Stereoselective total synthesis of 4-((3S,5R)-3,5-dihydroxynonadecyl)phenol

A stereoselective total synthesis of 4-((3S,5R)-3,5-dihydroxynonadecyl)phenol has been accomplished in two different synthetic approaches. In the first approach, Prins cyclization has been successfully utilized to produce the anti-1,3-diol unit, which was further converted into a required syn-1,3-diol through Mitsunobu reaction. The side chain was constructed through cross metathesis and hydrogenation sequence. In the second approach, the chiral syn-1,3-diol was prepared by a sequence of reactions such as alkylation of 1,3-dithane with (R)-epichlorohydrin, ring opening of the epoxide with vinylmagnesium bromide, and 1,3-syn-reduction of the β-hydroxyketone with NaBH₄ in the presence of diethylmethoxyborane.     

A Stereoselective Total Synthesis of Decarestrictine I

A convergent and stereoselective total synthesis of decarestrictine I, a polyketide natural product, is described. Both acid and alcohol fragments were prepared from the readily available L ‐malic acid via Still? Gennari olefination and Sharpless asymmetric epoxidation. The Steglich esterification and ring‐closing metathesis (RCM) are employed to combine both acid and alcohol fragments.     

Dendrimer influenced supramolecular structure formation of block copolymers: II. Dendrimer concentration dependence

The dendrimer concentration dependence of the supramolecular structure formation of polystyrene-block-poly(acrylic acid) in dioxane/THF was investigated as a function of water content. The distribution as well as the localization of the dendrimer units inside the formed aggregates were determined by comparative studies of turbidity measurements and transmission electron microscopy. The strong and specific interactions present between the amine groups of the dendrimer (PAMAM) and the carboxylic acid residues of PAA in the copolymer have a strong influence on the structure formation. The PAMAM concentration as well as the character of the terminal groups of the dendrimer influence the strength of these interactions and consequently affect the structure formation process. As shown by fluorescence quenching experiments, on all supramolecular hierarchical structure levels, and specifically in vesicles, the dendrimer is coated by the PAA chains of the block copolymer due to the strong interactions; since the PAA blocks are connected to the PS blocks, which form the corona, the dendrimer is surrounded by PS chains and is thus encapsulated into the hydrophobic regions of the block copolymer aggregates. A high-resolution transmission electron microscopy image of a micelle is shown, in which the individual dendrimer cores are seen to be localized in the center of these aggregates, and thus, the structure proposed in the previous publication (Kroeger, A.; Li, X.; Eisenberg, A. Langmuir 2007, 23, 10732) is confirmed. Furthermore, the sizes of the resulting aggregates depend on the relative concentration of dendrimer, expressed as RAm/Ac (the ratio of amine to acid groups). With increasing RAm/Ac values, not only the sizes of the micelles but also the vesicle dimensions, especially vesicle wall thicknesses, increase, and this effect suggests the encapsulation of the dendrimer into the vesicle walls. Thus, the constitution of the vesicle structure is determined precisely. This feature allows the potential incorporation of a wide range of species into the vesicle walls or the center of the micelle cores.     

Polyion complex nanomaterials from block polyelectrolyte micelles and linear polyelectrolytes of opposite charge. 2. Dynamic properties

The present study investigates the relationship between the aggregation state and dynamic properties of block ionomer complexes (BICs) based on amphiphilic ionic block copolymers. The polyion coupling of 4'-(aminomethyl)fluorescein (AMF)-labeled poly(sodium methacrylate) (PMANa) or polystyrene- block-poly(sodium carboxylates) with poly(N-ethyl-4-vinylpyridinium bromide), PEVP was studied at an excess of carboxylate groups [PEVP]/[COO(-)] TOTAL = 0.3 and detected by fluorescence quenching. The polyion interchange reactions included migration of PEVP between the following: (1) two linear polyanion chains, (2) linear polyanion chain and anionic polyion shell micelle, or (3) two anionic polyion shell micelles. Additionally, the interchange of AMF-labeled PMANa with unlabeled PMANa in the shell of polystyrene- block-PEVP micelles was studied. The interchange reactions were carried out at [PEVP]/[COO(-)] TOTAL = 0.15 and detected by fluorescence quenching (direct reaction) or ignition (reverse reaction). The rates of these reactions were compared using half-conversion times and, when possible, second-order reaction kinetic constants. The dependences of the rates on the ionic strength and polyion length observed for BICs were similar to those previously reported for regular interpolyelectrolyte complexes (IPECs) of linear polyions. However, the interchange reactions involving polyion shell micelles were much slower than those reactions observed in IPECs. The coupling reactions involving polyion shell micelles were also slower compared with the coupling of linear polyions. The observed phenomena were attributed to the aggregation state of polyion shell micelles and discussed using the collision model for polyion interchange reactions previously proposed for IPECs.     

Structural changes in glutamate cell swelling followed by multiparametric q-space diffusion MR of excised rat spinal cord

Diffusion in the extracellular and intracellular spaces (ECS and ICS, respectively) was evaluated in excised spinal cords, before and after cell swelling induced by glutamate, by high b-value q-space diffusion MR of specific markers and water. The signal decays of deuterated tetramethylammonium (TMA-d(12)) chloride, an exogenous marker of the ECS, and N-acetyl aspartate (NAA), an endogenous marker of the ICS, were found to be non-mono-exponential at all diffusion times. The signal decays of these markers were found to depend on the diffusion time and the cell swelling induced by the glutamate. It was found, for example, that the mean displacements of the apparent fast and slow diffusion components of TMA-d(12) are 7.21 +/- 0.11 and 1.16 +/- 0.05 microm, respectively at a diffusion time of 496 ms. After exposure of the spinal cords to 10 mM of glutamate, these values decreased to 6.62 +/- 0.13 and 1.01 +/- 0.05 microm, respectively. The mean displacement of NAA, however, showed a less pronounced opposite trend and increased after cell swelling induced by exposure to glutamate. q-Space diffusion MR of water was found to be sensitive to exposure to glutamate, and q-space diffusion MRI showed that a more pronounced decrease in the apparent diffusion coefficient and the mean displacement of water is observed in the gray matter (GM) of the spinal cord. All these changes demonstrate that diffusion MR is indeed sensitive to structural changes caused by cell swelling induced by glutamate. Multiparametric high b-value q-space diffusion MR is useful for obtaining microstructural information in neuronal tissues.     

Importance of monolayer quality for interpreting current transport through organic molecules: alkyls on oxide-free Si

We study the effect of monolayer quality on the electrical transport through n-Si/C(n)H(2n+1)/Hg junctions (n = 12, 14, and 18) and find that truly high quality layers and only they, yield the type of data, reported by us in Phys. Rev. Lett. 2005, 95, 266807, data that are consistent with the theoretically predicted behavior of a Schottky barrier coupled to a tunnel barrier. By using that agreement as our starting point, we can assess the effects of changing the quality of the alkyl monolayers, as judged from ellipsometer, contact angle, XPS, and ATR-FTIR measurements, on the electrical transport. Although low monolayer quality layers are easily identified by one or more of those characterization tools, as well as from the current-voltage measurements, even a combination of characterization techniques may not suffice to distinguish between monolayers with minor differences in quality, which, nevertheless, are evident in the transport measurement. The thermionic emission mechanism, which in these systems dominates at low forward bias, is the one that is most sensitive to monolayer quality. It serves thus as the best quality control. This is important because, even where tunneling characteristics appear rather insensitive to slightly diminished quality, their correct analysis will be affected, especially if layers of different lengths are also of different quality.     

Contour approximation of data: A duality theory

Given a dataset D partitioned in clusters, the joint distance function (JDF) J(x) at any point x is the harmonic mean of the distances between x and the cluster centers. The JDF is a continuous function, capturing the data points in its lower level sets (a property called contour approximation), and is a useful concept in probabilistic clustering and data analysis. In particular, contour approximation allows a compact representation of the data: for a dataset in Rⁿ with N points organized in K clusters, the JDF requires K centers and covariances (if Mahalanobis distances are used), for a total of Kn(n+3)/2 parameters, and a considerable reduction of storage if N?K,n. The JDF of the whole dataset, J(D)?∑{J(x):x∈D}, is a measure of the classifiability of the data, and can be used to determine the “right” number of clusters for D. A duality theory for the JDF J(D) is given, in analogy with Kuhn’s geometric duality theory for the Fermat-Weber location problem. The JDF J(D) is the optimal value of a primal problem (P), for which a dual problem (D) is given, with a sharp lower bound on J(D).     

Block polyelectrolytes in aqueous environments

Analogous to the self-assembly of low-molecular-weight amphiphiles in aqueous solutions, the formation of spherical micelle-like aggregates has been observed in systems of amphiphilic block copolymers in water. The aggregates, often called micelles due to structural similarities with surfactant associates, are found to exist above the critical micelle concentration (cmc). The micellization of amphiphilic block copolymers has been investigated using a wide range of techniques, such as size-exclusion chromatography (SEC), static and dynamic light scattering (SLS and DLS), small-angle x-ray scattering (SAXS), small-angle neutron scattering (SANS), transmission electron microscopy (TEM), viscometry, and steady-state fluorescence spectroscopy. The present lecture is a review of recent work in our laboratory concerning the micellization of ionic block copolymers. These high-molecular-weight amphiphiles may contain one or more of a variety of ionic blocks, such as poly(4-vinylpyridinium alkyl halides), poly(metal acrylates), poly(metal methacrylates) and sulfonated polystyrene. In water, such polymers are referred to as block polyelectrolytes, as they combine the colloidal behavior of block copolymers with the long-range electrostatic interactions of polyelectrolytes. Early work in this field has been reviewed by Selb and Gallot.¹     

Synthesis of porous C/Fe3O4 microspheres by spray pyrolysis with NaNO3 additive for lithium-ion battery applications

In this work, we successfully synthesized porous C/Fe₃O₄ microspheres by spray pyrolysis at 700ºC with a sodium nitrate (NaNO₃) additive in the precursor solution. Furthermore, we studied their electrochemical properties as anode material for Li-ion batteries. The systematic studies by various characterization techniques show that NaNO₃ catalyzes the carbonization of sucrose and enhances the crystallization of Fe₃O₄. Moreover, an aqueous etching can easily remove sodium compounds to produce porous C/Fe₃O₄ microspheres with large surface areas and pore volumes. The porous C/Fe₃O₄ microspheres exhibit a reversible capacity of ~780 mAh g^–1 in the initial cycles and ~520 mAh g^–1 after 30 cycles at a current density of 50 mA g^–1. Moreover, a reversible capacity of ~400 mAh g^–1 is attainable after 200 cycles, even at a high current density of 500 mA g^–1. The wide range of pores produced from the removal of sodium compounds might enable easy electrolyte penetration and facilitate fast Li-ion diffusion, while the N-doping can promote the electronic conductivity of the carbon. These features of porous C/Fe₃O₄ microspheres led to the improved electrochemical properties of this sample.

Graphical Abstract