1,425 research outputs found
Total Synthesis of Mycinolide IV and Path‐Scouting for Aldgamycin N
Proof‐of‐concept is provided that a large estate of 16‐membered macrolide antibiotics can be reached by a “unified” approach. The key building block was formed on scale by an asymmetric vinylogous Mukaiyama aldol reaction; its alkene terminus was then converted either into the corresponding methyl ketone by Wacker oxidation or into a chain‐extended aldehyde by catalyst‐controlled branch‐selective asymmetric hydroformylation. These transformations ultimately opened access to two structurally distinct series of macrolide targets. Notable late‐stage maneuvers comprise a rare example of a ruthenium‐catalyzed redox isomerization of an 1,3‐enyne‐5‐ol into a 1,3‐diene‐5‐one derivative, as well as the elaboration of a tertiary propargylic alcohol into an acyloin by trans‐hydrostannation/Chan‐Lam‐type coupling. Moreover, this case study illustrates the underutilized possibility of forging complex macrolactone rings by transesterification under essentially neutral conditions
Syzygies of torsion bundles and the geometry of the level l modular variety over M_g
We formulate, and in some cases prove, three statements concerning the purity
or, more generally the naturality of the resolution of various rings one can
attach to a generic curve of genus g and a torsion point of order l in its
Jacobian. These statements can be viewed an analogues of Green's Conjecture and
we verify them computationally for bounded genus. We then compute the
cohomology class of the corresponding non-vanishing locus in the moduli space
R_{g,l} of twisted level l curves of genus g and use this to derive results
about the birational geometry of R_{g, l}. For instance, we prove that R_{g,3}
is a variety of general type when g>11 and the Kodaira dimension of R_{11,3} is
greater than or equal to 19. In the last section we explain probabilistically
the unexpected failure of the Prym-Green conjecture in genus 8 and level 2.Comment: 35 pages, appeared in Invent Math. We correct an inaccuracy in the
statement of Prop 2.
Imaging in Inflammatory Bowel Diseases - Magnetic Resonance Imaging
Diagnostic imaging techniques play an important role in the diagnosis and management of patients with inflammatory bowel diseases (IBDs). The approach should be guided by considerations of diagnostic accuracy, concerns about patient exposure to ionizing radiation, local expertise and tolerance of the endoscopic and/or imaging technique. In regard to the clinical diagnostic value (sensitivity, specificity and accuracy), no significant differences exist between CT and MRI for the evaluation of the extent of inflammation, stricturing, penetrating disease or extraluminal complications such as abscesses. Due to the absence of radiation exposure, MRI of the intestine is recommended as the first-line imaging modality in patients with suspected or established IBD. The focus of this review is the latest developments in MRI techniques to detect IBDs. Specifically, the use of new indices for the grading of inflammation or assessing bowel damage as well as innovative experimental approaches such as diffusion-weighted imaging or magnetization-transfer MRI to evaluate and quantify the degree of intestinal inflammation and fibrosis in stricturing Crohn's disease are discussed
Radiological Imaging of the Small Bowel
Recently introduced endoscopy-based imaging methods such as double-balloon endoscopy or wireless capsule endoscopy can visualize the complete small bowel. These approaches are quite invasive diagnostic methods. Therefore, radiological small bowel imaging is also still considered as the primary imaging approach to diagnose pathological changes of the small bowel. In this review article the most important small bowel imaging modalities such as conventional fluoroscopy, computed tomography, magnetic resonance imaging and ultrasound are discussed. Additionally the most important diseases, which can affect the small bowel, are evaluated and the optimal imaging modalities are pointed out, respectively
Reorientation of Spin Density Waves in Cr(001) Films induced by Fe(001) Cap Layers
Proximity effects of 20 \AA thin Fe layers on the spin density waves (SDWs)
in epitaxial Cr(001) films are revealed by neutron scattering. Unlike in bulk
Cr we observe a SDW with its wave vector Q pointing along only one {100}
direction which depends dramatically on the film thickness t_{Cr}. For t_{Cr} <
250 \AA the SDW propagates out-of-plane with the spins in the film plane. For
t_{Cr} > 1000 \AA the SDW propagates in the film plane with the spins
out-of-plane perpendicular to the in-plane Fe moments. This reorientation
transition is explained by frustration effects in the antiferromagnetic
interaction between Fe and Cr across the Fe/Cr interface due to steps at the
interface.Comment: 4 pages (RevTeX), 3 figures (EPS
A VLA Search for Water Masers in Six HII Regions: Tracers of Triggered Low-Mass Star Formation
We present a search for water maser emission at 22 GHz associated with young
low-mass protostars in six HII regions -- M16, M20, NGC 2264, NGC 6357, S125,
and S140. The survey was conducted with the NRAO Very Large Array from 2000 to
2002. For several of these HII regions, ours are the first high-resolution
observations of water masers. We detected 16 water masers: eight in M16, four
in M20, three in S140, and one in NGC 2264. All but one of these were
previously undetected. No maser emission was detected from NGC 6357 or S125.
There are two principle results to our study. (1) The distribution of water
masers in M16 and M20 does not appear to be random but instead is concentrated
in a layer of compressed gas within a few tenths of a parsec of the ionization
front. (2) Significantly fewer masers are seen in the observed fields than
expected based on other indications of ongoing star formation, indicating that
the maser-exciting lifetime of protostars is much shorter in HII regions than
in regions of isolated star formation. Both of these results confirm
predictions of a scenario in which star formation is first triggered by shocks
driven in advance of ionization fronts, and then truncated approximately 10^5
years later when the region is overrun by the ionization front.Comment: 30 pages, 20 figures, 3 tables. Accepted for publication by ApJ. Full
resolution figures and PS and PDF versions with full-res figures available at
http://eagle.la.asu.edu/healy/preprints/hhc0
Prerequisites for setting up management system in municipal retail trade
The relevance of the problem is determined by the number of complex problems that exist in present Samara municipal retail trade system, which is manifested in the lack of regulation, a glut of sales area, poorly developed infrastructure. The purpose of this article is to form a quantitative base for determining the municipal needs in the floor space. Leading method to study this problem is the method of empirical observations and mathematical modeling. The study results: the article contains the analysis of sales area availability on the example in the urban district and presents a quantitative method to determine predictive values for retail trade turnover. Practical significance: the paper is aimed to develop managerial decisions in the sphere of municipal trade regulation. © 2016 Suraeva et al
Three embeddings of the Klein simple group into the Cremona group of rank three
We study the action of the Klein simple group G consisting of 168 elements on
two rational threefolds: the three-dimensional projective space and a smooth
Fano threefold X of anticanonical degree 22 and index 1. We show that the
Cremona group of rank three has at least three non-conjugate subgroups
isomorphic to G. As a by-product, we prove that X admits a Kahler-Einstein
metric, and we construct a smooth polarized K3 surface of degree 22 with an
action of the group G.Comment: 43 page
Revisiting the analytical solution approach to mixing-limited equilibrium multicomponent reactive transport using mixing ratios: identification of basis, fixing an error, and dealing with multiple minerals
Multicomponent reactive transport involves the solution of a system of nonlinear coupled partial differential equations. A number of methods have been developed to simplify the problem. In the case where all reactions are in instantaneous equilibrium and the mineral assemblage is constant in both space and time, de Simoni et al. (2007) provide an analytical solution that separates transport of aqueous components and minerals using scalar dissipation ofPeer ReviewedPostprint (published version
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