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131.
Synthesis of functionalized biodegradable polyesters 总被引:2,自引:0,他引:2
Williams CK 《Chemical Society reviews》2007,36(10):1573-1580
This tutorial review summarizes recent developments in the syntheses of functionalized aliphatic polyesters. These polymers are attracting attention as sustainable alternatives to petrochemicals and for applications in medicine. Two main syntheses are described: step polymerization using mild chemo/enzymatic catalysis and ring opening polymerization, which is usually initiated by metal complexes. The methods are compared and their utility illustrated with reference to interesting new materials. 相似文献
132.
Slattery JM Fish C Green M Hooper TN Jeffery JC Kilby RJ Lynam JM McGrady JE Pantazis DA Russell CA Willans CE 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(24):6967-6974
Abstraction of a Cl(-) ion from the P-chlorophospholes, R4C4PCl (R=Me, Et), produced the P--P bonded cations [R4C4P--P(Cl)C4R4]+, which reacted with PPh3 to afford X-ray crystallographically characterised phosphine-phosphenium cations [R4C4P(PPh3)]+ (R=Me, Et). Examination of the 31P-{1H} NMR spectrum of a solution (CH2Cl(2)) of [Et4C4P-(PPh3)]+ and PPh3 revealed broadening of the resonances due to both free and coordinated PPh3, and importantly it proved possible to measure the rate of exchange between PPh3 and [Et4C4P-(PPh3)]+ by line shape analysis (gNMR programmes). The results established second-order kinetics with DeltaS( not equal)=(-106.3+/-6.7) J mol(-1) K(-1), DeltaH( not equal)=(14.9+/-1.6) kJ mol(-1) and DeltaG( not equal) (298.15 K)=(46.6+/-2.6) kJ mol(-1), values consistent with a SN2-type pathway for the exchange process. This result contrasts with the dominant dissociative (S(N)1-type) pathway reported for the analogous exchange reactions of the [ArNCH2CH2N(Ar)P(PMe3)]+ ion, and to understand in more detail the factors controlling these two different reaction pathways, we have analysed the potential energy surfaces using density functional theory (DFT). The calculations reveal that, whilst phosphine exchange in [Et4C4P(PPh3)]+ and [ArNCH2CH2N(Ar)P(PMe3)](+) is superficially similar, the two cations differ significantly in both their electronic and steric requirements. The high electrophilicity of the phosphorus center in [Et4C4P]+, combined with strong pi-pi interactions between the ring and the incoming and outgoing phenyl groups of PPh3, favours the SN2-type over the SN1-type pathway in [Et4C4P(PPh3)]+. Effective pi-donation from the amide groups reduces the intrinsic electrophilicity of [ArNCH2CH2N(Ar)P]+, which, when combined with the steric bulk of the aryl groups, shifts the mechanism in favour of a dissociative SN1-type pathway. 相似文献
133.
The kinetics of the Al-catalyzed asymmetric Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction are presented. Structural identification of the catalytic precursor formed in situ between (S)-2,2'-dihydroxy-1,1'-binapthyl ((S)-BINOL), AlMe3, and 2-propanol was established through 1H and 27Al NMR spectroscopies, and APCIMS. All experimental evidence points toward the formation of a BINOL-chelated, pentacoordinate aluminum species in solution. Ligand-accelerated catalysis was confirmed for the phenolate/AlMe3/2-propanol system. The rate law for the catalytic reaction was determined to be nearly unimolecular dependent on aluminum, zero-order dependent on substrate, and inversely dependent on 2-propanol. At the low catalyst loading employed in the BINOL/AlMe3 system, the inherent reversibility of the MSPV reaction does not affect product yield or enantiomeric excess over time. Systematic ligand studies imply that while a tetrahedral geometry around the aluminum center may result in the most active MSPV reduction catalysts, the enantioselectivity of the reaction is enhanced when the aluminum center allows for a 2-point coordination of the substrate to achieve a pentacoordinate geometry with the fifth ligand weakly coordinated to the axial site of a pseudo square pyramid. 相似文献
134.
Isaac Abrahams Michael Lazell Majid Motevalli Charlotte K. Simon Alice C. Sullivan 《Chemistry of Heterocyclic Compounds》1999,35(8):954-964
Three structurally different metallasiloxanes were formed from reactions between in situ generated suspensions of Ph2Si(OH)2/BuLi (1∶2) in tetrahydrofuran (THF) with, metal dichlorides MgCl2·2THF, CrCl2, or CoCl2 followed by toluene/Py (Py=pyridine) work-up. The X-ray structures are reported for: [Mg{O(Ph2SiO)2}2]-μ-(LiPy)-μ-{(LiPy)3(OH)(Cl)] (1) incorporating two six-membered magnesiasiloxane rings and an MgLi3O3Cl cubane fragment, [{O(Ph2SiO)2}Co{O(Ph2SiO)3}-μ-(LiPy2)2] (2) with both six-and eight-membered cobaltasiloxane rings and [Cr{O(Ph2SiO)2}2-μ-(LiPy2)2] (3) with two six-membered chromiasiloxane rings. Structure assembly in these cases is apparently dictated by the metal dichloride. The compound [{O(Ph2SiO)2}Mg{O(Ph2SiO)3}-μ-(CoClPy)2]·Py (4) is formed from [{O(Ph2SiO)2}Mg{O(Ph2SiO)3}-μ-(LiPy2)2] and CoCl2 (1∶2). 相似文献
135.
An opinion function on a graph G = (V, E) is a function f: V → {−1, +1}. The vote of a vertex v is the sum of these function values over the closed neighborhood of v. A strict majority function on a graph G is an opinion function for which more than half of the vertices have a positive vote. The strict majority number of G is the minimum sum of the values in a strict majority function of G. We prove the conjecture of Cockayne and Mynhardt (Ars. Combin. 43 (1996), 235–245) that every tree has strict majority number at most 2. We also prove that every graph has strict majority number at most 4. Both bounds are sharp. © 1998 John Wiley & Sons, Inc. J Graph Theory 28: 49–56, 1998 相似文献
136.
Anne‐Marie Lamidey Lionel Fernon Laurent Pouysgu Charlotte Delattre Stphane Quideau Patrick Pardon 《Helvetica chimica acta》2002,85(8):2328-2334
The Echinacea‐derived immunostimulator and HIV‐1 integrase inhibitor (−)‐chicoric acid (=2,3‐bis{[3‐(3,4‐dihydroxyphenyl)‐1‐oxoprop‐2‐enyl]oxy}butanedioic acid; 1a ) was conveniently prepared via a silane‐promoted Pd‐mediated chemoselective hydrogenolysis of its perbenzylated derivative 12a , which was generated from an efficient and reliable carbodiimide‐mediated coupling reaction between the caffeic acid dibenzyl ether derivative 7 and commercially available (+)‐dibenzyl L ‐tartrate ( 9a ). The other naturally occurring dextrorotatory chicoric acid ( 1b ) can be similarly prepared. 相似文献
137.
Atiruj Theppawong Tim Van de Walle Dr. Charlotte Grootaert Prof. Dr. Kristof Van Hecke Nathalie Catry Prof. Dr. Tom Desmet Prof. Dr. John Van Camp Prof. Dr. Matthias D'hooghe 《ChemistryOpen》2019,8(2):236-247
Curcumin is known to display pronounced anticancer effects and a variety of other biological activities. However, the low bioavailability and fast metabolism of this molecule present an issue of concern with respect to its medicinal applications. To address this issue, structural modifications of the curcumin scaffold can be envisioned as a strategy to improve both the solubility and stability of this chemical entity, without compromising its biological activities. Previous work in our group targeted the synthesis of symmetrical azaheteroaromatic curcuminoids, which showed better solubility and cytotoxicity profiles compared to curcumin. In continuation of that work, we now focused on the synthesis of non-symmetrical nitrogen-containing curcuminoids bearing both a phenolic and an azaheteroaromatic moiety. In that way, we aimed to combine good solubility, antioxidant potential and cytotoxic properties into one molecule. Some derivatives were selected for further chemical modification of their rather labile β-diketone scaffold to the corresponding pyrazole moiety. In this way, thirteen new non-symmetrical aza-aromatic curcuminoids and four pyrazole-based analogues were successfully synthesized in a yield of 11–69 %. All newly synthesized analogues were evaluated for their antioxidant properties, reactive oxygen species (ROS) production, water solubility and anticancer activities. Several novel derivatives displayed good cytotoxicity profiles compared to curcumin, in combination with an improved water solubility and stability, and were thus identified as potential hit scaffolds for further optimization studies. 相似文献
138.
Objective and motivationTime-of-flight (TOF) tomography used by a clinical ultrasound tomography device can efficiently and reliably produce sound-speed images of the breast for cancer diagnosis. Accurate picking of TOFs of transmitted ultrasound signals is extremely important to ensure high-resolution and high-quality ultrasound sound-speed tomograms. Since manually picking is time-consuming for large datasets, we developed an improved automatic TOF picker based on the Akaike information criterion (AIC), as described in this paper.MethodsWe make use of an approach termed multi-model inference (model averaging), based on the calculated AIC values, to improve the accuracy of TOF picks. By using multi-model inference, our picking method incorporates all the information near the TOF of ultrasound signals. Median filtering and reciprocal pair comparison are also incorporated in our AIC picker to effectively remove outliers.ResultsWe validate our AIC picker using synthetic ultrasound waveforms, and demonstrate that our automatic TOF picker can accurately pick TOFs in the presence of random noise with absolute amplitudes up to 80% of the maximum absolute signal amplitude. We apply the new method to 1160 in vivo breast ultrasound waveforms, and compare the picked TOFs with manual picks and amplitude threshold picks. The mean value and standard deviation between our TOF picker and manual picking are 0.4 μs and 0.29 μs, while for amplitude threshold picker the values are 1.02 μs and 0.9 μs, respectively. Tomograms for in vivo breast data with high signal-to-noise ratio (SNR) (∼25 dB) and low SNR (∼18 dB) clearly demonstrate that our AIC picker is much less sensitive to the SNRs of the data, compared to the amplitude threshold picker.Discussion and conclusionsThe picking routine developed here is aimed at determining reliable quantitative values, necessary for adding diagnostic information to our clinical ultrasound tomography device - CURE. It has been successfully adopted into CURE, and allows us to generate such values reliably. We demonstrate that in vivo sound-speed tomograms with our TOF picks significantly improve the reconstruction accuracy and reduce image artifacts. 相似文献
139.
Probing hemoglobin structure by means of traveling-wave ion mobility mass spectrometry 总被引:1,自引:0,他引:1
Charlotte A. Scarff Vibhuti J. Patel Konstantinos Thalassinos James H. Scrivens 《Journal of the American Society for Mass Spectrometry》2009,20(4):625-631
Hemoglobin (Hb) is a tetrameric noncovalent complex consisting of two α- and two β-globin chains each associated with a heme group. Its exact assembly pathway is a matter of debate. Disorders of hemoglobin
are the most common inherited disorders and subsequently the molecule has been extensively studied. This work attempts to
further elucidate the structural properties of the hemoglobin tetramer and its components. Gas-phase conformations of hemoglobin
tetramers and their constituents were investigated by means of traveling-wave ion mobility mass spectrometry. Sickle (HbS)
and normal (HbA) hemoglobin molecules were analyzed to determine whether conformational differences in their quaternary structure
could be observed. Rotationally averaged collision cross sections were estimated for tetramer, dimer, apo-, and holo-monomers
with reference to a protein standard with known cross sections. Estimates of cross section obtained for the tetramers were
compared to values calculated from X-ray crystallographic structures. HbS was consistently estimated to have a larger cross
section than that of HbA, comparable with values obtained from X-ray crystallographic structures. Nontetrameric species observed
included apo- and holo- forms of α- and β-monomers and heterodimers; α- and β-monomers in both apo- and holo- forms were found to have similar cross sections, suggesting they maintain a similar fold
in the gas phase in both the presence and the absence of heme. Heme-deficient dimer, observed in the spectrum when analyzing
commercially prepared Hb, was not observed when analyzing fresh blood. This implies that holo-α-apo-β is not an essential intermediate within the Hb assembly pathway, as previously proposed. 相似文献
140.
Hind A. Maouel Vronique Alonzo Thierry Roisnel Houria Rebbah Eric Le Fur 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(7):i36-i38
The title synthesized hypophosphite has the formula V(H2PO2)3. Its structure is based on VO6 octahedra and (H2PO2)− pseudo‐tetrahedra. The asymmetric unit contains two crystallographically distinct V atoms and six independent (H2PO2)− groups. The connection of the polyhedra generates [VPO6H2]6− chains extended along a, b and c, leading to the first three‐dimensional network of an anhydrous transition metal hypophosphite. 相似文献