37 research outputs found
A Scalable Biomimetic Synthesis of Resveratrol Dimers and Systematic Evaluation of their Antioxidant Activities
An efficient synthetic route to the resveratrol oligomers quadrangularinâ
A and pallidol is reported. It features a scalable biomimetic oxidative dimerization that proceeds in excellent yield and with complete regioselectivity. A systematic evaluation of the natural products and their synthetic precursors as radicalâtrapping antioxidants has revealed that, contrary to popular belief, this mode of action is unlikely to account for their observed biological activity.HartnĂ€ckigkeit zahlt sich aus: Eine kurze Synthese der ResveratrolâOligomere Quadrangularinâ
A und Pallidol macht sich die StabilitĂ€t der von 2,6âDiâtertâbutylphenol abgeleiteten Radikalâ und der ChinonmethidâZwischenstufe zunutze. Untersuchungen dieser Verbindungen als antioxidative RadikalfĂ€nger ergaben, dass diese Eigenschaft höchstwahrscheinlich nicht die Ursache ihrer beobachteten biologischen AktivitĂ€t ist.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110868/1/3825_ftp.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/110868/2/ange_201409773_sm_miscellaneous_information.pd
Dust in Supernovae and Supernova Remnants II: Processing and survival
Observations have recently shown that supernovae are efficient dust factories, as predicted for a long time by theoretical models. The rapid evolution of their stellar progenitors combined with their efficiency in precipitating refractory elements from the gas phase into dust grains make supernovae the major potential suppliers of dust in the early Universe, where more conventional sources like Asymptotic Giant Branch (AGB) stars did not have time to evolve. However, dust yields inferred from observations of young supernovae or derived from models do not reflect the net amount of supernova-condensed dust able to be expelled from the remnants and reach the interstellar medium. The cavity where the dust is formed and initially resides is crossed by the high velocity reverse shock which is generated by the pressure of the circumstellar material shocked by the expanding supernova blast wave. Depending on grain composition and initial size, processing by the reverse shock may lead to substantial dust erosion and even complete destruction. The goal of this review is to present the state of the art about processing and survival of dust inside supernova remnants, in terms of theoretical modelling and comparison to observations
Intercepting Wacker Intermediates with Arenes: CâH Functionalization and Dearomatization
Unravelling Photochemical Relationships Among Natural Products from Aplysia dactylomela
[Image: see text] Aplydactone (1) is a brominated ladderane sesquiterpenoid that was isolated from the sea hare Aplysia dactylomela together with the chamigranes dactylone (2) and 10-epi-dactylone (3). Given the habitat of A. dactylomela, it seems likely that 1 is formed from 2 through a photochemical [2 + 2] cycloaddition. Here, we disclose a concise synthesis of 1, 2, and 3 that was guided by excited state theory and relied on several highly stereoselective transformations. Our experiments and calculations confirm the photochemical origin of 1 and explain why it is formed as the sole isomer. Irradiation of 3 with long wavelength UV light resulted in a [2 + 2] cycloaddition that proceeded with opposite regioselectivity. On the basis of this finding, it seems likely that the resulting regioisomer, termed â8-epi-isoaplydactoneâ, could also be found in A. dactylomela
A Photochemical Strategy for Lignin Degradation at Room Temperature
The
development of a room-temperature lignin degradation strategy
consisting of a chemoselective benzylic oxidation with a recyclable
oxidant ([4-AcNH-TEMPO]ÂBF<sub>4</sub>) and a catalytic reductive CâO
bond cleavage utilizing the photocatalyst [IrÂ(ppy)<sub>2</sub>(dtbbpy)]ÂPF<sub>6</sub> is described. This system was tested on relevant lignin model
substrates containing ÎČ-O-4 linkages to generate fragmentation
products in good to excellent yields
Unravelling Photochemical Relationships Among Natural Products from <i>Aplysia dactylomela</i>
Aplydactone
(<b>1</b>) is a brominated ladderane sesquiterpenoid that was
isolated from the sea hare <i>Aplysia dactylomela</i> together
with the chamigranes dactylone (<b>2</b>) and 10-<i>epi</i>-dactylone (<b>3</b>). Given the habitat of <i>A. dactylomela</i>, it seems likely that <b>1</b> is formed from <b>2</b> through a photochemical [2 + 2] cycloaddition. Here, we disclose
a concise synthesis of <b>1</b>, <b>2</b>, and <b>3</b> that was guided by excited state theory and relied on several highly
stereoselective transformations. Our experiments and calculations
confirm the photochemical origin of <b>1</b> and explain why
it is formed as the sole isomer. Irradiation of <b>3</b> with
long wavelength UV light resulted in a [2 + 2] cycloaddition that
proceeded with opposite regioselectivity. On the basis of this finding, it seems likely that the
resulting regioisomer, termed â8-<i>epi</i>-isoaplydactoneâ,
could also be found in <i>A. dactylomela</i>
Acanthurus nigros Gunther, a Valid Species of Surgeonfish, Distinct from the Hawaiian A. nigroris Valenciennes.
v. ill. 23 cm.QuarterlyThe Blueline Surgeonfish, Acanthurus nigroris Valenciennes, formerly considered as wide-ranging in the central and western Pacific, is restricted to the Hawaiian Islands. Acanthurus nigros GuÌ nther, type locality Vanuatu, is available for the sister species from the Pitcairn Islands west to the Great Barrier Reef and Caroline Islands. Although these two species are very similar in color, there are fin-ray and gill-raker differences, and the genetic difference (i.e., 4.12% mtDNA cytochrome b sequence divergence) alone warrants species recognition