Mechanical Stabilization of Helical Chirality in a Macrocyclic Oligothiophene
Kevin J. WeilandThomas BrandlKenneth AtzAlessandro PrescimoneDaniel HäußingerTomáš ŠolomekMarcel Mayor
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Abstract:
We introduce a design principle to stabilize helically chiral structures from an achiral tetrasubstituted [2.2]paracyclophane by integrating it into a macrocycle. The [2.2]paracyclophane introduces a three-dimensional perturbation into a nearly planar macrocyclic oligothiophene. The resulting helical structure is stabilized by two bulky substituents installed on the [2.2]paracyclophane unit. The increased enantiomerization barrier enabled the separation of both enantiomers. The synthesis of the target helical macrocycle 1 involves a sequence of halogenation and cross-coupling steps and a high-dilution strategy to close the macrocycle. Substituents tuning the energy of the enantiomerization process can be introduced in the last steps of the synthesis. The chiral target compound 1 was fully characterized by NMR spectroscopy and mass spectrometry. The absolute configurations of the isolated enantiomers were assigned by comparing the data of circular dichroism spectroscopy with TD-DFT calculations. The enantiomerization dynamics was studied by dynamic HPLC and variable-temperature 2D exchange spectroscopy and supported by quantum-chemical calculations.Keywords:
Chirality
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The results of the “Chirality measurement” circular dichroism along different viewing directions within molecules and phases—the anisotropy of circular dichroism can give suitable information in order to check helicity rules or to analyze the suprastructural chirality of films of organic materials, respectively. These results of the ACD spectroscopy (the CD of anisotropic phases and oriented molecules) show that in an oriented state different information about “chirality” will be gained from different viewing directions. ACD measurements of α,β-unsaturated ketosteroids, TADDOLes and binaphthol derivatives were analyzed. The chiral induction of cholesteric phases, the helical twisting power, is introduced as another chirality measurement which may possibly be a new relative method for the determination of the absolute configuration. Chirality 12:278–286, 2000. © 2000 Wiley-Liss, Inc.
Chirality
Helicity
Cotton effect
Vibrational Circular Dichroism
Dichroism
Axial Chirality
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Chiral stationary phase
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Tartaric acid
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Circular dichroism (CD) is a physical property observed in chiral molecules by inducing the difference of absorption between left- and right-handed circularly polarized light (CPL). Circular dichroism spectroscopy is widely used in the field of chemistry and biology to distinguish the enantiomers, which typically show either positive or severe side effects in biological applications depending on the molecular structures’ chirality. To effectively detect the chirality of molecules, diverse designs of nanostructured platforms are proposed based on optical resonances that can enhance the optical chirality and amplify the signal of circular dichroism. However, the underlying physics between the optical chirality and the resonance in a nanostructure is largely unexplored, and thus designing rules for optimal chiral detection is still elusive. Here, we carry out an in-depth analysis of chiral enhancement (C enhancement) in nanostructured surfaces to find the relationship between optical resonances and chirality. Based on the relations, we optimize the nanostructured metasurface to induce effective chiral detection of enantiomers for diverse conditions of molecule distribution. We believe that the proposed designing rules and physics pave the important pathway to enhance the optical chirality for effective circular dichroism spectroscopy.
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The enantiomers of 17 α-hydroxybenzylphosphonate diethylesters containing para, or ortho substituents or other aromatic rings (1-naphthyl, 2-naphthyl, and 2-thienyl) have been successfully separated by HPLC on a Whelk-O 1 chiral stationary phase which is superior to other CSPs. The effect of the substituents, particularly halogens, on the enantioselectivity was investigated and related to a chiral recognition model. The absolute configurations of 4-methyl and 2-methyl substituted α-hydroxybenzylphosphonates were obtained by measurement of the circular dichroism spectra of the isolated enantiomers. Chirality 10:100–105, 1998. © 1998 Wiley-Liss,Inc.
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Chiral derivatizing agent
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The enantiomers of U50,488, ligands highly selective for kappa-opioid receptors, have been prepared by a refined procedure and their optical purity demonstrated. The absolute configuration of (+)-trans-2-pyrrolidinyl-N-methylcyclohexylamine, a chemically versatile intermediate for synthesis of analogs of kappa-opioid receptor ligands with defined chirality, has been determined to be 1S,2S by X-ray crystallographic analysis. This intermediate has been used to synthesize the optically pure U50,488 enantiomers with known absolute configuration.
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κ-opioid receptor
Enantiomeric excess
Stereoisomerism
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In a recent contribution to this Journal (J. Nat. Prod. 2017, 80, 1531–1540), Noumeur et al. reported the isolation and structure elucidation of six novel polyketides named preussilides A–F, endowed with remarkable antiproliferative activity. The absolute configuration of the new compounds was established mainly by analyzing exciton-coupled electronic circular dichroism (ECD) spectra. However, the application of the exciton chirality method (ECM) was incorrect, because the chirality defined by transition moments was assigned in a wrong way. A correct application of the ECM, substantiated by time-dependent density functional theory (TDDFT) calculations of ECD spectra, led us to revise the absolute configuration of Preussilides A–F. A brief discussion on the criteria required for a correct application of the ECM is also presented.
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Absolute (philosophy)
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This chapter contains sections titled: Chirality Enantiomers and Racemic Compounds Absolute Configurations and System Descriptors Physical Properties of Enantiomers and Racemics Principles of Resolution and Preparation of Enantiomers Summary References
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Vibrational circular dichroism( VCD) was applied for study of the absolute configuration of chiral axial bis-carboline with N—O function. The computational results exhibite that(-)-biscarboline derivative should have a S absolute configuration. At the same time,electronic circular dichroism( ECD) and optical rotation( OR) methods were also used to study its absolute configuration. The results show that the three diffe-rent conclusion from VCD,ECD and OR methods should give a same conclusion,namely,this(-)-biscarboline derivative should have a S absolute configuration.
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Vibrational Circular Dichroism
Optical Rotation
Derivative (finance)
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