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Greener Organic Transformations by Plant-Derived Water Extract Ashes
Published in Ahindra Nag, Greener Synthesis of Organic Compounds, Drugs and Natural Products, 2022
The unique structural appearance, i.e., axial chirality, is the key feature of biaryls for the synthesis of chiral ligands, as, for example, BINOL, (S)-BINAP, (R)-SL-O103-1, (R)-BIPHEMP, and (R)-SLO106-1.90 Therefore, biaryl motifs containing some examples of natural products, drugs, and ligands are highlighted in Figure 10.5. Biaryl skeleton systems have received great attention due to several biological and pharmacological properties like anti-inflammatory, antibiotic, antibacterial, anticancer, antifungal, antimicrobial, antitumor, antagonist, antiproliferative, antihypertension, antituberculosis, and analgesic.81f,85f,91 Some examples of biologically active biaryl derivatives and electron-conducting materials are presented in Figure 10.5.
Aromatic Helicenes
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2020
Importantly, various synthetic methodologies were published to transform nonracemic biaryl precursors into nonracemic helicenes, thus demonstrating an excellent relay of stereochemical information when transforming axial chirality into helicity. An efficient synthetic route to highly enantioenriched 1-aza[6]helicenes 28 was developed by Fuchter et al. who succeeded in converting the separated atropisomers of the axially chiral biaryl (separated by semipreparative HPLC on a chiral column) through alkyne–arene cycloisomerization into azahelicene products (Weimar et al., 2013). Srebro-Hooper, Crassous, Guy et al. built an enantiopure backbone of the diazahelicene-like dibenzo[c]acridine derivative from the optically pure axially chiral bis-tetralone, which was obtained from racemate by preferential crystallization (it formed conglomerate) (Bensalah-Ledoux et al., 2016). Kamikawa et al. converted an enantiopure biaryl building block (obtained by resolution of the corresponding racemate by liquid chromatography on a chiral column) into enantiopure 6-aza[6]helicene in good yield by utilizing a palladium-catalyzed C-H annulation reaction (Kaneko et al., 2013). Nozaki et al. reported an efficient strategy for the synthesis of highly enantioenriched aza- and oxa[7]helicenes from the nonracemic biaryl precursor (4,4′-biphenanthryl-3,3′-diol) in good yields (Nakano et al., 2005).
Alcohols and Phenols as Hydrogen Bonding Catalysts
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
The ability of TADDOL-based alcohols to function as hydrogen bonding catalysts was utilized by the Yamamoto and Rawal groups in 2005 for the development of binaphthalene-derived alcohol catalysts possessing axial chirality [28]. Chiral 1,1′-biaryl-2,2′-dimethanol (BAMOL) derivatives were developed as hydrogen bond donors for the catalysis of HDA reactions of electron-rich diene 13 with aromatic and aliphatic aldehydes (Scheme 2.7). An initial catalyst screening revealed BAMOLs 25 and 26 as the best-performing catalysts in terms of both reactivity and selectivity. The resulting dihydropyrone products were obtained in good to high yields and with high levels of enantioselectivity. It is worth noting that a broad range of aliphatic aldehydes could be used as substrates in this methodology. The mechanism of action of the BAMOL catalysts was proposed by the authors to be similar to TADDOL-based hydrogen bonding catalysts. This proposal was supported by the crystal structure of 1:1 complex of BAMOL 27 with benzaldehyde (Scheme 2.7). The X-ray crystallographic analysis of this 1:1 complex revealed an intramolecular hydrogen bond between the two hydroxyls of BAMOL 27 resulting in the formation of a nine-membered ring. A second, intermolecular hydrogen bond is present between the free hydroxyl of BAMOL 27 and the carbonyl oxygen of benzaldehyde.
Enantioselective behavior of environmental chiral pollutants: A comprehensive review
Published in Critical Reviews in Environmental Science and Technology, 2022
Marina Arenas, Julia Martín, Juan Luis Santos, Irene Aparicio, Esteban Alonso
PCB congeners with free lateral positions in one of the phenyl rings are metabolized to methylsulfonyl-PCBs (MeSO2-PCBs) and hydroxylated PCBs (OH-PCBs) (Norström et al., 2006). Similar to their PCB precursors, MeSO2-PCB congeners exhibit axial chirality. Therefore their enantioselective transformation and toxic effects in living organisms cannot be excluded. Of the metabolites of PCBs containing MeSO2, the 3- and 4-substituted compounds have been found to persist in biota (Letcher et al., 1998). Wiberg et al. (1998) found para and meta substituted MeSO2-CB91, MeSO2-CB95, MeSO2-CB132, MeSO2-CB149, and MeSO2-CB174 in biota. Lastly, Kato et al. reported that PCB91 and PCB95 and their metabolites with more unsubstituted carbons in the meta-position bind better to the cytochrome (CYP) enzymes than metabolites with more substitutions in the meta-position (Kato et al., 1995; 1997). OH-PCBs are also considered a new class of contaminants with higher polarity than their parent compounds, which makes them more toxic and mobile. Different behaviors of OH-PCBs have been observed in living organisms (poplars). Zhai et al. (2014) observed that 5-OH-PCB 95 showed a high enantioselective biotransformation while 5- OH-PCB 91 were equally transported and metabolized in whole poplars. The fact that 5-OH-PCB91 has two unsubstituted carbon atoms at the meta-position of the non-phenyl ring, and that 5-OH-PCB95 has one unsubstituted carbon would explain the different behaviors of these compound in poplars.
Cation–π interaction of thallium (I) with [7]helicene: an experimental and theoretical study
Published in Molecular Physics, 2022
Petr Vaňura, David Sýkora, Stanislav Böhm, Tereza Uhlíková
Ortho-fused aromatic molecules can be rendered helical or spirally coiled due to the inability of conjoined rings to occupy the same plane. This is a case for molecules, where the number of benzene rings exceeds 4. These molecules termed the helicenes display a non-planar topology with C2-symmetric axis perpendicular to the axis of helicity because of the steric repulsive interaction between terminal aromatic rings [3–6]. Helicenes are good π-donors and can form charge-transfer complexes with many π-acceptors. In addition, the π–π interactions play an important role in determining both the properties and the self-assembly behaviour of helicenes in either the solution or the solid state. Helicenes are notable for having chirality despite lacking both asymmetric carbons and chiral centres. Instead, there is axial chirality, which results from the handedness of the helicity itself. The helical topology of carbohelicenes causes a high optical rotation, high circular dichroism values and several enhanced physical-organic properties [7].
Twist-bend nematic phases of banana-shaped molecules with an axial chirality
Published in Liquid Crystals, 2019
Consider a bent-core molecule, or a banana-shaped molecule, connecting two cuboids (mesogens) of the length , width , and height (Figure 1(a)). Let be the bend angle between the arm’s unit vectors and on the orthonormal molecular axes . The fixed angle between the vectors and is given by . The molecular axis is also an axis of reflection-rotation by an angle (torsion angle), which is the fixed angle for the arm vector and is the opposite sign () for the arm vector . Then the banana-shaped molecule has two possible conformations with the “axial chirality” [36], and there are mutual enantiomers. The two conformations are taken to have intrinsically equal weights, thus rendering the molecule statistically achiral. This model for the bent-core molecules has been introduced by Vanakaras and Photinos [49]. The unit vectors along the arm of the bent-core molecule are given by