Search results for "Absolute"

Totalitarismo y lenguaje: el campo de concentración como espacio multilingüe y la figura del intérprete concentracionario

2020

espanolEn este trabajo se examinan las relaciones entre el poder absoluto y el lenguaje. Los campos de concentracion del regimen nacionalsocialista se entienden como el espacio donde se materializa la maxima expresion del totalitarismo. Al convertirse en sociedades forzadamente multilingues desde el inicio de la guerra, la lengua alemana puede convertirse, mas que nunca, en un instrumento mas de opresion al servicio del sistema: se deforma para convertirse practicamente en un protolenguaje violento yrudimentario, y su conocimiento por parte de los reclusos se relaciona siempre con las perspectivas de supervivencia. Ademas, se desarrolla una lengua franca que incorpora rasgos de varios idiom…

Synthesis of l -[4-11 C]Asparagine by Ring-Opening Nucleophilic 11 C-Cyanation Reaction of a Chiral Cyclic Sulfamidate Precursor

2018

The development of a convenient and rapid method to synthesize radiolabeled, enantiomerically pure amino acids (AAs) as potential positron emission tomography (PET) imaging agents for mapping various biochemical transformations in living organisms remains a challenge. This is especially true for the synthesis of carbon-11-labeled AAs given the short half-life of carbon-11 (11 C, t1/2 =20.4 min). A facile synthetic pathway to prepare enantiomerically pure 11 C-labeled l-asparagine was developed using a partially protected serine as a starting material with a four-step transformation providing a chiral five-membered cyclic sulfamidate as the radiolabeling precursor. Its structure and absolute…

Acetogenins from Annonaceae

1997

In 1982, Jolad et al. isolated uvaricin, a new antitumor agent, from the roots of Uvaria acuminata (Annonaceae), a bis-tetrahydrofuranoid fatty acid lactone (1) related to polyketides or acetogenins. However, it contained a number of original structural characteristics, particularly a linear acetogenin, a bis-tetrahydrofuran pattern flanked by hydroxyls and a terminal unsaturated lactone. Two years later, Dabrah and Sneden (2, 3 and Cortes et al. (4) described four new products presenting the same structural characteristics. Because these products formed a new class of natural compounds, and are only found in species belonging to the family of Annonaceae, they are commonly called acetogenin…

Stereochemistry of terpene derivatives. Part 4:☆ Fragrant terpenoid derivatives with an unsaturated gem-dimethylbicyclo[3.1.0]hexane system

2005

Abstract Starting from (+)-3-carene 1 several chiral fragrant compounds with the bicyclo[3.1.0]hexane system 4–6 and 10–20 were synthesized. These compounds are structural analogues of naturally occurring fragrant compounds, such as ionones and damascones, and possess either an endo- or an exo-cyclic double bond in the bicyclo[3.1.0]hexane moiety. The absolute configuration of selected products was confirmed by X-ray crystallography and circular dichroism analysis.

Homochiral self-assembly of biocoordination polymers: anion-triggered helicity and absolute configuration inversion

2015

The different natures of the weakly coordinating anions – triflate or perchlorate – in the Cu2+-mediated self-assembly of cytidine monophosphate nucleotide play a fundamental role in the homochiral resolution process, yielding one-dimensional copper(II) coordination polymers of opposite helicity that can be easily inverted, in a reversible way, by changing the nature of the anion as revealed by circular dichroism experiments both in solution and in the solid state.

Synthesis of cyclic dipeptide templates, their incorporation into peptides and studies on their conformational and biological properties.

2009

This study investigated the diastereoselective synthesis of three dipeptide templates 1, 2 and 3, which may be regarded as conformationally restricted analogs of H-Gly-Xaa-OH, in which Xaa constitutes an aromatic amino acid. Bond formation between α-C of Gly and the aromatic moiety was achieved by proton-catalyzed intramolecular electrophilic aromatic substitution. The absolute configuration of the dipeptide templates was determined by single-crystal X-ray crystallography or by nuclear Overhauser enhancement measurements. A protective group strategy was elaborated to allow their incorporation into peptide sequences by liquid phase as well as by solid-phase peptide synthesis. The templates w…

Determination of the absolute configuration of (−)-abietic acidviaits (4R,5R,9R,10R)-7,13-abietadien-18-ylp-bromobenzoate derivative

2006

The absolute configuration of the title bromo derivative of abietic acid, C27H35BrO2, has been determined. The structural analysis confirms the absolute stereochemistry for (−)-abietic acid proposed by Bose & Struck [(1959). Chem. Ind. (London), pp. 1628–1630] on the basis of optical rotatory dispersion measurements. The mol­ecule exhibits a trans anti 6/6/6 tricyclic hydro­carbon skeleton, with the cyclo­hexane ring in the expected chair form and the two cyclo­hexene rings, the double bonds of which are conjugated, in half-chair conformations.

(1S,2R,4S)-1-[(Benzyl­amino)­meth­yl]-4-(prop-1-en-2-yl)cyclo­hexane-1,2-diol

2010

The title compound, C17H25NO2, was synthesized by epoxidation of the double bond of (S)-perillyl alcohol [(S)-4-isopropenyl-1-cyclohexenylmethanol], followed by the oxirane ring-opening by benzylamine using [Ca(CF3CO2)2] as catalyst under solvent-free condition at 313 K. The molecular conformation is stabilized by an intramolecular O—H...N hydrogen bond. In the crystal, molecules are linked by intermolecular N—H...O hydrogen bonds, forming chains parallel to the a axis, which are further connected by O—H...O hydrogen bonds into sheets parallel to (010). The absolute configuration of the molecule is known from the synthetic procedure.

1988

Monte Carlo simulations are presented for binary (AB) symmetric polymer mixtures (chain lengths NANBN) for the case that an attractive interaction ϵ exists between monomers of the same kind, and the limiting case that one species (B) is very diluted. It is shown that with increasing interaction strength ϵ/(kB · T), T being the absolute temperature, the minority chains collapse to a very dense configuration, while the majority chains stay nearly Gaussian. Both chain radii, structure factors and numbers of nearest neighbour contacts are discussed.

Stereospecific CC-bond formation with rabbit muscle aldolase - A chemoenzymatic synthesis of (+)-exo-brevicomin

1990

Abstract (+)-(1S,5R,7S)-Exo-brevicomin 9, a sex pheromone of the western pine bark beetle, is synthesized using an aldol reaction catalyzed by fructose-1,6-diphosphate aldolase (EC 4.1.2.13) from rabbit muscle as the key step by which the absolute configuration of the target is established.