Browsing by Author "Laschat, Sabine (Prof. Dr.)"

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    Darstellung und Charakterisierung von chiralen ionischen Flüssigkeiten mit mesogenen Eigenschaften
    (2008) Tosoni, Martin; Laschat, Sabine (Prof. Dr.)
    Ziel der vorliegenden Arbeit war die Entwicklung von chiralen ionischen Flüssigkeiten mit mesogenen Eigenschaften, wobei Imidazolium-, Pyridinium-, und Benzimidazolium-Derivate als kationische Grundkörper gewählt wurden. Es wurden von einigen chiralen ionischen Flüssigkeiten die elektrischen Leitfähigkeiten in Lösung, sowie die Zersetzungsspannungen bestimmt. Es stellte sich heraus, dass unter gewissen Voraussetzungen mesogene Phasen beobachet werden konnten. Hierbei handelte es sich meist um smektische Mesophasen.
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    Diastereoselective alkylation of bi- and tricyclic lactimethers as a pathway to biologically relevant diketopiperazines
    (2007) Hendea, Daniela; Laschat, Sabine (Prof. Dr.)
    Bi- and tricyclic diketopiperazines prove to be useful templates for diastereoselective alkylation. This synthetic route should allow a novel access to diketopiperazine alkaloids, such as tryprostatins and quinocarcins. Tryprostatins are secondary metabolites of an Aspergillus fumigatus marine strain and are inhibitors of the mammalian cell cycle. Quinocarcin has moderate activity against Gram(+)-bacteria and its citrate salt has shown potent antitumor activity against several tumor cell lines. Retrosynthetically, tryprostatin B 20b could be obtained from indole-derived bicyclic diketopiperazine 65, by regioselective prenylation. Breaking the diketopiperazine 65 further down, it derives from two building blocks, the proline-derived lactim ether 63 and bromomethyl indole 66 that could be assembled by stereoselective alkylation at C-3 of diketopiperazine molecule 73. The direct approach to the total synthesis of tryprostatin B 20b was preceded by the investigation of the stereoselective alkylation on a model system, the lactimether derived from the diketopiperazine 73, with different electrophiles, leading to the alkylated lactim ethers 75. In order to confirm the preliminary stereochemical assignment of the lactim ethers 75 and the corresponding diketopiperazines 76 based on NMR and NOE experiments, the diketopiperazines cis-76a,d,e,h were prepared from methyl L-alaninate (77a), L-leucinate (77b), L-valinate (77c) and L-phenylalaninate (77d). L-proline derivative 69 was condensed with the amino acid esters 77 in the presence of EDC and HOBT to give intermediate dipeptides which were immediately cyclized to the diketopiperazines cis-76. The cis-diketopiperazines resulted by cyclization were compared by 1H, 13C-NMR, optical rotation and X-ray crystal structures with the corresponding diketopiperazines obtained by alkylation, leading to the correct assignment of the diastereomers 76. Based on these alkylation experiments, the alkylation reaction of the lactim ether 63 with the bromoindole derivative 66 was carried out under similar reaction conditions. The second major part consists in the synthetic approach to the quinocarcin system. Retrosynthetically, the quinocarcin system 83 can be obtained via allylsilane addition to an iminium ion, in the allyl-alkylated molecule 124. Breaking the molecule 126 down, it derives from two building blocks, the trycliclic diketopiperazine 134 and (E)-4-bromo-2-butenyl-trimethylsilane 128 that could be assembled by stereoselective alkylation at C-3 of diketopiperazine molecule 134.
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    Funktionalisierte Pentalen-Fragmente in der Naturstoffsynthese : asymmetrische Deprotonierung und α-Funktionalisierung als Schlüsselschritte
    (2012) Lutz, Vanessa; Laschat, Sabine (Prof. Dr.)
    Ziel der Arbeit war, eine allgemeine Syntheseroute zur Darstellung von mehrfach enantioselektiv funktionalisierten Pentalen-Derivaten für die Naturstoffsynthese auszuarbeiten. Hierfür wurden Weisssche Diketone als einfache Vorläufermoleküle entsprechend modifiziert. Schlüsselschritte der erarbeiteteten Synthesen stellten die asymmetrische Desymmetrisierung mit chiralen Basen sowie die anschließende alpha-Funktionalisierung mit verschiedenen Elektrophilen dar. Als chirale Basen wurden Lithiumamide verwendet, als Elektrophile wurden Alkylhalogenide eingesetzt.
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    Lumineszierende Flüssigkristalle auf Basis multiresonanter Bor-dotierter Heteroaromaten
    (2025) Knöller, Julius A.; Laschat, Sabine (Prof. Dr.)
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    Synthesis, characterization and electronic applications of novel calamitic azulenes
    (2023) Schulz, Finn; Laschat, Sabine (Prof. Dr.)
    Due to its inherent dipole moment and unique electronic properties the azulene moiety is of interest in the development of novel organic semiconductors for optoelectronic applications. Molecular orientation of the applied organic semiconductors is crucial for the performance of organic electronics and can be controlled to some extent e.g., through self-assembly of liquid crystals. We developed a divergent synthetic route starting from 2,6-dibromoazulene to access novel, calamitic mesogens and investigated these azulenes on their mesomorphic behavior and -for selected examples- on their suitability as organic semiconductors. Azulenes with only one side chain tethered to the 7-membered ring exclusively formed soft-crystalline smectic E-phases (SmE), with 2-(6-(dodecyloxy)azulen-2-yl)benzo[b]thiophene(12O-Az-BT) exhibiting the broadest phase width between 51 °C and 270 °C during cooling. Additionally, derivatives with two side chains and a lateral substituent at the 5-membered ring were prepared. Based on 6-dodecyloxy-2-(4-dedecyloxyphenyl)azulene-1-carbonitrile (12O-AzCN-PhO12), parameters such as chain length, size, and polarity of the lateral substituent, and (hetero)aromatic moiety in 2-position were systematically varied to understand the complex structure-property-relationship governing mesomorphic properties of our novel azulenes. Mainly SmC phases were observed for derivatives with unhindered interaction of the aromatic azulene cores. Upon disturbance of the stacking due to bulky substituents, less polar side chains or the electronic influence of heteroatoms, less ordered SmA phase were the preferred mesophase. Some azulene derivatives with a SmA-SmC phase sequence were found to show de Vries-like properties, which are heavily investigated in the context of next generation liquid crystal displays. The best values for important de Vries parameters, namely a maximum layer shrinkage of only 0.16 % resulting in reduction factors of down to 0.14 compete with state of the art deVries materials. Additionally, 6-hexadecyloxy-2-(4-octyloxyphenyl)azulene-1-carbonitrile (16O-AzCN-PhO8) exhibited a SmA-SmC-SmA phase sequence. The rarely observed SmA re-entrant phase was explained by a reorientation of the azulene cores which was also identified as a key factor for de Vries-like behavior. A selected series of azulenes modified with thiophene units was investigated on their suitability as semiconducting layer in organic field effect transistor (OFET) devices. Spin-coating of these azulene derivatives at mesophase temperature resulted in uniform polycrystalline thin films. In the corresponding OFET devices, 12O-Az-BT achieved the best hole mobility with μ+ = (3.1 ± 0.5)∙10-3cm2V-1s-1.