Asymmetric aldol reactions on a soluble polymeric support

[reaction: see text] The combinatorial synthesis of small, nonpeptidic compounds is of increasing interest in current medicinal chemistry. To meet this demand, efficient entries, preferentially on polymeric supports, to pharmacologically interesting classes of compounds such as polyketides are necessary. Therefore, we have developed a synthetic protocol allowing for the asymmetric synthesis of diketides on the soluble support MeOPEG-5000. The strategy employed mainly allows for repeated aldolizations, thus providing access to functionalized polyketides of varying degree of oligomerization.     

Polynomials with two values

This paper investigates the minimal degree of polynomialsfR[x] that take exactly two values on a given range of integers {0,...n}. We show that thegap, defined asn-deg(f), isO(n ⁵⁴⁸). The maximal gap forn128 is 3. As an application, we obtain a bound on the Fourier degree of symmetric Boolean functions.     

Self-assembled nanotubes that reversibly bind acetic acid guests

A large bis-urea macrocycle was synthesized and assembled into columnar nanotubes containing a sizable cavity. This purely organic nanotube is held together primarily by hydrogen bonding and yet shows remarkable thermal stability up to 180 degrees C in the presence and absence of acetic acid guest. This enables the nanotube to be used as reusable organic zeolite.     

Novel mixed-valent (V/VI) triple perovskite ruthenates: observation of a complex low-temperature structural and magnetic transition

Two new mixed-valent triple perovskites, Ba(3)MRu(2)O(9) (M = Li, Na), were grown from reactive hydroxide fluxes. They crystallize in the hexagonal space group P6(3)/mmc, where Ru(V) and Ru(VI) are disordered on only one crystallographic site. Upon cooling, single crystals of Ba(3)NaRu(2)O(9) undergo a complex symmetry-breaking structural transition at ca. 225 K from room-temperature hexagonal symmetry to a low-temperature orthorhombic symmetry, space group Cmcm. Accompanying this structural transition is a rather abrupt decrease in the magnetic susceptibility at 210 K followed by a steady decrease in the susceptibility with decreasing temperature. Interestingly, the lithium analogue does not display any structural transition down to 100 K. The structural transition in Ba(3)NaRu(2)O(9) generates three crystallographically unique Ru sites in the low-temperature structure as compared to only one distinct site in the room-temperature structure. On the basis of an analysis of the Ru-Ru distances in the face-sharing bi-octahedra, the structural transition also appears to involve charge ordering of Ru(V) and Ru(VI), causing all Ru(V) to occupy one set of bi-octahedra and all Ru(VI) to occupy another set.     

Infrarotspektroskopische Charakterisierung von Carbons?uren durch ihre Trimethylsilylester

Zusammenfassung 91 Carbonsäuren wurden mit Dialkoxytrimethylsiloxy-N-trimethylsilylphosphoriminen (besonders Diäthoxytrimethylsiloxy-N-trimethylsilylphosphorimin, DASPI) oder mit Trimethylsilylmethyl acetamid, TMSMA, silyliert. Die C=O-Frequenzen der entsprechenden Trimethylsilylester können zur Ermittlung des Substitutionstyps am-C-Atom der Carbonsäuren dienen.-Aminosäuren bilden eine Ausnahme. Diese Säuren wurden wegen ungenügender Silylierung mit DASPI nur mit TMSMA silyliert. Folgende Frequenzbereiche der Silylesterbande wurden gefunden: aliphatisch, aliphatisch-verzweigt 1713 bis 1733 (1722±4) cm^–1;-heterosubstituiert, einschließlich-Acylamino- 1721–1754 (1737±11) cm^–1, in den meisten Fällen von einer zweiten, längerwelligen Bande begleitet im Abstand von 10–25 cm^–1:-unge-sättigt 1687–1736 (1701±13) cm^–1; aromatisch 1673–1721 (1705+10) cm^–1;-Amino- 1702–1731 (1719±7) cm^–1. (In Klammern: Mittelwerte mit Standardabweichung.)

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Infrared-spectroscopic characterisation of carboxylic acids by their trimethylsilyl esters

91 Carboxylic acids were silylated by dialkyltrimethylsilyl-N-trimethylsilyl phosphorimidates (mainly diethyltrimethylsilyl-N-trimethylsilyl phosphorimidate, DESPI) or trimethylsilylmethyl acetamide, TMSMA, and the C=O frequencies of the corresponding trimethylsilyl esters were investigated. Exceptions were -amino acids. Due to uncomplete silylation by DESPI these were silylated by TMSMA, but investigations in the region lower than 1690 cm^–1 could not be made because of the strong absorption by the amide I band of TMSMA.The following frequency regions of the silyl ester band were found: aliphatic, aliphatic-branched 1713–1733 (1722±4) cm^–1;-heterosubstituted, including-acylamino- 1721–1754 (1737±11) cm^–1 and, in most cases a second, lower frequency band at a distance of 10–25 cm^–1 originating from conformational isomers:-unsa-turated 1687–1736 (1701±13) cm^–1; aromatic 1673–1721 (1705±10) cm^–1;-amino-1702–1731 (1719±7) cm^–1. (In brackets: mean value ± standard deviation.)

     

Multidimensional frameworks assembled from silver(I) coordination polymers containing flexible bis(thioquinolyl) ligands: role of the intra- and intermolecular aromatic stacking interactions

The two flexible multidentate ligands 1,3-bis(8-thioquinolyl)propane (C3TQ) and 1,4-bis(8-thioquinolyl)butane (C4TQ) were reacted with AgX (X = CF(3)SO(3)(-) or ClO(4)(-)) to give four new complexes: ([Ag(C3TQ)](ClO(4)))(n)() 1, ([Ag(C3TQ)](CF(3)SO(3)))(n)() 2, ([Ag(2)(C4TQ)(CF(3)SO(3))(CH(3)CN)](CF(3)SO(3)))(n)() 3, and ([Ag(C4TQ)](ClO(4)))(n)() 4. All complexes have been characterized by elemental analysis, IR, and (1)H NMR spectroscopy. Single-crystal X-ray analysis showed that chain structures form for all complexes in which the quinoline rings interact via various intra- (1) or intermolecular (2, 3, and 4) pi-pi aromatic stacking interactions, which in the latter cases results in multidimensional structures. Additional weak interactions, such as Ag.O and Ag.S contacts and C-H.O hydrogen bonding, are also present and help form stable, crystalline materials. It was found that the (CH(2))(n) spacers (n = 3 or 4) affect the orientation of the two terminal quinolyl rings, thereby significantly influencing the specific framework structure that forms. If the same ligand is used, on the other hand, then the different counteranions have the greatest effect on the final structure.     

Design and optimization for variable rate selective excitation using an analytic RF scaling function

At higher B(0) fields, specific absorption rate (SAR) deposition increases. Due to maximum SAR limitation, slice coverage decreases and/or scan time increases. Conventional selective RF pulses are played out in conjunction with a time independent field gradient. Variable rate selective excitation (VERSE) is a technique that modifies the original RF and gradient waveforms such that slice profile is unchanged. The drawback is that the slice profile for off-resonance spins is distorted. A new VERSE algorithm based on modeling the scaled waveforms as a Fermi function is introduced. It ensures that system related constraints of maximum gradient amplitude and slew rate are not exceeded. The algorithm can be used to preserve the original RF pulse duration while minimizing SAR and peak b1 or to minimize the RF pulse duration. The design is general and can be applied to any symmetrical or asymmetrical RF waveform. The algorithm is demonstrated by using it to (a) minimize the SAR of a linear phase RF pulse, (b) minimize SAR of a hyperbolic secant RF pulse, and (c) minimize the duration of a linear phase RF pulse. Images with a T1-FLAIR (T1 FLuid Attenuated Inversion Recovery) sequence using a conventional and VERSE adiabatic inversion RF pulse are presented. Comparison of images and scan parameters for different anatomies and coils shows increased scan coverage and decreased SAR with the VERSE inversion RF pulse, while image quality is preserved.     

The number of Hamiltonian decompositions of regular graphs

A Hamilton cycle in a graph Γ is a cycle passing through every vertex of Γ. A Hamiltonian decomposition of Γ is a partition of its edge set into disjoint Hamilton cycles. One of the oldest results in graph theory is Walecki’s theorem from the 19th century, showing that a complete graph K _n on an odd number of vertices n has a Hamiltonian decomposition. This result was recently greatly extended by Kühn and Osthus. They proved that every r-regular n-vertex graph Γ with even degree r = cn for some fixed c > 1/2 has a Hamiltonian decomposition, provided n = n(c) is sufficiently large. In this paper we address the natural question of estimating H(Γ), the number of such decompositions of Γ. Our main result is that H(Γ) = r ^(1+o(1))nr/2. In particular, the number of Hamiltonian decompositions of K _n is \({n^{\left( {1 + o\left( 1 \right)} \right){n^2}/2}}\).