Zur konfiguration und konformation von aklavinon-I und aklavinon-II,zwei neuen anthracyclinonen aus kulturansätzen von streptomyces galilaeus

By means of NMR and CD measurements the configuration and conformation of aklavinone-I and aklavinone-II, two new anthracyclinones from Streptomyces galilaeus, have been derived. Aklavinone-II is the 7-epimer of aklavinone with the absolute configuration 7R, 9R, 10R. For aklavinone-I the absolute configuration 7R, 9S, 10R or its enantiomer (10-epi-aklavinone) is proposed.     

Supramolecular chirogenesis in bis(zinc porphyrin): An absolute configuration probe highly sensitive to guest structure

The role of the ligand's structure and absolute configuration in the supramolecular chirality induction in achiral bis(zinc porphyrin) has been studied. The amines with bulkier substituents resulted in stronger CD signals due to increased helical displacement in the anti conformer. All the amines with an R absolute configuration gave a negative first Cotton effect and positive second Cotton effect, while the ligands with an S absolute configuration produced CD signals with opposite signs due to formation of the left- and right-handed screw diastereomers, respectively.     

NMR and chiroptical examination of the diastereoisomers of (S)-Eu-EOB-DTPA

The solution structure of the diastereoisomers of (S)-Eu--EOB--DTPA has been analysed by (1)H NMR, CD and CPL spectroscopy. Two major species exist which possess very similar (1)H NMR paramagnetic shifts and emission spectra, consistent with a 9-coordinate structure involving one bound water. Circularly polarised luminescence data are consistent with a common Lambda-helicity for each isomer; the isomers differ only in the absolute configuration of the central nitrogen atom.     

Isolation of an anthrabenzoxocinone 1.264-C from Streptomyces sp. FXJ1.264 and absolute configuration determination of the anthrabenzoxocinones

The anthrabenzoxocinones (ABXs) are hexacyclic aromatic compounds with various bioactivities. Some ABXs with two stereogenic carbon centers have been isolated from different actinomycetes. However, none of them have an absolute configuration assigned, which precludes further studies on the structure–activity relationship and biosynthetic mechanisms of the ABXs. An ABX compound 1.264-C was isolated from Streptomyces sp. FXJ1.264. The absolute configuration of 1.264-C was characterized by X-ray crystallographic, electronic, and experimental circular dichroism (CD) analyses. Based on the reported CD and specific rotation data, the absolute configurations of several other ABXs were also assigned.     

A CD exciton chirality method for determination of the absolute configuration of threo-beta-aryl-beta-hydroxy-alpha-amino acid derivatives

The absolute configuration of threo-beta-aryl-beta-hydroxy-alpha-amino acids was studied by CD exciton chirality method using 7-diethylaminocoumarin-3-carboxylate as a red-shifted chromophore. Bischromophoric derivatives for a series of threo-beta-aryl-beta-hydroxy-alpha-amino acids (3a-h) were prepared and their CD spectra measured in CH2Cl2. By combining the data of CD and NMR coupling constants, we are able to correlate their preferred conformer (B) and the positive CD to the corresponding (2S,3R)-absolute configuration. These results are consistent with those obtained from serine and threonine derivatives, which represent the simplest form of beta-hydroxy-alpha-amino acids. This CD method could thus become a general method for determining the absolute configuration of threo-beta-aryl-beta-hydroxy-alpha-amino acids.     

Structure of brefeldin A

On the basis of X-ray crystal structure analysis, CD. measurements, and asymmetric synthesis the absolute configuration of brefeldin A has been determined.     

A general and nonempirical approach to the determination of the absolute configuration of 1-aryl-1,2-diols

We describe herein a simple, general, and reliable nonempirical approach, based on the exciton coupling method, to assign the absolute configuration of the benzylic stereogenic center of 1-aryl-1,2-diols. According to this method, it is only necessary to prepare the 4-biphenylboronic esters of the diols and to record their CD spectra in the 230-300 nm range, i.e., in the range corresponding to the long-axis (1)L(a) transition of the biphenyl chromophore. From the sign of the CD couplet or Cotton effect at 260 nm it is possible to know the chirality defined by the aryl and biphenyl chromophore transitions and then to determine the absolute configuration of the benzylic carbon. By this approach, simple rules have been formulated which allow us to establish the absolute configuration of many classes of 1-aryl-1,2-diols.     

Conformational studies by dynamic NMR. 93.(1) Stereomutation,enantioseparation, and absolute configuration of the atropisomers of diarylbicyclononanes

The structure of 9-(4-methoxyphenyl)-9-(3-methylphenyl) bicyclo[3.3.1]nonane (1) has been determined by single-crystal X-ray diffraction, and the rotation barriers about the two aryl-C9 bonds have been measured by variable temperature NMR spectroscopy in solution. In the case of 9-(4-methoxyphenyl)-9-(1-naphthyl)bicyclo[3.3.1]nonane (3), the barrier involving the naphthyl-C9 rotation was found to be so high as to allow the physical separation of the two atropisomers by enantioselective HPLC at ambient temperature: the absolute configuration could be established on the basis of the corresponding CD spectra. It was also observed that the Ar-C9 rotation barriers of monoaryl-substituted nonanes are much lower than those of the corresponding diaryl-substituted nonanes.     

长梗千里光呋喃雅槛兰型倍半萜的结构研究

我们曾从甘肃省漳县采集的长梗千里光全草中分离并鉴定出两个呋喃雅槛兰型倍半萜. 在此基础上, 本文对其脂溶性化学成分进行深入研究, 除了发现倍半萜外, 又分离并鉴定出2个呋喃雅槛兰型倍半萜. 通过IR， UV， MS， CD和NMR等数据分析, 确定了4个化合物的结构, 其中, 化合物3首次在长梗千里光中分离得到, 化合物4是一种新化合物, 通过圆二光谱及核磁共振和NOE差谱研究其立体化学结构, 并按Dreiding models构筑该化合物分子的立体化学结构, 确定该化合物为Non-steroid型构象.     

Direct determination of absolute configuration of carboxylic acids by cyclooctapyrrole

Octaphyrin O1 has been found to be an effective sensor for the direct determination of absolute configuration of a variety of carboxylic acids at mM concentrations at room temperature based on CD exciton chirality method.     

Chiroptical properties of 2-substituted piperidines : A simple helicity rule

The CD spectra of fifteen 2-substituted piperidines 2 and their N-methylated derivatives 1 have been determined. The sign of the Cotton effect observed for the n→σ^* transition of nitrogen is shown to be correlated, for a given absolute configuration, with the axial or equatorial orientation of the nitrogen lone pair, and to be determined by the screw sense of the helicity between the nitrogen lone pair and the 2-substituent R in 1 and 2: for compounds of the absolute D configuration, a positive helicity gives a positive CD and vice versa.     

Absolute Konfiguration der Equisetumalkaloide (+)-Palustrin,Palustridin ((+)-Monohydrochlorid) sowie der (−)-Dihydropalustraminsäure und weiterer Derivate. 16. Mitteilung über Schachtelhalmalkaloide

Determination of the absolute configuration of alkaloids from Equisetum sp.,e.g. (+)-palustrin, palustridin ((+)-hydrochloride), and (?)-dihydropalustramic acid and derivatives The absolute configuration of the title alkaloids has been determined using chiroptical methods (ORD., superposition of CD. values of appropriate models and a modified Horeau method). (+)-Palustrin is (13R, 17 S, 1′S)-17-(1-hydroxypropyl)-1,5,10-triazabicyclo [11.4.0]heptadec-15-en-11-on.     

Direct determination of absolute configuration of monoalcohols by bis(magnesium porphyrin)

Ethane bridged bis(Mg porphyrin) has been found to be an effective sensor for the direct determination of the absolute configuration of a variety of monoalcohols at millimolar concentrations and room temperature based on the CD exciton chirality method. Examination of a variety of synthetic and natural monoalcohols, including alpha-substituted aliphatic and aromatic alkanols with one chiral center, as well as alicyclic alcohols with 2-5 chiral centers, gave consistent results. In the case of alcohols with multiple chiral centers, the absolute configuration of the carbon alpha to OH is always read out.     

The absolute configuration of α-hydroxyphosphonates

The absolute configuration and CD spectra of a series of chiral α-hydroxyphosphonates and phosphoric acids have been determinated. The hydroxyphosphonates were prepared via a quinine catalysed 1,2-addition reaction of phosphite to aldehyde.     

Chiral liquid chromatography contribution to the determination of the absolute configuration of enantiomers

The review covers examples in which chiral HPLC, as a source of pure enantiomers, has been combined with classical methods (X-ray, vibrational circular dichroism (VCD), enzymatic resolutions, nuclear magnetic resonance (NMR) techniques, optical rotation, circular dichroism (CD)) for the on- or off-line determination of absolute configuration of enantiomers. Furthermore, it is outlined that chiral HPLC, which associates enantioseparation process and classical purification process, opens new perspectives in the classical determination of absolute configuration by chemical correlation or chemical interconversion methods. The review also contains a discussion about the various approaches to predict the absolute configuration from the retention behavior of the enantiomers on chiral stationary phases (CSPs). Some examples illustrate the advantages and limitations of molecular modeling methods and the use of chiral recognition models. The assumptions underlying some of these methods are critically analyzed and some possible emerging new strategies are outlined.     

Absolute configuration of glycosyl sulfoxides

A series of alkyl glycosyl sulfoxides were synthesized and analyzed by NMR and CD. The study of the configuration of the sulfur atom revealed several types of spectroscopic behavior that can be used as a criterion for this purpose. The study also pointed to CD as the preferential technique, showing clear advantages over NMR methods. A general rule for determining the absolute configuration of glycosyl sulfoxides by CD is proposed.     

A convenient method for the assignment of the absolute configuration to cyclic amines

We have assigned the absolute configuration to trans-2,5-dimethylpyrrolidine and trans-2,6-dimethyl-piperidine through radical chlorination in ω-1 position of optically active amines, 2 and 5, of known absolute configuration, followed by cyclization. CD curves of 1f and 4g derivatives indicate that the reaction is stereospecific. The absolute configuration of trans-2,5-dimethyl-Δ³-pyrròline has also been determined.     

Vibrational circular dichroism study of optically pure cryptophane-A

Vibrational circular dichroism (VCD) measurements and density functional theory (DFT) calculations were used to obtain the absolute configuration of optically pure cryptophane-A molecule. This large molecule (120 atoms) that possess a globular shape, but no chiral centers, exceeds the molecular size of published structures for which VCD has been used to determine the absolute configuration. VCD spectra recorded in CDCl(3) solution for the two resolved enantiomers are near mirror images, and very good agreement between the observed IR and VCD spectra and intensity calculations performed at the DFT (B3PW91/6-31G) level establish, besides the absolute configuration, the preferential anti conformation of the aliphatic linkers of the chloroform-cryptophane-A complex. Experiments performed in CD(2)Cl(2) and C(2)D(2)Cl(4) solutions show no significant modifications in the IR and VCD spectra, indicating that the conformation of the aliphatic linkers is similar for empty (C(2)D(2)Cl(4) solution) and encaged (CDCl(3) and CD(2)Cl(2) solutions) cryptophane-A molecules.     

Chiral cyclohexane based fluorescent chemosensors for enantiomeric discrimination of aspartate

Some new chiral cyclohexyl based fluorescent anion receptors have been synthesized and their absolute configuration has been determined by using circular dichroism (CD). Complexation experiments have been carried out with several dicarboxylates, and stoichiometries and complexation constants for the corresponding complexes have been determined. The chiral discrimination ability of these ligands for chiral dicarboxylates has been studied and the best results have been obtained with TMA aspartate.     

Erratum

Abstract

1,3-oxathianes and 1,3-dithianes, with and without p-bromophenyl group in position 2, have been oxidized to sulphoxides. Both starting materials and oxidation products have been resolved by chromatography, and CD spectra have been recorded. Absolute configuration for the p-bromophenyl compounds have been assigned on the basis of known absolute configurations for analogous 1,3-oxathiolanes.