Browsing by Author "Laschat, Sabine"

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Open Access
Application of intramolecular Heck reactions to the preparation of steroid and terpene intermediates having cis A-B ring fusions - model studies for the total synthesis of complex cardenolides
(1994) Laschat, Sabine; Narjes, Frank; Overmann, Larry E.
The cis-fused tricyclic dienone 13 is the major product formed from intramolecular Heck cyclization of the dienyl triflate 12 (Scheme I). Similarly, the cis-hexahydrophenanthridine 22 is formed in good yield from Heck cyclization of the aryl triflate 21. This latter conversion demonstrates that allylic ether substitution is compatible with intramolecular Heck chemistry and suggests applications of this chemistry in the synthesis of highly oxidized cardenolides.
Open Access
Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids
(2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, Sabine
Covalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.
Open Access
Carbohydrates as chiral templates : diastereoselective synthesis of N-glycosyl-N-homoallylamines and β-amino acids from imines
(1991) Laschat, Sabine; Kunz, Horst
Complexing properties and chirality of carbohydrates were utilized in diastereoselective reactions of 0-pivaloylated N-galactosylimines with allylsilanes and -stannaries. With dyltrimethylsilane in the presence of SnCl4, imines 2 of aromatic and heteroaromatic aldehydes were converted to homdylamines 3, giving ratios of diastereomers higher than 7:1. No addition products derived from α-anomeric aromatic imines were formed. Aliphatic homdylamines 3 were synthesized by using allyltributylstannane in the presence of SnCl4. Both α- and β-anomeric aliphatic imines reacted with the allylstannane. They gave the same ratio of diastereomers and showed the same sense of asymmetric induction.
Open Access
Carbohydrates as chiral templates : stereoselective synthesis of (R)- and (S)-α-aminophosphonic acid derivatives
(1992) Laschat, Sabine; Kunz, Horst
The stereoselective synthesis of diethyl (S)- or (R)-α-[(O-pivaloyl-hexapyranosyl) amino]benzylphosphonates is achieved via Lewis acid catalyzed addition of diethyl phosphite to O-pivaloylated N-benzylidene -β-D-galactosylamine or N-benzylidene-α-D-arabinopyranosylamine. The process can also be performed by a one-pot procedure selectively giving (S)-aminophosphonic acid derivatives from galactosylamine and (R)-aminophosphonic acid derivatives from β-L-fucosylamine as the chiral auxiliaries.
Open Access
Carbohydrates as chiral templates : stereoselective synthesis of (R)-homoallyl amines using L-fucose as the auxiliary formally enantiomeric to D-galactose
(1990) Laschat, Sabine; Kunz, Horst
(R)-Homoallyl amines are synthesized with high asymmetric induction using the diastereoselective Lewis acid induced addition of allylsilanes and allylstannanes to Schiff bases of O-acyl protected fucopyranosylamine.
Open Access
Carbohydrates as chiral templates : stereoselective synthesis of chiral homoallyl amines and β-amino acids
(1990) Laschat, Sabine; Kunz, Horst
Enantiomerically pure homoallyl amines and β-amino acids are synthesized using the diastereoselective Lewis acid induced addition of allylsilanes and allylstannanes to Schiff bases of O-aryl protected galactopyranosylamine.
Open Access
Charged triazole cross-linkers for hyaluronan-based hybrid hydrogels
(2016) Martini, Maike; Hegger, Patricia S.; Schädel, Nicole; Minsky, Burcu B.; Kirchhof, Manuel; Scholl, Sebastian; Southan, Alexander; Tovar, Günter E. M.; Boehm, Heike; Laschat, Sabine
Polyelectrolyte hydrogels play an important role in tissue engineering and can be produced from natural polymers, such as the glycosaminoglycan hyaluronan. In order to control charge density and mechanical properties of hyaluronan-based hydrogels, we developed cross-linkers with a neutral or positively charged triazole core with different lengths of spacer arms and two terminal maleimide groups. These cross-linkers react with thiolated hyaluronan in a fast, stoichiometric thio-Michael addition. Introducing a positive charge on the core of the cross-linker enabled us to compare hydrogels with the same interconnectivity, but a different charge density. Positively charged cross-linkers form stiffer hydrogels relatively independent of the size of the cross-linker, whereas neutral cross-linkers only form stable hydrogels at small spacer lengths. These novel cross-linkers provide a platform to tune the hydrogel network charge and thus the mechanical properties of the network. In addition, they might offer a wide range of applications especially in bioprinting for precise design of hydrogels.
Open Access
Columnar propeller‐like 1,3,5‐triphenylbenzenes : probing the effect of chlorine on the Suzuki cross‐coupling and liquid crystalline properties
(2020) Grunwald, Marco André; Wöhrle, Tobias; Forschner, Robert; Baro, Angelika; Laschat, Sabine
Suzuki cross‐couplings either between chlorinated N‐methyliminodiacetic acid (MIDA)‐protected aryl boronates and 1,3,5‐tribromobenzene or between chlorinated aryl bromides and phenyltrisboronic species to star‐shaped 1,3,5‐triphenylbenzenes with different substitution patterns and chloro substituents at the outer phenyl rings were studied. The chlorinated precursors required for the respective reaction were synthesized and characterized. Depending on the used coupling reaction target triphenylbenzenes were isolated in yields between 42 % and 88 %. Their mesomorphic properties were influenced by the substitution pattern and number of peripheral chlorine atoms. Triphenylbenzene with 3,5‐alkoxy substitution and H in para‐position self‐assembled into either columnar hexagonal (Colh) mesophases or a soft crystal. While threefold chloro substitution in meta‐position of the outer phenyl rings led to stable room temperature Colho phases, triphenylbenzenes with threefold para‐chloro or 3,5‐dichloro substitution were non‐mesomorphic. Based on X‐ray diffraction data a helical packing model for the observed phases similar to that of related alkoxy‐substituted triphenylbenzenes was proposed.
Open Access
A complementary experimental and theoretical approach for probing the surface functionalization of ZnO with molecular catalyst linkers
(2023) Kousik, Shravan R.; Solodenko, Helena; YazdanYar, Azade; Kirchhof, Manuel; Schützendübe, Peter; Richter, Gunther; Laschat, Sabine; Fyta, Maria; Schmitz, Guido; Bill, Joachim; Atanasova, Petia
The application of ZnO materials as solid-state supports for molecular heterogeneous catalysis is contingent on the functionalization of the ZnO surface with stable self-assembled monolayers (SAMs) of catalyst linker molecules. Herein, experimental and theoretical methods are used to study SAMs of azide-terminated molecular catalyst linkers with two different anchor groups (silane and thiol) on poly and monocrystalline (0001, ) ZnO surfaces. Angle-resolved and temperature-dependent X-ray photoelectron spectroscopy (XPS) is used to study SAM binding modes, thermal stabilities, and coverages. The binding strengths and atomistic ordering of the SAMs are determined via atom-probe tomography (APT). Density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations provide insights on the influence of the ZnO surface polarity on the interaction affinity and conformational behavior of the SAMs. The investigations show that SAMs based on 3-azidopropyltriethoxysilane possess a higher binding strength and thermal stability than the corresponding thiol. SAM surface coverage is strongly influenced by the surface polarity of ZnO, and the highest coverage is observed on the polycrystalline surface. To demonstrate the applicability of linker-modified polycrystalline ZnO as a catalyst support, a chiral Rh diene complex is immobilized on the azide-terminal of the SAM and its coverage is evaluated via XPS.
Open Access
Coumarin-4-ylmethyl- and p-hydroxyphenacyl-based photoacid generators with high solubility in aqueous media: synthesis, stability and photolysis
(2020) Adatia, Karishma K.; Halbritter, Thomas; Reinfelds, Matiss; Michele, Andre; Tran, Michael; Laschat, Sabine; Heckel, Alexander; Tovar, Günter E. M.; Southan, Alexander
(Coumarin‐4‐yl)methyl (c4m) and p‐hydroxyphenacyl (pHP)‐based compounds are well known for their highly efficient photoreactions, but often show limited solubility in aqueous media. To circumvent this, we synthesized and characterized the two new c4m and pHP‐based photoacid generators (PAGs), 7‐[bis(carboxymethyl)amino]‐4‐(acetoxymethyl)coumarin (c4m‐ac) and p‐hydroxyphenacyl‐2,5,8,11‐tetraoxatridecan‐13‐oate (pHP‐t), and determined their solubilities, stabilities and photolysis in aqueous media. The two compounds showed high solubilities in water of 2.77 mmol L−1±0.07 mmol L−1 (c4m‐ac) and 124.66 mmol L−1±2.1 mmol L−1 (pHP‐t). In basic conditions at pH 9, solubility increased for c4m‐ac to 646.46 mmol L−1±0.63 mmol L−1, for pHP‐t it decreased to 34.68 mmol L−1±0.62 mmol L−1. Photochemical properties of the two PAGs, such as the absorption maxima, the maximum molar absorption coefficients and the quantum yields, were found to be strongly pH‐dependent. Both PAGs showed high stabilities s24h ≥95 % in water for 24 h, but decreasing stability with increasing pH value due to hydrolysis. The present study contributes to a clearer insight into the synthesis, solubilities, stabilities, and photolysis of c4m and pHP‐based PAGs for further photochemical applications when high PAG concentrations are required, such as in polymeric foaming.
Open Access
Counterion effects on the mesomorphic and electrochemical properties of guanidinium salts
(2024) Ebert, Max; Lange, Alyna; Müller, Michael; Wuckert, Eugen; Gießelmann, Frank; Klamroth, Tillmann; Zens, Anna; Taubert, Andreas; Laschat, Sabine
Ionic liquid crystals (ILCs) combine the ion mobility of ionic liquids with the order and self-assembly of thermotropic mesophases. To understand the role of the anion in ILCs, wedge-shaped arylguanidinium salts with tetradecyloxy side chains were chosen as benchmark systems and their liquid crystalline self-assembly in the bulk phase as well as their electrochemical behavior in solution were studied depending on the anion. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS) experiments revealed that for spherical anions, the phase width of the hexagonal columnar mesophase increased with the anion size, while for non-spherical anions, the trends were less clear cut. Depending on the anion, the ILCs showed different stability towards electrochemical oxidation and reduction with the most stable being the PF6 based compound. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest a possible contribution of the guanidinium cation to the oxidation processes.
Open Access
Diastereoselective synthesis of α-Hydroxy- and α-amino-indolizidines and -quinolizidines : evidence for a novel cyclization/hydride migration mechanism in the TiCl4-induced reaction of prolinalbenzylimines by deuterium labeling studies
(1994) Laschat, Sabine; Grehl, Matthias
Lewis acid-catalyzed cyclization of prolinal and 2-piperidine-carbaldehyde benzylimines 11, 12 results in the diastereoselective formation of α-amino-β-alkyl-substituted indolizidines 15, 17, 19, 21 and -quinolizidines 16, 18, 20, respectively. Both diastereoselectivity and constitution depend on the Lewis acid. FeCl3 yields α,β- trans-α-(benzylamino)-β-isopropenyl derivatives 15 and 16, probably by a cationic cyclization via carbenium ions 32a, b. In contrast, TiCl4 yields ,α,β- cis-α-(benzylideneamino)-β-isopropyl derivatives 19 and 20 by a novel cyclization/intermolecular hydride transfer mechanism, which was supported by deuterium labeling studies. Compounds 15, 16, 19, and 20 were converted to the diastereomeric acetamides 24, 25 and 28, 29. By an analogous cyclization of the aldehydes 8 and 9 only α ,β-cis-α-hydroxy--isopropenylindolizidines 51 and -quinolizidines 52 were obtained irrespective of the Lewis acid used. The structures of 30 and 52 were elucidated by X-ray analysis.
Open Access
Diastereoselective synthesis of amino-substituted indolizidines and quinolizidines by the intramolecular hetero-ene reaction of prolinal imine and 2-piperidine carbaldimine
(1994) Laschat, Sabine; Grehl, Matthias
We have concerned ourselves with the question of the extent to which diastereoselectively substituted indolizidines and quinolizidines can be constructed with such hetero-ene reactions particularly in view of the fact that a range of these alkaloid derivatives are effective glucosidase inhibitors and are thus of increasing interest because of their Anti-HIV activity.
Open Access
Diastereoselektive Synthese von Amino-substituierten Indolizidinen und Chinolizidinen aus Prolinalimin bzw. 2-Piperidincarbaldimin durch intramolekulare Hetero-En-Reaktion
(1994) Laschat, Sabine; Grehl, Matthias
Wir haben uns mit der Frage beschäftigt, inwieweit mit solchen Hetero-En-Reaktionen diastereoselektiv substituierte Indolizidine und Chinolizidine hergestellt werden können, insbesondere im Hinblick darauf, daß eine Reihe dieser Alkaloid-Derivate wirksame Glucosidase-Inhibitoren sind und wegen ihrer Anti-HIV-Aktivitat zunehmendes Interesse gewinnen.Im folgenden berichten wir über eine neue formal intramolekulare Lewis-Säure-katalysierte Imino-En-Reaktion zum diastereoselektiven Aufbau von 7-Alkenyl-8-amino-substituierten Indolizidinen und von 2-Alkyl-1-amino-substituierten Chinolizidinen, bei der sich die relative Konfiguration der beiden neugebildeten Stereozentren durch die Wahl der Lewis-Säure steuern läßt.
Open Access
Efficient and spatially controlled functionalization of SBA‐15 and initial results in asymmetric Rh‐catalyzed 1,2‐additions under confinement
(2021) Beurer, Ann‐Katrin; Kirchhof, Manuel; Bruckner, Johanna R.; Frey, Wolfgang; Baro, Angelika; Dyballa, Michael; Giesselmann, Frank; Laschat, Sabine; Traa, Yvonne
Selectively functionalized mesoporous silica may considerably advance heterogeneous catalysis through the controlled immobilization of highly selective complex catalysts inside the mesopores. However, spatially controlled functionalization and the precise analytical verification are still a challenge. In this publication, we report a method, which ensures a selective functionalization of the mesopore surface with a clickable linker and thus makes it possible to study confinement effects during catalyzed reactions. First, we passivate the silanol groups on the particle surface and in the pore entrances of the mesoporous silica material SBA‐15 with 1,1,1‐trimethyl‐N‐(trimethylsilyl)silanamine. Then we remove the template by solvent extraction and functionalize the pore walls with 3‐azidopropyltriethoxysilane before we click the catalyst. In initial experiments of asymmetric Rh‐catalyzed 1,2‐addition, we investigate the performance of a catalyst clicked selectively in the mesopores and compare it to the dissolved catalyst as well as to the catalyst immobilized exclusively on the external surface of SBA‐15.
Open Access
Enantioenriched boron C,N‐chelates via chirality transfer
(2023) Stöckl, Yannick; Tait, Ethan J.; Frey, Wolfgang; Wegner, Sascha; Claasen, Birgit; Zens, Anna; Laschat, Sabine
Molecules stereogenic only at tetrahedral boron atoms show great promise for applications, for example as chiroptical materials, but are scarcely investigated due to their synthetic challenge. Hence, this study reports a two-step synthesis of enantioenriched boron C,N-chelates. First, the diastereoselective complexation of alkyl/aryl borinates with chiral aminoalcohols furnished boron stereogenic heterocycles in up to 86 % yield and d.r. >98 : 2. Treatment of these O,N-complexes with chelate nucleophiles was surmised to transfer the stereoinformation via the ate-complex into the C,N-products. This chirality transfer succeeded by substitution of the O,N-chelates with lithiated phenyl pyridine to give boron stereogenic C,N-chelates in up to 84 % yield and e.r. up to 97 : 3. The chiral aminoalcohol ligands could be recovered after isolation of the C,N-chelates. The chirality transfer tolerated alkyl, alkynyl and (hetero-)aryl moieties at boron and could be further extended by post-modification: transformations such as catalytic hydrogenations or sequential deprotonation/electrophilic trapping were feasible while maintaining the stereochemical integrity of the C,N-chelates. Structural aspects of the boron chelates were studied by variable temperature NMR and X-ray diffraction.
Open Access
Hierarchical silica inverse opals as a catalyst support for asymmetric molecular heterogeneous catalysis with chiral Rh‐diene complexes
(2021) Deimling, Max; Kousik, Shravan R.; Abitaev, Karina; Frey, Wolfgang; Sottmann, Thomas; Koynov, Kaloian; Laschat, Sabine; Atanasova, Petia
The efficacy of heterogeneous catalysis relies heavily on diffusion and distribution of reactants within catalyst supports. However, the presence of confinement, essential for reaction selectivity, drastically slows down molecular transport. Here, macro‐mesoporous silica inverse opal (SiO2-IO) films were used as a model system to study the rather unexplored molecular infiltration behavior using a probe molecule resembling a catalyst via confocal laser scanning microscopy (CLSM). CLSM analysis revealed homogeneous tracer distribution in SiO2-IO and attachment to both transport and mesopores. Bulk macro‐mesoporous SiO2-IO support was used for the attachment of mono‐ and bis‐functionalized chiral Rh‐diene complexes, and the catalytic activity and selectivity with respect to the support was studied. Lower enantioselectivity was observed with the bis‐functionalized ligand due to ligand entanglement and reduced accessibility of the active site, while the mono‐functionalized ligand gave an excellent enantioselectivity of 94 %ee in the asymmetric 1,2‐addition of triphenylboroxine to N‐tosylimines and could be recycled up to three times.
Open Access
Intramolecular hetero-Diels-Alder reaction of N-arylimines : applications to the synthesis of octahydroacridine derivatives
(1993) Laschat, Sabine; Lauterwein, Jürgen
A new intramolecular Lewis acid catalyzed hetero-Diels-Alder reaction of N-arylimines 5 with nonactivated olefins tethered to the 2-azadiene system was developed in order to prepare 1,2,3,4,-4a,9,9a,l0-octahydroacridined erivatives 6. Both reactivity and cis/ trans selectivity of 6 were mainly dependent on the substitution pattern in the 3-position of the cyclization precursor 5. The type of Lewis acid plays only a minor role in determination of the cis/trans ratio. The synthetic utility of this new cyclization was increased by performing it as a one-pot reaction. Both absolute configuration and preferred conformation were assigned by detailed NMR studies.
Open Access
Intramolecular hetero-Diels-Alder reaction of prolinal-derived N-arylimines : Lewis acid-dependent reversal of the diastereoselectivity
(1994) Linkert, Frank; Laschat, Sabine
Intramolecular Lewis acid-catalyzed hetero-Diels-Alder reaction of L-prolinal-derived N-arylimine 1 yielded diastereoselectively either the cis,cis-azacyclopenta[a]anthracene derivative 2a or the trans,trans-isomer 2b, depending on the Lewis acid.
Open Access
Liquid crystalline self-assembly of mixtures of rod- and wedge-shaped MIDA boronates
(2025) Schilling, Christopher; Bauch, Soeren; Wuckert, Eugen; Zens, Anna; Bruckner, Johanna R.; Laschat, Sabine
Rod-like MIDA boronates form smectic mesophases, while wedge-shaped MIDA boronates self-assemble into columnar mesophases. However, the phase behavior of mixtures is less understood. In order to obtain further insight on the molecular self-assembly of MIDA boronate mixtures two series of binary mixtures of rod-like and wedge-shaped mesogens were prepared. The phase behavior was studied using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). The study revealed a strong dependency of the mesophase structure on the mesogen composition. Usage of a less bulky columnar mesogen suppressed the formation of columnar mesophases in the mixture and led to a decrease in melting and clearing temperatures. The phase behavior is discussed in terms of the packing parameter model typically applied for lyotropic liquid crystals.