10,955 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
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
Asymmetric α-amidoalkylation. Synthesis of α-substituted piperidines of high enantiomeric purity
5,6-Dihydro-2H-pyran-3(4H)-on als Baustein zur Synthese pyrananellierter Heterocyclen
Während 5,6-Dihydro-2H-pyran-3(4H)-on (3) sich mit ortho-substituierten Phenylcarbonylverbindungen nur in Einzelfällen regioselektiv zu pyrananellierten Heterocyclen umsetzt - z. B. zum Pyrano[2,3-b]chinolin 7c -, gelingt das besser mit dem aus 3 hergestellten Enamin 15d, dem Silylenolether 18 und dem daraus gewonnenen Lithiumenolat 14. Diese Pyranderivate mit 2,3-oder 3,4-Doppelbindungen eignen sich zur gezielten Darstellung von 2- oder 4-substituierten 3-Pyranonen - z.B. 2, 21a, 21b, 23a-c, 26a-c, 31a-c, 32, sowie 35a-c - und von Pyrano[3,2-b]- oder -[3,4-b]chinolinen, -chinolonen, -chromonen und -thiochromonen 6a, 30a-c und 38a-d
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