helicene
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By means of selective deuterium labeling of 1-aza[6]helicene combined with resolution of the enantiomers, chiral discrimination in silver(I)-bound dimers of the type [LAgL']+ is probed by electrospray mass spectrometry. The analysis of the results reveals a pronounced preference for the formation of homochiral dimers (P,P and M,M, respectively) over the statistically preferred heterochiral variant (P,M), which is fully consistent with previous data about the formation of homochiral dimers in the condensed phase. Further, competitive experiments with mixtures of 1- and 2-aza[6]helicene suggest a largely preferred coordination of 1-aza[6]helicene to the silver(I) cation.
Correlated ab initio calculations on large systems, such as the popular MP2 (or RI-MP2) method, suffer from the intramolecular basis set superposition error (BSSE). This error is typically manifested in molecules with folded structures, characterized by intramolecular dispersion interactions. It can dramatically affect the energy differences between various conformers as well as intramolecular stabilities, and it can even impair the accuracy of the predictions of the equilibrium molecular structures. In this study, we will present two extreme cases of intramolecular BSSE, the internal stability of [n]helicene molecules and the relative energies of various conformers of phenylalanyl-glycyl-phenylalanine tripeptide (Phe-Gly-Phe), and compare the calculated data with benchmark values (experimental or high-level theoretical data). As a practical and cheap solution to the accurate treatment of the systems with large anticipated value of intramolecular BSSE, the recently developed density functional method augmented with an empirical dispersion term (DFT-D) is proposed and shown to provide very good results in both of the above described representative cases. (c) 2007 Wiley Periodicals, Inc.
Carbohelicenes are a group of helical-shaped polycyclic aromatic hydrocarbons. This study examined the effect of hexahelicene (or [6]helicene) and of its imidazolium derivative, 1-butyl-3-(2-methyl[6]helicenyl)-imidazolium bromide (I[6]H), on the activity of the aryl hydrocarbon receptor (AhR) and expression of cytochrome P450 1A1 (CYP1A1) in human hepatoma HepG2 cells. An MTT viability assay showed that both [6]helicene and I[6]H were cytotoxic to HepG2 cells after 24 h of exposure, with IC50 values of 0.9 and 8.4 μM, respectively. Using a gene reporter assay performed in transiently transfected HepG2 cells, we found that 1 μM [6]helicene, unlike I[6]H, significantly increased the activity of AhR to 2.1-fold compared to the control after 24 h of exposure. Moreover, [6]helicene induced a small but significant increase in the level of CYP1A1 mRNA. On the other hand, neither the protein level nor activity of CYP1A1 were affected by [6]helicene in HepG2 cells. The effect of [6]helicene on the AhR pathway was thus much lower than that of 2,3,7,8-tetrachlorodibenzo-p-dioxin, a potent AhR activator. We conclude that [6]helicene is a poor activator of the AhR pathway in HepG2 cells, and that the possible activation of the AhR pathway in vivo remains to be investigated.
The review deals with the chemistry of helically chiral polyaromatics represented by helicenes. Attention is paid to synthetic methods that can be used for the preparation of helicenes, heterohelicenes and helicene analogues. Of them, the Co- or Ni-catalysed [2+2+2] cycloisomerisation of aromatic triynes to form the helical scaffold, a method developed at the Institute is reviewed. After six decades of helicene chemistry, the interest in these unique aromatic compounds is gradually increasing. As the synthesis of helicenes has recently witnessed a significant progress, their applications in various branches of chemistry, physics and nanoscience can be expected.
- Klíčová slova
- heliceny,
- MeSH
- acetylen chemie MeSH
- katalýza MeSH
- pojmy organické chemie MeSH
- polycyklické sloučeniny * chemie MeSH
- stereoizomerie * MeSH
- vztahy mezi strukturou a aktivitou * MeSH
- Publikační typ
- práce podpořená grantem MeSH
In this short communication we report optimized procedures for the chiral separation of non-charged [6]helicene (1) and cationic derivative 1-butyl-3-(2-methyl[6]helicenyl)-imidazolium bromide (2) using high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) methods. The possibility of using capillary electrophoresis (CE) was also tested. The satisfactory results were obtained with SFC, where the highly selective resolution of four enantiopure 1 and 2 helicenes was achieved in a single run within 5min. The semi-preparative procedure for the isolation of P and M enantiomers of compound 2, including circular dichroism data, is reported here for the first time. The results could be used in further separations and analytical applications targeting carbohelicenes vs. positively charged helicene derivatives.
A series of diverse aromatic azadienetriyne and azatriynes was synthesised. These compounds were subjected to transition metal-mediated [2+2+2] cycloisomerisation to form pentacyclic or hexacyclic helically chiral azahelicene or azahelicene-like structures mostly in moderate yields. Introducing stereogenic centre(s) into selected azatriynes, cyclisation proceeded in a stereoselective fashion providing aza[5]helicenes or aza[6]helicene-like compounds in up to a 100:0 diastereomeric ratio. Gibbs energy differences between corresponding pairs of diastereomers (calculated at the DFT(B3LYP)/TZV+P level) were in good agreement with the experimental data and allowed for the prediction of the stereochemical outcome of the reaction. This study presents for the first time asymmetric synthesis of azahelicene derivatives in high optical purities.
Nonaqueous capillary electrophoresis (NACE) using methanol (MeOH) as a solvent of the BGEs and quantum mechanical density functional theory (DFT) have been applied to determine the thermodynamic acidity (ionization) constants (pKa ) of mono- and diaza[5]helicenes, mono- and diaza[6]helicenes, and their dibenzo derivatives in MeOH and water. First, the mixed acidity constants, pKa,MeOHmix${\rm{p}}K_{{\rm{a,MeOH}}}^{{\rm{mix}}}$ , of ionogenic pyridinium groups of azahelicenes and their derivatives in MeOH were obtained by nonlinear regression analysis of pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by NACE in a large series of methanolic BGEs within a wide conventional pH range (pHMeOH 1.6-12.0) and at ambient temperature (21-26°C) in a home-made CE device. Prior to mixed acidity constant calculation, the effective mobilities were corrected to reference temperature (25°C) and constant ionic strength (25 mM). Then, the mixed acidity constants were recalculated to the thermodynamic acidity constants pKa,MeOH by the Debye-Hückel theory of nonideality of electrolyte solutions. Finally, from the methanolic thermodynamic pKa,MeOH values, the aqueous thermodynamic pKa,H2O${\rm{p}}{K_{{\rm{a,}}{{\rm{H}}_{\rm{2}}}{\rm{O}}}}$ constants were estimated using the empirical relations between methanolic and aqueous acidity constants derived for structurally related pyridine derivatives. Depending on the number and position of the nitrogen atoms in their molecules, the analyzed azahelicenes were found to be weak to moderate bases with methanolic pKa,MeOH in the range 2.01-8.75 and with aqueous pKa,H2O${\rm{p}}{K_{{\rm{a,}}{{\rm{H}}_{\rm{2}}}{\rm{O}}}}$ in the range 1.67-8.28. The thermodynamic pKa,MeOH obtained by the DFT calculations were in a good agreement with those determined experimentally by NACE.
The oxidative photocyclization of aromatic Schiff bases was investigated as a potential method for synthesis of phenanthridine derivatives, biologically active compounds with medical applications. Although it is possible to prepare the desired phenanthridines using such an approach, the reaction has to be performed in the presence of acid and TEMPO to increase reaction rate and yield. The reaction kinetics was studied on a series of substituted imines covering the range from electron-withdrawing to electron-donating substituents. It was found that imines with electron-withdrawing substituents react one order of magnitude faster than imines bearing electron-donating groups. The 1H NMR monitoring of the reaction course showed that a significant part of the Z isomer in the reaction is transformed into E isomer which is more prone to photocyclization. The portion of the Z isomer transformed showed a linear correlation to the Hammett substituent constants. The reaction scope was expanded towards synthesis of larger aromatic systems, namely to the synthesis of strained aromatic systems, e.g., helicenes. In this respect, it was found that the scope of oxidative photocyclization of aromatic imines is limited to the formation of no more than five ortho-fused aromatic rings.
... Helicenes 1163 -- 14.8. Molecules with Planar Chirality 1166 a. Introduction 1166 b. ...
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