Stereoselective migration of sterically hindered organoboranes in cyclic and acyclic systems. A stereoselective allylic C-H activation reaction

The thermal migration of cyclic and acyclic organoboranes were studied. In most cases, a stereoselective 1,2-dyotropic migration was observed, allowing the stereocontrol of three contiguous chiral centers. Scope and limitations of this thermal migration are presented.     

Problems, Models and Complexity. Part II: Application to the DLSP

In Part I of this study, we suggest to identify an operations research (OR) problem with the equivalence class of models describing the problem and enhance the standard computer-science theory of computational complexity to be applicable to this situation of an often model-based OR context. The Discrete Lot-sizing and Scheduling Problem (DLSP) is analysed here in detail to demonstrate the difficulties which can arise if these aspects are neglected and to illustrate the new theoretical concept. In addition, a new minimal model is introduced for the DLSP which makes this problem eventually amenable to a rigorous analysis of its computational complexity.     

GnRH binding RNA and DNA Spiegelmers: a novel approach toward GnRH antagonism

Mirror-image oligonucleotide ligands (Spiegelmers) that bind to the pharmacologically relevant target gonadotropin-releasing hormone I (GnRH) with high affinity and high specificity have been identified using the Spiegelmer technology. GnRH is a decapeptide that plays an important role in mammalian reproduction and sexual maturation and is associated with several benign and malignant diseases. First, aptamers that bind to D-GnRH with dissociation constants of 50-100 nM were isolated out of RNA and DNA libraries. The respective enantiomers of the DNA and RNA aptamers were synthesized, and their binding to L-GnRH was shown. These Spiegelmers bind to L-GnRH with similar affinity to that of the corresponding aptamers that bind to D-GnRH. We further demonstrated dose-dependent inhibition of GnRH-induced Ca(2+) release in Chinese hamster ovary cells that were stably transfected with the human GnRH receptor.     

Simultaneous determination of alkylphenol ethoxylates and their biotransformation products by liquid chromatography/electrospray ionisation tandem mass spectrometry

Reversed-phase LC-MS/MS is used to determine major estrogenic alkylphenol ethoxylates (APEOs) and their biotransformation products. It allows the simultaneous analysis of eight APEOs, alkylphenoxy carboxylates (APECs) and alkylphenols (APs) in sewage treatment plant (STP) effluents in the same extract after solid-phase enrichment on polymeric Oasis HLB. As precursor ions, [APEO + NH4]+, [APEC - H]- and [AP - H]- were monitored. Instrumental limits of detection (LOD) were 2-600 pg, corresponding to sample concentrations of 0.04-12 ng l(-1), without correction for overall method recoveries. Matrix-induced signal suppression during electrospray ionisation (ESI) and extraction as well as overall method recoveries were assessed and the suitability of deuterated surrogates as internal standards was evaluated.     

Convexity in finite metric spaces

A subsetS of a metric space (X,d) is calledd-convex if for any pair of pointsx,y S each pointz X withd(x,z) +d(z,y) =d(x,y) belongs toS. We give some results and open questions concerning isometric and convexity-preserving embeddings of finite metric spaces into standard spaces and the number ofd-convex sets of a finite metric space.     

Solid-state NMR spectroscopy: an advancing tool to analyse the structure and properties of metal–organic frameworks

Metal–organic frameworks (MOFs) gain increasing interest due to their outstanding properties like extremely high porosity, structural variability, and various possibilities for functionalization. Their overall structure is usually determined by diffraction techniques. However, diffraction is often not sensitive for subtle local structural changes and ordering effects as well as dynamics and flexibility effects. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is sensitive for short range interactions and thus complementary to diffraction techniques. Novel methodical advances make ssNMR experiments increasingly suitable to tackle the above mentioned problems and challenges. NMR spectroscopy also allows study of host–guest interactions between the MOF lattice and adsorbed guest species. Understanding the underlying mechanisms and interactions is particularly important with respect to applications such as gas and liquid separation processes, gas storage, and others. Special in situ NMR experiments allow investigation of properties and functions of MOFs under controlled and application-relevant conditions. The present minireview explains the potential of various solid-state and in situ NMR techniques and illustrates their application to MOFs by highlighting selected examples from recent literature.

Metal–organic frameworks (MOFs) gain increasing interest due to their outstanding properties like extremely high porosity, structural variability, and various possibilities for functionalization.     

Über halogen- und stickstoffhaltige Derivate aliphatischer Carbonsäuren, 10. Mitt. (Über Reaktionen mit Aluminiumalkylen, 13. Mitt.): Über die Reduktion von tertiären α-Nitrocarbonsäureestern mit LiAlH4, Diisobutylaluminiumhydrid und Diäthylaluminiumhydrid

Zusammenfassung Die Reduktion tertiärer -Nitroester mit LiAlH₄ verläuft anomal. Infolge Abspaltung der Carboxylgruppe sind Aminoalkane Hauptprodukte dieser Reduktion. Daneben entstehen in geringer Menge unter Erhaltung des Kohlenstoffgerüstes -Hydroxylaminoalkohole. Diese Reaktion wird mit der Reduktion von sekundären -Nitroestern und von -Oximinoestern verglichen.Mit Dialkylaluminiumhydriden erleiden die tertiären Nitroester ebenfalls in der Hauptsache Reduktion unter C-C-Spaltung zu Aminoalkanen. Eine Nebenreaktion führt jedoch über die Stufe der Hydroxylaminoalkohole hinaus zu Aminoalkoholen. Außerdem tritt alkylierung am Stickstoffatom zu Alkylaminoalkanen bzw.-alkoholen auf.

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Halogen and nitrogen containing derivatives of aliphatic carboxylic acids, X (reactions with aluminium alkyls, XIII): Reduction of tertiary -nitro carboxylic acid esters with LiAlH₄, diisobutyl aluminium hydride, and diethyl aluminium hydride

The reduction of tertiary -nitro esters with LiAlH₄ proceeds anomalously. As a result of a decarboxylation amino alkanes are the main products of this reductions. Besides -hydroxylamino alcohols are obtained in small amounts by conservation of the carbon skeleton. This reaction is compared with the reduction of secondary -nitro esters and of -oximino esters.Tertiary -nitro esters are reduced by dialkyl aluminium hydrides to amino alkanes under cleavage of the C-C-bond. In a side-reaction one obtains amino alcohols via the stage of hydroxylamino alcohols. Besides the nitrogen atom is alkylated leading to alkylamino alkanes respectively alkohols.

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9. Mitt.:H. Reinheckel, Mh. Chem.99, 2215 (1968).

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12. Mitt.:H. Reinheckel undR. Gensike, J. Organometal. Chem.13, 45 (1968).