension exhibits considerably richer long-term dynamical behavior, which
includes a high level of multistability. In this paper, we establish the
existence of certain heteroclinic connections between the homogeneous
equilibrium state, which represents a perfect copolymer mixture, and all local
and global energy minimizers. In this way, we show that not every solution
originating near the homogeneous state will converge to the global energy
minimizer, but rather is trapped by a stable state with higher energy. This
phenomenon can not be observed in the one-dimensional Cahn-Hillard equation,
where generic solutions are attracted by a global minimizer
Analytical description of interference between two misaligned and mismatched complete Gaussian beams
A typical application for laser interferometers is a precision measurement of
length changes that result in interferometric phase shifts. Such phase changes
are typically predicted numerically, due to the com- plexity of the overlap
integral that needs to be solved. In this paper we will derive analytical
representations of the interferometric phase and contrast (aka. fringe
visibility) for two beam interferometers, both homodyne and heterodyne. The
fundamental Gaussian beams can be arbitrarily misaligned and mismatched to each
other. A limitation of the analytical result is that both beams must be
detected completely, which can experimentally be realized by a sufficiently
large single-element photodetector.Comment: 8 pages, 2 figure
Probabilistic validation of homology computations for nodal domains
Homology has long been accepted as an important computable tool for
quantifying complex structures. In many applications, these structures arise as
nodal domains of real-valued functions and are therefore amenable only to a
numerical study based on suitable discretizations. Such an approach immediately
raises the question of how accurate the resulting homology computations are. In
this paper, we present a probabilistic approach to quantifying the validity of
homology computations for nodal domains of random fields in one and two space
dimensions, which furnishes explicit probabilistic a priori bounds for the
suitability of certain discretization sizes. We illustrate our results for the
special cases of random periodic fields and random trigonometric polynomials.Comment: Published at http://dx.doi.org/10.1214/105051607000000050 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
Isomerization of N-acyl-1,2,5,6-tetrahydropyridines to N-acyl-enamines by palladium on carbon
Chromosome Centromeres: Structural and Analytical Investigations with High Resolution Scanning Electron Microscopy in Combination with Focused Ion Beam Milling
Whole mount mitotic metaphase chromosomes of different plants and animals were investigated with high resolution field emission scanning electron microscopy (FESEM) to study the ultrastructural organization of centromeres, including metacentric, acrocentric, telocentric, and holocentric chromosome variants. It could be shown that, in general, primary constrictions have distinctive ultrastructural features characterized by parallel matrix fibrils and fewer smaller chromomeres. Exposure of these structures depends on cell cycle synchronization prior to chromosome isolation, chromosome size, and chromosome isolation technique. Chromosomes without primary constrictions, small chromosomes, and holocentric chromosomes do not exhibit distinct ultrastructural elements that could be directly correlated to centromere function. Putative spindle structures, although rarely observed, spread over the primary constriction to the bordering pericentric regions. Analytical FESEM techniques, including specific DNA staining with Pt blue, staining of protein as a substance class with silver-colloid, and artificial loosening of fixed chromosomes with proteinase K, were applied, showing that centromere variants and ultrastructural elements in the centromere differ in DNA and protein distribution. Immunogold localization allowed high-resolution comparison between chromosomes with different centromere orientations of the distribution of centromere-related histone variants, phosphorylated histone H3 (ser10), and CENH3. A novel application of FESEM combined with focused ion beam milling (FIB) provided new insights into the spatial distribution of these histone variants in barley chromosomes. Copyright (C) 2009 S. Karger AG, Base
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