bluuebunny/crossref_metadata_2025_split huggingface dataset:
Noncovalent Assembly of Functional Groups on Calix[4]Arene Molecular Boxes
Authors: Timmerman Peter , Vreekamp Remko H. , Hulst Ron , Verboom Willem , Reinhoudt David N. , Rissanen Kari , Udachin Konstantin A. , Ripmeester John 
Published: July 2007

AbstractCalix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well‐defined box‐like assemblies in the presence of two equivalents of 5,5‐diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10−4M. This paper reports the first X‐ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen‐bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new 1,3‐bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 1H NMR spectroscopic studies on the assembly behavior of these functionalized calix[4]arenes shows that 1) polar substituents (e.g. nitro, cyano) hardly affect the stability of the hydrogen‐bonded assembly; 2) hydrogen bond donating or accepting groups, like amino and acetamido, can disturb assembly of the boxes under certain conditions by destabilizing the calix[4]arene pinched cone conformation as a result of intramolecular hydrogen bond formation; and 3) sterically bulky groups (e.g. tBu) can significantly inhibit the formation of the hydrogen‐bonded assembly, but this effect very much depends on the exact positions of the groups.
Opposite Stereochemical Effects Exerted by CeCl3 and TiCl4 on the Lewis Acid Mediated Reduction of α‐Alkyl‐β‐ketophosphine Oxides with Metallic Hydrides: A Highly Stereoselective Protocol for the Synthesis of syn and anti α‐alkyl‐β‐Hydroxyphosphine Oxides
Authors: Bartoli Giuseppe , Bosco Marcella , Sambri Letizia , Dalpozzo Renato , Marcantoni Enrico 
Published: July 2007

AbstractA general, highly efficient methodology for obtaining both syn and anti β‐hydroxyphosphine oxides by reduction of the corresponding β‐ketophosphine oxides is described. The nature of the Lewis acid was found to be pivotal in determining the outcome of these reactions. Strongly chelating TiCl4 led to the anti isomer in high diastereoisomeric excess in noncoordinating solvents (CH2Cl2) at ‐78°C with BH3/py as reducing agent, while nonchelating CeCl3 gave a high excess of the syn isomer in coordinating solvents (THF) at the same temperature with LiBH4 as reducing agent. In the latter case, CeCl3 is essential in achieving high yields and stereoselectivity, since it allows the reaction to be performed at low temperatures. Otherwise, higher temperatures (0°C) are required, which lower both yields and selectivities. Moreover, each step of the protocol for the synthesis of stereodefined disubstituted olefins from alkylphosphine oxides (Warren's modification of the Horner procedure) has been optimized, and the optimized procedure has been applied to the synthesis of muscalure, the pheromone of the domestic fly.
Structure, Stability, Dynamics, High‐Field Relaxivity and Ternary‐Complex Formation of a New Tris(aquo) Gadolinium Complex
Authors: Nonat Aline , Fries Pascal H. , Pécaut Jacques , Mazzanti Marinella 
Published: July 2007

AbstractThe tripodal hexadentate picolinate ligand dpaa3− (H3dpaa=N,N′‐bis[(6‐carboxypyridin‐2‐yl)methyl]glycine) has been synthesised. It can form 1:1 and 1:2 lanthanide/ligand complexes. The crystal structure of the bis(aquo) lutetium complex [Lu(dpaa)(H2O)2] has been determined by X‐ray diffraction studies. The number of water molecules was determined by luminescence lifetime studies of the terbium and europium complexes. The tris(aquo) terbium complex shows a fairly high luminescence quantum yield (22 %). The [Gd(dpaa)(H2O)3] complex displays a high water solubility and an increased stability (pGd=12.3) with respect to the analogous bis(aquo) complex [Gd(tpaa)(H2O)2] (pGd=11.2). Potentiometric and relaxometric studies show the formation of a soluble GdIII hydroxo complex at high pH values. A unique aquohydroxo gadolinium complex has been isolated and its crystal structure determined. This complex crystallises as a 1D polymeric chain consisting of square‐shaped tetrameric units. In heavy water, the [Gd(dpaa)(D2O)3] complex shows a quite high HOD proton relaxivity at high field (11.93 s−1 mM−1 at 200 MHz and 298 K) because of the three inner‐sphere water molecules. The formation of ternary complexes with physiological anions has been monitored by relaxometric studies, which indicate that even under conditions favourable to the formation of adducts with oxyanions, the mean relaxivity remains higher than those of most of the currently used commercial contrast agents except for the citrate. However, the measured relaxivity (r̄1=7.9 s−1 mM−1) in a solution containing equimolar concentrations of [Gd(dpaa)(D2O)3] and citrate is still high. The interaction with albumin has been investigated by relaxometric and luminescence studies. Finally, a new versatile method to unravel the geometric and dynamic molecular factors that explain the high‐field relaxivities has been developed. This approach uses a small, uncharged non‐coordinating probe solute, the outer‐sphere relaxivity of which mimics that of the water proton. Only a routine NMR spectrometer and simple mathematical analysis are required.
The Nature of the AgI⋅⋅⋅AgIInteraction in Different Ag(NH3)2Dimers Embedded in Supramolecular Solids
Authors: Zheng Shao‐Liang , Volkov Anatoliy , Nygren Cara L. , Coppens Philip 
Published: July 2007

AbstractAn isolated silver(I) ammonia monomer, a dimer, and a novel dimer containing an intercalated water molecule have been embedded as guests in supramolecular frameworks, [Ag(NH3)2][(H2thpe)(H3thpe)]⋅MeCN (1), [{Ag(NH3)2}2][(H2thpe)2]⋅4.25 H2O (2), and [{Ag(NH3)2}‐H2O‐{Ag(NH3)2}][(H2thpe)2]⋅benzene (3) (H3THPE=tris(hydroxyphenyl)ethane). The [{Ag(NH3)2}2]2+dimer is not stable as an isolated entity, but is stabilized by hydrogen bonding in the supramolecular framework. The water‐intercalated silver(I) ammonia dimer, which constitutes a novel species, is also subject by hydrogen bonding in concentrated solutions. The destabilization energy of the dimer relative to isolated monomers is calculated to be ≈300 kJ mol−1by both perturbation methods and DFT theory. For the water‐intercalated dimer it is calculated to be ≈200 kJ mol−1according to the BSSE‐corrected MP2 calculation. The different aggregate states show a dramatic variation of absorption and emission properties, in accordance with the concentration dependent red‐shift observed in solutions. Natural‐bond‐orbital analysis shows that the disilver‐ammonium‐aquo “sandwich” cation in3is stabilized by interaction between the π lone pair orbital on the oxygen atom of the water molecule and AgIN σ antibonding molecular orbital.
Iterative Tandem Catalysis of Secondary Diols and Diesters to Chiral Polyesters
Authors: van As Bart A. C. , van Buijtenen Jeroen , Mes Tristan , Palmans Anja R. A. , Meijer E. W. 
Published: July 2007

AbstractThe well‐known dynamic kinetic resolution of secondary alcohols and esters was extended to secondary diols and diesters to afford chiral polyesters. This process is an example of iterative tandem catalysis (ITC), a polymerization method where the concurrent action of two fundamentally different catalysts is required to achieve chain growth. In order to procure chiral polyesters of high enantiomeric excess value (ee) and good molecular weight, the catalysts employed need to be complementary and compatible during the polymerization reaction. We here show that Shvo's catalyst and Novozym 435 fulfil these requirements. The optimal polymerization conditions of 1,1′‐(1,3‐phenylene) diethanol (1,3‐diol) and diisopropyl adipate required 2 mol % Shvo's catalyst and 12 mg Novozym 435 per mmol alcohol group in the presence of 0.5 M 2,4‐dimethyl‐3‐pentanol as the hydrogen donor. With these conditions, chiral polyesters were obtained with peak molecular weights up to 15 kDa, an ee value up to 99 % and with 1–3 % ketone end groups. Also with the structural isomer, 1,4‐diol, a chiral polyester was obtained, albeit with lower molecular weight (8.3 kDa) and slightly lower ee (94 %). Aliphatic secondary diols also resulted in enantio‐enriched polymers but at most an ee of 46 % was obtained with molecular weights in the range of 3.3–3.7 kDa. This low ee originates from the intrinsic low enantioselectivity of Novozym 435 for this type of secondary aliphatic diols. The results presented here show that ITC can be applied to procure chiral polyesters with good molecular weight and high ee from optically inactive AA–BB type monomers.
Neonatal maternal separation alters adult eyeblink conditioning and glucocorticoid receptor expression in the interpositus nucleus of the cerebellum
Authors: Wilber Aaron A. , Southwood Christopher J. , Sokoloff Greta , Steinmetz Joseph E. , Wellman Cara L. 
Published: July 2007

AbstractNeonatal maternal separation alters learning and memory. Glucocorticoids also modulate adult learning and memory, and neonatal maternal separation alters forebrain glucocorticoid receptor (GR) concentrations. We used eyeblink classical conditioning to assess the effect of neonatal maternal separation on associative learning. We assessed delay eyeblink conditioning, GR expression, and total neuron number in the interpositus nucleus, a critical site of plasticity in eyeblink conditioning, in adult rats that had undergone either standard animal facilities rearing, handling for 15 min, or maternal separation for either 15 or 60 min per day on postnatal days 2–14. At 2–3 months of age, delay eyeblink classical conditioning was assessed. Brains were processed for GR immunohistochemistry, and GR expression in the interpositus nucleus was assessed using a computer‐based densitometry system. Neuron counts and nuclear volumes were obtained from an alternate series of thionin‐stained sections. Maternal separation significantly impaired eyeblink conditioning in male but not female rats. Handling and maternal separation did not significantly affect interpositus neuron number and volume. However, prolonged maternal separation significantly increased GR expression in the posterior interpositus in males, and increases were correlated with eyeblink conditioning. In female rats, maternal separation and handling did not significantly alter interpositus neuron number, volume, or GR protein expression, and GR expression did not correlate with eyeblink conditioning. Thus, neonatal maternal separation produces adult deficits in eyeblink conditioning and alterations in GR expression in its neural substrate in a sex‐dependent manner. © 2007 Wiley Periodicals, Inc. Develop Neurobiol, 2007.
Ion‐Selective Electrode Potentiometry in Environmental Analysis
Authors: De Marco Roland , Clarke Graeme , Pejcic Bobby 
Published: July 2007

AbstractThis review will illustrate how it is possible to develop ion‐selective electrode (ISE) methodologies that meet the stringent requirements (i.e., high selectivities and very low detection limits) for the analysis of important analytes in the environment, and will present a variety of examples on the application of ISEs in environmental analysis. Despite the experimental biases that have limited the analytical performance of ISEs through apparently high detection limits and modest selectivities, there has been a plethora of research in the application of ISEs in the monitoring of environmentally important trace metals and anions in natural waters and soils. Most popular has been the analysis of free metals in natural waters, as this parameter is known to be a master variable in the uptake and toxicology of trace metals on aquatic biota reflecting the bioavailability of trace metals in the environment. Furthermore, as copper is a major trace metal in coastal waters due to its extensive use in antifouling paints on sea vessels and structures, there are many reports in the literature on the use of the copper ISE in assays of either free copper or the copper complexing capacity of natural waters and soil peats. Moreover, there have been a variety of studies showing a strong correlation between free copper levels and the toxicity of copper on a variety of marine and fresh water organisms. Nevertheless, there are numerous reports in the literature that have used ISEs to monitor important anions such as fluoride, phosphate, sulfate, nitrate, nitrite, chloride, cyanide, etc., as well as other important cations such as ammonium and chromium(VI) in waste and natural waters. In conclusion, this review will present new and interesting perspectives on the application of ISEs in environmental analysis using approaches such as real‐time remote monitoring of water quality, shipboard monitoring of environmentally important analytes using flow analysis instrumentation, the use of robust all‐solid‐state ISEs in submersible instruments for long‐term deployment in the field, and innovative analytical approaches such as backside calibration and switchtrodes that avoid standard addition analysis and the concomitant perturbation in analyte speciation in natural samples.
Fenton oxidation of TCE vapors in a foam reactor
Authors: Kan Eunsung , Kim Seongyup , Deshusses Marc A. 
Published: July 2007

AbstractOxidation of dilute TCE vapors in a foam reactor using Fenton's reagent was investigated. The new process relies on rapid mass transfer from the gas undergoing treatment to a fine aqueous foam and oxidation by Fenton's reagents. Laboratory investigations demonstrated that TCE elimination capacity was as high as 56–61 g TCE m−3reactorh−1 with a removal efficiency of 62% and TCE mineralization of 83% at gas retention time of 30 s and TCE inlet concentration of 0.6 g m−3. Compared to oxidation in wet scrubbers packed with commercially available plastic packings and using the same composition of Fenton's reagents, the foam reactor configuration provided a higher rate absorption and greater elimination capacity of TCE vapors. The treatment performance far exceeded those of current bioreactors and was comparable to those of advanced oxidation techniques such as TiO2/UV and ozone/UV, suggesting that Fenton oxidation in foam reactors may be a promising technique for treating TCE and other recalcitrant vapors. © 2007 American Institute of Chemical Engineers Environ Prog, 2007
Astrocyte glycogen and brain energy metabolism
Authors: Brown Angus M. , Ransom Bruce R. 
Published: July 2007

AbstractThe brain contains glycogen but at low concentration compared with liver and muscle. In the adult brain, glycogen is found predominately in astrocytes. Astrocyte glycogen content is modulated by a number of factors including some neurotransmitters and ambient glucose concentration. Compelling evidence indicates that astrocyte glycogen breaks down during hypoglycemia to lactate that is transferred to adjacent neurons or axons where it is used aerobically as fuel. In the case of CNS white matter, this source of energy can extend axon function for 20 min or longer. Likewise, during periods of intense neural activity when energy demand exceeds glucose supply, astrocyte glycogen is degraded to lactate, a portion of which is transferred to axons for fuel. Astrocyte glycogen, therefore, offers some protection against hypoglycemic neural injury and ensures that neurons and axons can maintain their function during very intense periods of activation. These emerging principles about the roles of astrocyte glycogen contradict the long held belief that this metabolic pool has little or no functional significance. © 2007 Wiley‐Liss, Inc.
On the feasibility of the microscale approach for a multistep biotransformation: sitosterol side chain cleavage
Authors: Marques Marco PC , de Carvalho Carla CCR , Claudino Mauro JC , Cabral Joaquim MS , Fernandes Pedro 
Published: July 2007

AbstractBACKGROUND: The millilitre/microlitre‐scale approach is widely used for screening purposes and one‐pot biotransformations, but it has seldom been applied to whole cell multistep biotransformations and two‐liquid biphasic processes. The present study aims to contribute to filling this gap by using 24‐well microtitre plates as reactors for the side chain cleavage of sitosterol with resting cells of Mycobacterium sp. NRRL B‐3805 and comparing results with other set‐ups.RESULTS: High catalytic activity in microtitre plates and aqueous medium was observed at 250 rpm shaking frequency and 25 mm shaking diameter with 80% headspace and a mixture of Mg2+, Zn2+, Mn2+ and Co2+ salts. Data obtained with microtitre plates were reproduced in other set‐ups when aqueous medium was used, whereas higher biotransformation rates were recorded in stirred reactors when biphasic systems were employed.CONCLUSION: Results suggest that microtitre plates may be used for the characterisation of complex bioconversion systems to establish representative screening systems, particularly if based in aqueous phase systems. Probing experiments also demonstrated the feasibility of using microlitre/millilitre shaken bioreactors to evaluate organic/aqueous two‐liquid bioconversion systems, although the enhanced mass transfer patterns resulting from the biphasic system hampered the translation of the data to the more common stirred reactor configuration. Copyright © 2007 Society of Chemical Industry
Inter‐ and intramolecular hetero Diels‐Alder reactions, 52. Effect of high pressure on the kinetics and selectivity of the intramolecular hetero Diels‐Alder reaction of an 1‐oxa‐1,3‐butadiene
Authors: Buback Michael , Abeln Johannes , Hübsch Thomas , Ott Christian , Tietze Lutz F. 
Published: July 2007

AbstractThe intramolecular hetero Diels‐Alder reaction of the benzylidene‐isoxazolone 3 giving the adducts 4 and 5 is studied in dichlormethane under high pressure up to 5 kbar. The kinetics is measured by on‐line FT‐IR spectroscopy up to 3 kbar. The cycloaddition shows a clear pressure‐induced increase in diastereoselectivity. For the difference in activation volume ΔΔV≠ of the reactions affording the two diastereomers, a value of −(1.6 ± 0.2) cm3 · mol−1 is obtained. The activation volume ΔV is determined to be ‐(19.4 ± 0.5) cm3 · mol−1 at 343 K. The activation enthalpy and entropy, as derived from the overall rate coefficient k measured at 1000 bar, are (62.9 ± 1.7) kJ · mol−1 (at 343 K) and ‐(135 ± 11) J · mol−1 · K−1.
Photochemistry of δ‐hydroxybutyl α,β‐unsaturated γ,δ‐epoxy nitriles. Formation of spiro ketals
Authors: Ishii Keitaro , Kotera Masashi , Nakano Takao , Zenko Tatsuya , Sakamoto Masanori , Iida Ikuo , Nishio Takehiko 
Published: July 2007

AbstractPhotocyclization reactions of δ‐hydroxybutyl α,β‐unsaturated γ,δ‐epoxy nitriles 4a, b at 254 nm lead to spiro ketals 5a, b and 6a, b arising from the carbonyl ylide intermediate A. The spiro ketal 6b containing an endocyclic double bond is transformed to spiro ether 16 by an acid‐catalyzed 1,3‐sigmatropic rearrangement. The ratio of spiro ketals 5 and 6 has been shown to depend on solvent polarity and acidity of additives. Application to the synthesis of pheromones is presented. X‐ray crystal structure analyses are performed on dispiro compound 18 and spiro ester 25.
Bausteine von Oligosacchariden, CX. Synthese von potentiellen Inhibitoren für die N‐Acetylglucosaminyltransferase I
Authors: Paulsen Hans , Springer Matthias , Reck Folkert , Brockhausen Inka , Schachter Harry 
Published: July 2007

Building Units of Oligosaccharides, CX. — Synthesis of Potential Inhibitors of N‐Acetylglucosaminyltransferase IModified derivatives of the trisaccharide octyl α‐D‐Man‐(1→3)‐[α‐D‐Man(1→6)]‐β‐D‐Man were synthesized by attaching reactive groups via a pentyl spacer to the 4′‐OH and 6′‐OH group. Glycosylation steps were carried out by using the trichloroacetimidate method with suitable building blocks. The compounds substituted at 6′‐OH are of special interest. The epoxide 27 and the diazirine 25 are reversible inhibitors of N‐acetylglucosaminyltransferase I (GlcNAc‐T I). Product 26 with a spacer‐linked iodoacetamide is an irreversible inhibitor of GlcNAc‐T I.
Ungesättigte 6‐Alkylpyrone aus dem Schleimpilz Ceratiomyxa fruticulosa (Myxomycetes)
Authors: Velten Robert , Josten Ingrid , Steglich Wolfgang 
Published: July 2007

Unsaturated 6‐Alkylpyrones from the Slime Mould Ceratiomyxa fruticulosa (Myxomycetes)From plasmodia of the common slime mould Ceratiomyxa fruticulosa five 6‐alkyl‐4‐methoxypyran‐2‐ones with varying degrees of unsaturation in the side chain have been isolated. Their structures have been determined by spectroscopic and chemical investigations.
Teil‐ und perfluorierte Thioketone und Thioaldehyde Chemische Speicherung, in‐situ‐Freisetzung und ungewöhnliche 1,3‐dipolarophile Reaktivität gegenüber Bis(trimethylstannyl)diazomethan (BTSD) und C,N‐Bis(triisopropylsilyl)nitrilimin (NI)
Authors: Hasserodt Jens , Pritzkow Hans , Sundermeyer Wolfgang 
Published: July 2007

Partially and Perfluorinated Thioketones and Thioaldehydes: Chemical Storage, In Situ Generation and Surprising Reactivity towards Bis(trimethylstannyl)‐diazomethane (BTSD) and C,N‐Bis(triisopropylsilyl)nitrilimine (NI)The hetero‐Diels‐Alder anthracene adducts 1a‐d are synthesized in a one‐pot reaction from the corresponding partially and perfluorinated carbonyl compounds in good yields. By their thermolysis in solution in presence of BTSD or NI the 1,3‐dipolar 1:1 cycloadducts 12a, d, 14a, b, d are obtained without any detectable by‐products. The method can be of wide applicability predominantly for any aliphatic CS‐unsaturated species, which are not accessible in substance due to tendency towards polymerisation, but can be stabilized as anthracene adducts. Other 1,3‐dipolar cycloaddition products of BTSD and NI are obtained from reaction with iso‐lable hexafluorothioacetone (6f) or bis(trifluoromethyl)sulfine (7f). Comparative studies were carried out with hexafluoroacetone (8), 12a, d, e, 13 are consecutive products from a 1,2‐metallotropic migration of the primary addition compounds. 14a, b, d, 15, 16 are useful thiadiazolines with respect to the synthesis of active substances because of their seldom found peripheral functionality. Desilylation was achieved by fluoride support. An X‐ray structure determination of 12a and 14d was carried out. In addition trifluoromethylsulfine (7c) was trapped at low temperatures. It dimerizes at ambient temperature to unsymmetrical 1,2‐dithietane 11.
Synthese von Naturstoffen mit Alkyl‐methoxy‐1,4‐benzochinon‐Struktur
Authors: Pfeifer Jochen , Gerlach Hans 
Published: July 2007

Synthesis of Natural Compounds with Alkyl‐methoxy‐1,4‐benzoquinone StructureThe 1,4‐benzoquinones1–7occurring in various plants and having interesting biological properties have been synthesized from the tetrahydropyranyl ethers17and21. Reaction withn‐butyllithium gave selectively theortho‐lithiated compounds. These could be alkylated with the haloalkenes13–16and haloalkanes to give the substituted phenols18–20and22–25in high yield after methanolysis. Alkylations of the methoxyisopropylether8of 10‐undecyn‐1‐ol with bromobutane and bromohexane gave9and10. These alkynols could be selectively hydrogenated with Lindlar‐Palladium catalyst to give the (Z)‐alkenols11and12which could be converted into the bromoalkenes13, 14and the iodoalkenes15, 16. Oxidation of the phenols18–20and22–25with molecular oxygen in the presence of salcomine as catalyst produced the substituted 1,4‐benzoquinones1–3and26–29in high yield. Selective hydrolysis with perchloric acid converted26–29into the natural products4–7.