Stereoselective Synthesis of Enantiomerically Pure Natural Products—Estrone as Example

Natural products have been synthesized for billions of years in animals, plants, and microorganisms. As a rule they occur enantiomerically pure. Their chiral character corroborates their use in metabolism or as biologically active agents. Natural products may be insufficient in quality or quantity. They have recently begun to become accessible, either unchanged or modified, by biological synthesis; here, too, they are obtained enantiomerically pure. In the last twenty years chemical synthesis has become a major concern of organic chemists. Their target compounds are primarily enantiomerically pure natural products or biologically active variants thereof.     

Chiral allyl cations are captured by furan with 100% stereoselectivity: synthesis of enantiopure 2-alkoxy-8-oxabicyclo

A low-temperature (-95 degrees C) protocol for intermolecular cycloadditions of furan to chiral silyloxyallyl cations in dichloromethane is described. Key precursors are open-chain, mixed a-ketoacetals, which are chiral. The resulting [4+3] cycloadducts are densely functionalized and are isolated as single enantiomers in high chemical yield. The yield of the cycloadducts increases with increasing dilution. Three and four stereogenic centres are created in one single step.     

Fragmentation studies on monensin A and B by accurate-mass electrospray tandem mass spectrometry

Monensin A and B were studied by electrospray ionisation tandem mass spectrometry (ESI-MS/MS) and the fragment ions were confirmed by accurate-mass measurements. Analyses were performed on both a quadrupole time-of-flight (QTOF) and a Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer. The analysis revealed that fragment ions were produced by Grob-Wharton fragmentations and pericyclic rearrangements in addition to various simple neutral losses. A study of the protonated and sodiated sodium salt revealed different fragmentation pathways for these species, thus complementary structural information could be gained. A complete fragmentation pathway of monensin A and B protonated sodium salt [(M-H+Na)+H])+) and sodiated sodium salt [(M-H+Na)+Na](+) is proposed. MS(3) analysis confirmed the separate fragmentation pathways.     

Einige Verbesserungen von Nachweisen in der organischen Tüpfelanalyse

Zusammenfassung Es werden Verbesserungen von Nachweisen von Alkoholen, Chloral, Salicylalkohol, Salicin, Sulfoxylatverbindungen und bestimmten reduzierenden organischen Verbindungen beschrieben, durch die man zu mikroanalytischer Empfindlichkeit und zum Teil auch zu erhöhter Selektivität gelangt.

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Summary Improvements of tests for alcohols, chloral, salicylalcohol, salicine, sulfoxylate compounds and certain reducing organic compounds are described, which lead to microanalytical sensibility and sometimes also to enhanced selectivity.

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Résumé On décrit des améliorations des méthodes d'identification des alcools, du chloral, de l'alcool salicylique, de la salicine, de composés sulfoxyliques et de combinaisons organiques réductrices définies. Elles atteignent une sensibilité microanalytique et leur sélectivité est parfois accrue.

     

Application of Phosphorus-31 Nuclear Magnetic Resonance Spectrosocopy for the Study of Human Diabetic Cataractogenesis

Intact human Senses incubated at 5.5 mM (normal) and 35.5 mM glucose were examined by phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy. Lense in 35.5 mM glucose showed an altered metabolic steady-state characterized by a lowered adenosine triphosphate/inorganic orthophosphate ratio. ³¹P NMR spectroscopy can be used to measure metabolic changes in the lens. This model offers an important means to study dynamic metabolism in the human lens in the setting of diabetic cataractogenesis.     

Dependence of the activity of a rhizopus lipase on microemulsion composition

Enzymatic hydrolysis of a model triglyceride, palm oil, was carried out with lipase fromRhizopus sp. in microemulsions with varying water content. The microemulsions were based on a nonionic surfactant, pentaethylene glycol monododecyl ether (C12 EO5), buffered water solution and an oil component consisting of isooctane and palm oil at a weight ratio of 20:1. The structure of the microemulsions was characterized using Fourier transform pulsed-gradient spin-echo¹H NMR. The rate of reaction decreased as the water content of the reaction medium was increased. The self-diffusion coefficient of water, D_w was found to be constant within the interval 1–20% water. The difference in reactivity is believed to be due to a difference in structure of the palisade layer between water and hydrocarbon microdomains. The nonionic surfactant was demonstrated to be unsuitable for enzymatic reactions since only partial hydrolysis was obtained in all experiments. The surfactant, however, did not cause enzyme deactivation, even at very high concentrations.     

One Way Traffic: Base-to-Backbone Hole Transfer in Nucleoside Phosphorodithioate

The directionality of the hole-transfer processes between DNA backbone and base was investigated by using phosphorodithioate [P(S⁻)=S] components. ESR spectroscopy in homogeneous frozen aqueous solutions and pulse radiolysis in aqueous solution at ambient temperature confirmed initial formation of G^.+-P(S⁻)=S. The ionization potential of G-P(S⁻)=S was calculated to be slightly lower than that of guanine in 5′-dGMP. Subsequent thermally activated hole transfer from G^.+ to P(S⁻)=S led to dithiyl radical (P-2S^.) formation on the μs timescale. In parallel, ESR spectroscopy, pulse radiolysis, and density functional theory (DFT) calculations confirmed P-2S^. formation in an abasic phosphorodithioate model compound. ESR investigations at low temperatures and higher G-P(S⁻)=S concentrations showed a bimolecular conversion of P-2S^. to the σ²-σ*¹-bonded dimer anion radical [-P-2S 2S-P-]⁻ [ΔG (150 K, DFT)=−7.2 kcal mol⁻¹]. However, [-P-2S 2S-P-]⁻ formation was not observed by pulse radiolysis [ΔG° (298 K, DFT)=−1.4 kcal mol⁻¹]. Neither P-2S^. nor [-P-2S 2S-P-]⁻ oxidized guanine base; only base-to-backbone hole transfer occurs in phosphorodithioate.     

Microswimmers – From Single Particle Motion to Collective Behavior

Locomotion of autonomous microswimmers is a fascinating field at the cutting edge of science. It combines the biophysics of self-propulsion via motor proteins, artificial propulsion mechanisms, swimming strategies at low Reynolds numbers, the hydrodynamic interaction of swimmers, and the collective motion and synchronisation of large numbers of agents. The articles of this Special Issue are based on the lecture notes of an international summer school, which was organized by the DFG Priority Programme 1726 “Microswimmers – From Single Particle Motion to Collective Behaviour” in the fall of 2015. The minireviews provide a broad overview of the field, covering both elementary and advanced material, as well as selected areas from current research.