1,721,155 research outputs found
Structure/reactivity studies on an alpha-lithiated benzylsilane: Chemical interpretation of experimental charge density
No full textModern organic synthesis (e.g., of natural products) is virtually impossible without employment of enantiomerically enriched compounds. In many cases, alkyllithium compounds are key intermediates for the generation of these stereogenic substances. In recent years, the lithiated carbon atom in silicon-substituted benzyllithium compounds has become a focus of interest because it is possible to maintain its stereogenic information. Starting from a highly enantiomerically enriched benzylsilane, (R,S)-2.quinuclidine could be obtained, and the absolute configuration at the metalated carbon atom was determined by X-ray diffraction analysis. In solution, a quartet was found in the (13)C NMR spectrum for the metalated carbon atom because of coupling between carbon and lithium, indicating a fixed lithium carbon contact at room temperature. After reaction of (R, S)-2.quinuclidine with trimethylchlorostannane, the trapped product (S,S)-4 was obtained with a dr >= 98:2 with inversion of the configuration at the metalated carbon. Multipole refinement against high-resolution diffraction data and subsequent topological analysis of the benchmark system (R,S)-2.quinuclidine provide insight in the electronic situation and thus the observed stereochemical course of the transformations. Surprisingly, the negative charge generated at the carbanion hardly couples into the phenyl ring. The neighboring silicon atom counterbalances this charge by a pronounced positive charge. Therefore, the a-effect of the silicon atom is caused not just by a polarization of the electron density but also by an electrostatic bond reinforcement. Furthermore, the experimentally determined electrostatic potential unequivocally explains the observed back side attack of an electrophile under inversion of the stereogenic center with high diastereomere ratios
Insight into the Bonding and Aggregation of Alkyllithiums by Experimental Charge Density Studies and Energy Decomposition Analyses
No full textAntibacterial Azaphilones from an Endophytic Fungus, Colletotrichum sp BS4
No full textThree new compounds, colletotrichones A-C (1-3), and one known compound, chermesinone B (4a), were isolated from an endophytic fungus, Colletotrichum sp. BS4, harbored in the leaves of Buxus sinica, a well-known boxwood plant used in traditional Chinese medicine (TCM). Their structures were determined by extensive spectroscopic analyses including 1D and 2D NMR, HRMS, ECD spectra, UV, and IR, as well as single-crystal X-ray diffraction, and shown to be azaphilones sharing a 3,6a-dimethy1-9-(2-methylbutanoyl)-9H-furo[2,3-h]isochromene-6,8-dione scaffold. Owing to the remarkable antibacterial potency of known azaphilones coupled to the usage of the host plant in TCM, we evaluated the antibacterial efficacy of the isolated compounds against two commonly dispersed environmental strains of Escherichia coli and Bacillus subtilis, as well as against two human pathogenic clinical strains of Staphylococcus aureus and Pseudomonas aeruginosa. Compound 1 exhibited marked antibacterial potencies against the environmental strains that were comparable to the standard antibiotics. Compound 3 was also active against E. coli. Finally, compound 2a exhibited the same efficacy as streptomycin against the clinically relevant bacterium S. aureus. The in vitro cytotoxicity of these compounds on a human acute monocytic leukemia cell line (THP-1) was also assessed. Our results provide a scientific rationale for further investigations into endophyte-mediated host chemical defense against specialist and generalist pathogens
Epigenetic Modulation of Endophytic <i>Eupenicillium</i> sp. LG41 by a Histone Deacetylase Inhibitor for Production of Decalin-Containing Compounds
No full textAn endophytic fungus, Eupenicillium sp. LG41, isolated from the Chinese medicinal plant Xanthium sibiricum, was subjected to epigenetic modulation using an NAD(+)-dependent histone deacetylase (HDAC) inhibitor, nicotinamide. Epigenetic stimulation of the endophyte led to enhanced production of two hew decalin-containing compounds, eupenicinicols C and D (3 and 4), along with two biosynthetically related known compounds, eujavanicol A (1) and eupeniciincol A (2). The structures and stereochemistry of the new compounds were elucidated by extensive spectroscopic analysis using LC-HRMS, NMR, optical rotation, and ECU calculations, as well as single-crystal X-ray diffraction. Compounds 3 and 4 exist in chemical equilibrium with two and three, cis/trans isomers, respectively, as revealed by LC-MS analysis. Compound 4 was active against Staphylococcus aureus with an MIC of 0.1 mu g/mL and demonstrated marked cytotoxicity against the human acute monocytic leukemia cell line (THP-1). We have shown that the HDAC inhibitor, nicotinamide, enhanced the production of compounds 3 and 4 by endophytic Eupenicillium sp. LG41, facilitating their isolation, structure elucidation, and evaluation of their biological activities
Stereoselective synthesis of glycosides through novel catalytic method development
No full textCarbohydrates are one of the most prevalent natural product class with a wide range of structural and functional properties. Further, glycoside bond formation is one of the most important process in carbohydrate chemistry, particularly the production of O(S)-glycosides, which are abundant in bioactive compounds. To achieve this, glycosyl donors are synthesized and converted into reactive glycosylating species using catalysis. In this thesis, various activation strategies have been established to activate different glycosyl donors to synthesize a range of O(S)-glycosides, some of which are bioactive compounds. A phosphonochalcogenide (PCH) catalyzed strategy was developed to catalyze a stereoselective α-iminoglycosylation of iminoglycals with a wide range of glycosyl acceptors with remarkable protecting group tolerance in chapter 3. Mechanistic research revealed the catalyst's unexpected role in serially activating both the glycosyl donor and acceptor in the upstream and downstream stages of the reaction via chalcogen bonding (ChB). The dynamic interaction of chalcogens with substrates brings up new mechanistic possibilities based on repetitive ChB catalyst engagements and disengagements in multiple elementary steps. This research addressed the overall shortage of robust catalytic iminoglycosylations and provided a feasible approach for biologically relevant sp2-iminoglycosidic scaffolds. This methodology will demonstrate the enormously underexploited potential of sigma hole-based activation in broadening the frontiers of stereoselective carbohydrate and glycomimetic synthesis in the future. A synergistic chiral Rh(I) and organoboron-catalyzed protocol was introduced to catalyze site selective carbohydrate functionalization to synthesize biologically relevant anomeric aryl naphthalene glycosides with excellent enantioselectivity, diastereoselectivity and regioselectivity in chapter 4. The proper choice of an organoboron catalyst and ligands are critical to the success of this protocol. Following further investigation of this approach, my study revealed that structurally related allylic carbonate substrates were also well tolerated to furnish functionalized carbohydratesl with outstanding regio- and diastereoselectivity. This successful methodology would stimulate more effort in the development of chiral transition catalytic systems for demanding site-selective functionalizations of carbohydrates with prochiral electrophiles
Synthesis and reactivity of mono‐ and polycationic phosphines and amines
Full text linkLigands depicting π‐acceptor properties are able to facilitate those elementary steps of a catalytic cycle that require strong Lewis acidity at the metal center. Unfortunately, commercially available π‐acceptor ligands, e.g. PF3, P(CF3)3, PCl3 are either very toxic or moisture sensitive. Therefore, the design of new π‐acceptor ligands is still a challenge in the field of ligand design. The approach outlined in this thesis is based on the introduction of positively charged substituents as a way to impart π‐acceptor properties on the resulting ligands. In addition, it circumvents the moisture sensitivity issue avoiding any labile P‐X bond in the ligand structure.Continuing the study of the structure and reactivity of the cyclopropenium‐derived phosphines developed within our group, a second generation of strong π‐acceptor ligands was prepared by incorporating polyfluorinated aromatic substituents or by replacing the cyclopropenium moiety with the more electron‐withdrawing dihydroimidazolium unit. The superior catalytic activity of the Pt(II) complexes derived from these ligands in a hydroarylation reaction was demonstrated as well as the application of the new catalyst in the synthesis of natural products Chrysotoxene and Epimedoicarisoside A.As an extension of this work on cationic phosphines the analogous cationic amines were synthesized and fully characterized. The solid‐state structure analyses reveal unprecedented chemical environments around the central nitrogen atom. In contrast to their phosphorus analogues, the nitrogen atom in these cations adopts a trigonal planar environment, despite the computationally calculated lone pair of electrons and negative charge at nitrogen.Finally, in order to avoid stability issues often associated with polycationic ligands, chelating architectures, employing two electron‐rich cyclopropenimines were used for the stabilization of dicationic phosphines for the first time. A series of dications were synthesized in good yields by applying the –onium substituent transfer methodology. Despite their highly charged nature, these cations were able to coordinate Au(I) and Ag(I) metal centers and could also be oxidized to the phosphorus(V) compounds
Development of cyclopentadienyl ligands for asymmetric C‒H activation and synthesis of natural-product inspired compound collections
Full text linkEntwicklung Übergangsmetall-freier Cycloisomerisierungen in Hexafluorisopropanol
Full text linkDie zielgerichtete Entwicklung Metall-freier Reaktionen, um damit Übergangsmetalle in chemischen Prozessen zu substituieren, ist Thema aktueller Forschung. Dies hat den Hintergrund, dass Metall-Kontaminationen in Erzeugnissen der Industrie stark reguliert sind, da sie das Potential bergen, Menschen und Umwelt zu schädigen. Zudem wenden sich viele Zweige der Industrie nachhaltigeren Prozessen zu. Metalle sind in dieser Hinsicht ambivalent, da sie zwar sehr gut rezykliert werden können, aber die Gewinnung und Reinigung mit Belastungen verbunden ist.
Diese Arbeit zeigt eine Möglichkeit auf, wie typischerweise Metall-katalysierte Reaktionen durch BRØNSTEDT-Säuren ersetzbar sind. Dazu wird das von AUBÉ publizierte System von Acetylchlorid (AcCl) in Hexafluorisopropanol (HFIP) verwendet und auf Trimethylsilylchlorid (TMSCl) erweitert. Die Reaktion der beiden Reagenzien mit HFIP erzeugt in situ HCl. Dieses kann genutzt werden um Propargylamide zu cyclisieren. Für Propargylamide mit terminalen Dreifachbindungen werden Oxazole (und ihre Hydrochloride) gewonnen, die Reaktion von Propargylamiden mit internen Dreifachbindungen ergeben in der Mehrzahl Oxazoline mit (Z) konfigurierten exo-ständigen Doppelbindung
Selektive Transformationen von Alkoxysilanen: Reaktivitätsuntersuchungen an Si–N,O-funktionalisierten Silanen
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