18 research outputs found
Synthetic approaches toward sesterterpenoids
Sesterterpenoids account for many bioactive natural products, often with unusual and complex structural features, which makes them attractive targets for synthetic chemists. This review surveys efforts undertaken toward the synthesis of sesterterpenoids, focusing on completed total syntheses and covering ca. 50 natural products in tota
X-ray Thread G0.13-0.11: A Pulsar Wind Nebula?
We have examined Chandra observations of the recently discovered X-ray thread
G0.13-0.11 in the Galactic center Radio Arc region. Part of the {\sl Chandra}
data was studied by Yusef-Zadeh, Law, & Wardle (2002), who reported the
detection of 6.4-keV line emission in this region. We find, however, that this
line emission is {\sl not} associated with G0.13-0.11. The X-ray spectra of
both G0.13-0.11 and the point-like source embedded are well characterized by a
power law. The 2 -- 10 keV band luminosities of these two components are (G0.13-0.11) and (point source) at the Galactic center distance of 8 kpc. The
morphological, spectral, and luminosity properties strongly indicate that
G0.13-0.11 represents the leading-edge of a pulsar wind nebula, produced by a
pulsar (point source) moving in a strong magnetic field environment. The main
body of this pulsar wind nebula is likely traced by a bow-shaped radio feature,
which is apparently bordered by G0.13-0.11 and is possibly associated with the
prominent nonthermal radio filaments of the Radio Arc. We speculate that young
pulsars may be responsible for various unique nonthermal filamentary radio and
X-ray features observed in the Galactic center region.Comment: 15 pages, accepted for publication in the Astrophysical Journa
A platform for the discovery of new macrolide antibiotics.
The chemical modification of structurally complex fermentation products, a process known as semisynthesis, has been an important tool in the discovery and manufacture of antibiotics for the treatment of various infectious diseases. However, many of the therapeutics obtained in this way are no longer effective, because bacterial resistance to these compounds has developed. Here we present a practical, fully synthetic route to macrolide antibiotics by the convergent assembly of simple chemical building blocks, enabling the synthesis of diverse structures not accessible by traditional semisynthetic approaches. More than 300 new macrolide antibiotic candidates, as well as the clinical candidate solithromycin, have been synthesized using our convergent approach. Evaluation of these compounds against a panel of pathogenic bacteria revealed that the majority of these structures had antibiotic activity, some efficacious against strains resistant to macrolides in current use. The chemistry we describe here provides a platform for the discovery of new macrolide antibiotics and may also serve as the basis for their manufacture
Chandra Observations of the Pleiades Open Cluster: X-ray Emission from Late-B to Early-F Type Binaries
We present the analysis of a 38.4 ks and a 23.6 ks observation of the core of
the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the
Chandra X-ray Observatory detected 99 X-ray sources in a 17'X17' region,
including 18 of 23 Pleiades members. Five candidate Pleiades members have also
been detected, confirming their cluster membership. Fifty-seven sources have no
optical or near-infrared counterparts to limiting magnitudes V=22.5 and J=14.5.
The unidentified X-ray sources are probably background AGN and not stars. The
Chandra field of view contains seven intermediate mass cluster members. Five of
these, HII 980 (B6 + G), HII 956 (A7 + F6), HII 1284 (A9 + K), HII 1338 (F3 +
F6), and HII 1122 (F4 + K), are detected in this study. All but HII 1284 have
high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties
comparable to non-flaring K-type stars. Since all five stars are visual or
spectroscopic binaries with X-ray properties similar to F-G stars, the
late-type binary companions are probably producing the observed coronal X-ray
emission. Strengthening this conclusion is the nondetection by Chandra of two A
stars, HII 1362 (A7, no known companion) and HII 1375 (A0 + A SB) with X-ray
luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7
stars with cooler companions. Despite the low number statistics, the Chandra
data appear to confirm the expectation that late-B and A stars are not strong
intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual
increase in coronal temperature with decreasing mass from F4 to K. M stars
appear to have somewhat cooler coronae than active K stars.Comment: 22 pages, 4 figures, 7 tables, to appear in Ap
Sloan Digital Sky Survey Imaging of Low Galactic Latitude Fields: Technical Summary and Data Release
The Sloan Digital Sky Survey (SDSS) mosaic camera and telescope have obtained
five-band optical-wavelength imaging near the Galactic plane outside of the
nominal survey boundaries. These additional data were obtained during
commissioning and subsequent testing of the SDSS observing system, and they
provide unique wide-area imaging data in regions of high obscuration and star
formation, including numerous young stellar objects, Herbig-Haro objects and
young star clusters. Because these data are outside the Survey regions in the
Galactic caps, they are not part of the standard SDSS data releases. This paper
presents imaging data for 832 square degrees of sky (including repeats), in the
star-forming regions of Orion, Taurus, and Cygnus. About 470 square degrees are
now released to the public, with the remainder to follow at the time of SDSS
Data Release 4. The public data in Orion include the star-forming region NGC
2068/NGC 2071/HH24 and a large part of Barnard's loop.Comment: 31 pages, 9 figures (3 missing to save space), accepted by AJ, in
press, see http://photo.astro.princeton.edu/oriondatarelease for data and
paper with all figure
Evolution of a Unified Strategy for Complex Sesterterpenoids: Progress toward Astellatol and the Total Synthesis of (-)-Nitidasin.
A Unified Approach to <i>trans</i>-Hydrindane Sesterterpenoids
A synthetic approach to several sesterterpenoids containing
an
isopropyl <i>trans</i>-hydrindane system is presented. Its
most remarkable feature is the stereochemical diversification of a
common precursor through the choice of different hydrogenation conditions
A Unified Approach to <i>trans</i>-Hydrindane Sesterterpenoids
A synthetic approach to several sesterterpenoids containing
an
isopropyl <i>trans</i>-hydrindane system is presented. Its
most remarkable feature is the stereochemical diversification of a
common precursor through the choice of different hydrogenation conditions
Gas-Solid Reaction of Carbon Dioxide with Alanates
An empirical study of the gasâsolid
reaction of carbon dioxide
(CO<sub>2</sub>) with alanates is presented. This investigation was
triggered by reports of hazards related to the reaction of lithium
aluminum hydride with carbon dioxide, together with the recent emergence
of alanates as potential hydrogen storage materials. Furthermore,
the reduction of CO<sub>2</sub> by hydrides is an alternative to the
conventional CO<sub>2</sub> reduction employing hydrogen gas in combination
with a catalyst. Experimentally this work was carried out by studying
the decomposition of alanate samples in a carbon dioxide atmosphere
with thermogravimetric and ex situ IR spectroscopic techniques. It
is shown that alanates react with CO<sub>2</sub> at atmospheric pressure
in two distinct temperature regions, yielding methane, hydrogen gas,
and metal oxides as the major products. The experimental findings
allowed us to postulate a mechanism for the complex metal hydride
reduction of CO<sub>2</sub>, involving alane as a highly reactive
intermediate. It is suggested that nucleophilic attack of alane hydride
ions onto the carbonyl carbon of CO<sub>2</sub> leads to sequential
formation of aluminum formate and methoxy species which get converted
into methane and metal oxides as the final products
Evolution of a Unified Strategy for Complex Sesterterpenoids: Progress toward Astellatol and the Total Synthesis of (â)âNitidasin
Astellatol and nitidasin belong to a subset of sesterterpenoids that share a sterically encumbered trans-hydrindane motif with an isopropyl substituent. In addition, these natural products feature intriguing polycyclic ring systems, posing significant challenges for chemical synthesis. Herein, the evolution of our stereoselective strategy for isopropyl trans-hydrindane sesterterpenoids is detailed. These endeavors included the synthesis of several building blocks, enabling studies toward all molecules of this terpenoid subclass, and of advanced intermediates of our initial route toward a biomimetic synthesis of astellatol. These findings provided the basis for a second-generation and a third-generation approach toward astellatol that eventually culminated in the enantioselective total synthesis of (-)-nitidasin. In particular, a series of substrate-controlled transformations to install the ten stereogenic centers of the target molecule was orchestrated and the carbocyclic backbone was forged in a convergent fashion. Furthermore, the progress toward the synthesis of astellatol is disclosed and insights into some observed yet unexpected diastereoselectivities by detailed quantum-mechanical calculations are provided