Showing papers by "Bernd Plietker published in 2019"

The Polycyclic Polyprenylated Acylphloroglucinol Antibiotic PPAP 23 Targets the Membrane and Iron Metabolism in Staphylococcus aureus.

Abstract: Recently, a series of endo-type B polycyclic polyprenylated acylphloroglucinols (PPAP) derivatives with high antimicrobial activities were chemically synthesized. One of the derivatives, PPAP 23, which showed high antimicrobial activity and low cytotoxicity, was chosen for further investigation of its bactericidal profiles and mode of action. PPAP 23 showed a better efficacy in killing methicillin resistant Staphylococcus aureus (MRSA) and decreasing the metabolic activity of 5-day-old biofilm cells than vancomycin. Moreover, S. aureus did not appear to develop resistance against PPAP 23. The antimicrobial mechanism of PPAP 23 was investigated by RNA-seq combined with phenotypic and biochemical approaches. RNA-seq suggested that PPAP 23 signaled iron overload to the bacterial cells because genes involved in iron transport were downregulated and iron storage gene was upregulated by PPAP 23. PPAP 23 affected the membrane integrity but did not induce pore formation; it inhibited bacterial respiration. PPAP 23 preferentially inhibited Fe-S cluster enzymes; it has a mild iron chelating activity and supplementation of exogenous iron attenuated its antimicrobial activity. PPAP 23 was more effective in inhibiting the growth of S. aureus under iron-restricted condition. The crystal structure of a benzylated analog of PPAP 23 showed a highly defined octahedral coordination of three PPAP ligands around a Fe (3+) core. This suggests that PPAPs are generally capable of iron chelation and are able to form defined stable complexes. PPAP 23 was found to induce reactive oxygen species (ROS) and oxidative stress. Fluorescence microscopic analysis showed that PPAP 23 caused an enlargement of the bacterial cells, perturbed the membrane, and dislocated the nucleoid. Taken together, we postulate that PPAP 23 interacts with the cytoplasmic membrane with its hydrophobic pocket and interferes with the iron metabolism to exert its antimicrobial activity in Staphylococcus aureus.

β‐Ketoesters as Mono‐ or Bisnucleophiles: A Concise Enantioselective Total Synthesis of (−)‐Englerin A and B

Abstract: A short enantioselective total synthesis of englerin A, a guaiane sesquiterpene with significant in vitro antitumor activity, is reported. Key features of this total synthesis are an organocatalytic asymmetric decarboxylative aldol reaction, a neighboring-group-participating [4+3] cycloaddition, a novel one-pot Heck coupling/regioselective 1,4-hydrosilylation/Tamao-Fleming oxidation cascade, and a kinetic CBS reduction, generating the optically pure natural product in 6.7 % overall yield over twelve steps starting from methylglyoxal. Selective saponification of the more reactive glycolic ester moiety of englerin A also gave (-)-englerin B.

Mild, Selective Ru‐Catalyzed Deuteration Using D2O as a Deuterium Source

Abstract: A method for the selective deuteration of polyfunctional organic molecules using catalytic amounts of [RuCl2 (PPh3 )3 ] and D2 O as a deuterium source is presented. Through variation of additives like CuI, KOH, and various amounts of zinc powder, orthogonal chemoselectivities in the deuteration process are observed. Mechanistic investigation indicates the presence of different, defined Ru-complexes under the given specific conditions.

Scope and Limitations of TBA[Fe]-Catalyzed Carbene Transfer to X−H-bonds – Indication of a Mechanistic Dichotomy

Abstract: An in‐depth study of carbene transfer reactions into polar X−H‐bonds catalyzed by the nucleophilic Fe‐complex Bu4N[Fe(CO)3(NO)] (TBA[Fe]) is presented. Whereas alcohols are unreactive, thiols and secondary amines are reactive, silanes showed modest activities. Hammett correlations indicate the presence of an X−H‐acidity dependent mechanistic dichotomy.

Nucleophilic Iron Complexes in Proton-Transfer Catalysis: An Iron-Catalyzed Dimroth Cyclocondensation.

Abstract: The nucleophilic iron complex Bu4 N[Fe(CO)3 (NO)] (TBA[Fe]) is an active catalyst in C-H-amination but also in proton-transfer catalysis. Herein, we describe the successful use of this complex as a proton-transfer catalyst in the cyclocondensation reaction between azides and ketones to the corresponding 1,2,3-triazoles. Cross-experiments indicate that the proton-transfer catalysis is significantly faster than the nitrene-transfer catalysis, which would lead to the C-H amination product. An example of a successful sequential Dimroth triazole-indoline synthesis to the corresponding triazole-substituted indolines is presented.

A Cyanide-Free Synthesis of Acylcyanides through Ru-Catalyzed C(sp3)-H-Oxidation of Benzylic Nitriles

Abstract: A practical method for generation of acylcyanides devoid of any external cyanide sources is presented that relies on a mild Ru-catalyzed selective C-H-oxidation of benzylic nitriles. The starting materials are smoothly generated through condensation of the corresponding carboxylic acid amides using silanes. The obtained acylcyanides can be employed in a plethora of transformation as exemplified to some larger extend in the sequence of C-H-oxidation-Tischenko-rearrangement for the generation of structurally diverse benzoyloxycyanohydrines.

Decentralized city district hydrogen storage system based on the electrochemical reduction of carbon dioxide to formate

Abstract: High fluctuations of renewable energy sources, such as wind and solar energy, require storage capacity to maintain supply reliability. For long term storage energy carriers to substitute fossil fuels must be found. The reduction of carbon dioxide to liquid substances such as formic acid or formate with electrons from renewable energy sources seem to be a promising approach. This paper tries to find answers for the following question: Under which conditions is it possible to use electrochemical carbon dioxide reduction to formate as urban seasonal energy storage?