Coordination polymers of silver(I) with ditopic cross-conjugated dienone

The reaction of the silver salts AgX (a: X = BF₄^-, b: X = ClO₄^-, c: X = OTf^–) with α,α'-di(3/4-pyridylmethylene)cycloalkanones (L¹–L³) and piperidones (L⁴–L⁷) results in the formation of coordination products of general composition [AgX(L ⁿ )(solvent)] and [AgX(L ⁿ )] (L ⁿ = L¹–L⁷). All complexes were characterized by elemental analysis and IR-spectroscopy. The structures of [Ag(ClO₄)(L¹)(MeC≡N)]_∞ (1b · MeC≡N) and [Ag(ClO₄)(L¹)]_∞ (1b) in the solid state are reported. In both structures {Ag(L¹)}⁺ building units are linked to each other via Ag–N_pyridine primary bonds resulting in the formation of infinite chains. In both structures the ligands L¹ are fixed in transoid conformations, thus forming zig-zag polar chains. The structure of 1b · MeC≡N consists of pairs of tightly and loosely stacked chains. The tightly packed chains are weakly coupled by perchlorate anions acting as μ-bridges in between Ag(I) centers as well as by π–π-stacking interactions of unsaturated fragments of the respective ligands. In contrast, polar 2D layers composed of {Ag(L¹)} _m ^m+ chains, which interdigitate via multiple weak interactions by Ag–O contacts, are found in the solid structure of 1b. The dissolution of coordination products in coordinating solvents like MeCN or DMSO leads to the decomposition of complexes due to formation of silver-solvent coordination compounds. The coordination products 1–5 are stable in solid state against exposure to the ambient light, whereas solutions of the compounds, especially in DMSO-d₆, appeared to be photochemically labile. As revealed by NMR spectroscopic studies, the organic components undergo trans-cis isomerization.     

Efficient alkylation of ketones with primary alcohols catalyzed by ruthenium(II)/P,N ligand complexes

An efficient catalytic system containing [RuCl₂(η⁶-p-cymene)]₂ and one P,N ligand, N-diphenylphosphino-2-aminopyridine (L1) was loaded in catalyzing the alkylation of ketones with primary alcohols for a diverse array of substrates. Other five P,N ligands based on pyridin-2-amine and pyrimidin-2-amine were also examined in this reaction to explore the influence of steric hindrance and electronic effects. Monitoring by ¹H NMR and ESI-MS reveals a stable cationic L1-coordinated ruthenium hydride intermediate, identified as [Ru(η⁶-p-cymene)(κ²-L1)H]⁺. Organic intermediates consistent with a three-step dehydrogenation, alkylation and hydrogenation pathway were also observed. The final step in this reaction, the ruthenium-catalysed transfer hydrogenation reduction of α,β-unsaturated ketone with benzyl alcohol was performed separately.     

Stereoselective aza Diels-Alder reaction on solid phase: a facile synthesis of hexahydrocinnoline derivatives

As part of our continuing studies of polymer-supported pericyclic reactions for preparing biologically interesting heterocyclic compounds, we have introduced a traceless solid-phase synthesis of hexahydrocinnolines. We developed a method in which mild reaction conditions can be used for the hetero-Diels-Alder reactions on a polymeric support. The dienoic 3-vinyl-2-cyclohexenol attached to a Wang resin through an ether linkage undergoes [4 + 2] cycloaddition reaction with several azadienophiles. The highly stereoselective Diels-Alder reaction showed preferential formation of a single cycloadduct resulting from an anti attack of the dienophile on the polymer-bound diene. Trifluoroacetic acid-mediated cleavage of the polymer-bound cycloadducts yields fused nonaromatic hexahydrocinnolines in moderate yields in three steps.     

Effects of caffeine on stereoselectivities of high cell density biotransformations of cyclic beta-keto esters with Saccharomyces cerevisiae

Caffeine affects the stereoselectivity of microbial high cell density reductions with commercial grade Saccharomyces cerevisiae (Baker's yeast). Cyclic beta-keto esters ethyl 2-oxocyclopentanoate (1) and ethyl 2-oxocyclohexanoate (3) were shown to be reduced with increased diastereoselectivity (1: 90.1 --> 92.1% de, 3: 75.0 --> 90.0% de) after addition of caffeine. Effects on enantioselectivity were less pronounced (1: 97.3 --> 98.5% ee, 3: 90.1 --> 92.1% ee). The observations are ascribed to the action of caffeine on cellular calcium homeostasis. These effects are accompanied by caffeine-induced cell-death, which preferably takes effect on pre-stressed cells which were found to decrease diastereoselectivity.     

Electrical signaling in Aloe vera induced by localized thermal stress

Action potentials in higher plants are theorized as the information carriers in intercellular and intracellular communication in the presence of environmental stressors. Among the most common stressors is heat shock. Under stressful conditions, the response reactions of plant tissues and organs can be local or transmitted over long distances. In this article, the speeds of propagation of thermally induced action potentials in green plants are discussed, and their speeds were found to be comparable to those occurring in various mammalian species. These rapid action potentials in green plants were recorded in real time using modern data acquisition techniques. According to our measurements, a single application of localized heat stress induces fast action potentials in Aloe vera (67 m/s). Electrical signals propagated along all leaves of the A. vera plants were studied. Possible pathways for electrical signal propagation in vascular plants are also discussed.     

REVEALING MICROSTRUCTURE OF FINE PARTICLES BY TEM

This paper introduces a method of cutting nano-scaled thin films from fine particles by using the technique of metal encapsulation. The interior morphology and microstructure of fine particles are then examined by TEM or HREM.     

Spontaneous Self‐Assembly of γ‐Cyclodextrins in Dilute Solutions with Tunable Sizes and Thermodynamic Stability

The behavior in dilute solution of phosphate‐functionalized γ‐cyclodextrin macroanions with eight charges on the rim was explored. The hydrophilic macroions in mixed solvents show strong attraction between each other, mediated by the counterions, and consequently self‐assemble into blackberry‐type hollow spherical structures. Time‐resolved laser light scattering (LLS) measurements at high temperature ruled out the possibility of hydrogen bonding as the main driving force in the self‐assembly and indicated the good thermodynamic stability of assemblies regulated by the charge. The transition from single macroions to blackberries can be tuned by adjusting the content of organic solvent. The sizes of blackberries vary with the charge density of γ‐cyclodextrin by adjusting pH. It is the first report that pure cyclodextrins can generate supramolecular structures by themselves in dilute solution. The unique solution behavior of macroions provides a new opportunity to assemble cyclodextrin into functional materials and devices.     

4,5‐Dihydro‐1,2,3‐oxadiazole: A Very Elusive Key Intermediate in Various Important Chemical Transformations

4,5‐Dihydro‐1,2,3‐oxadiazoles are postulated to be key intermediates in the industrial synthesis of ketones from alkenes, in the alkylation of DNA in vivo, and in the decomposition of N‐nitrosoureas; they are also a subject of great interest for theoretical chemists. In the presented report, the formation of 4,5‐dihydro‐1,2,3‐oxadiazole and the subsequent decay into secondary products have been studied by NMR monitoring analysis. The elusive properties evading characterization have now been confirmed by ¹H, ¹³C, and ¹⁵N NMR spectroscopy, and relevant 2D experiments at very low temperatures. Our experiments with suitably substituted N‐nitrosoureas using thallium(I) alkoxides as bases under apolar conditions answer important questions on the existence and the secondary products of 4,5‐dihydro‐1,2,3‐oxadiazole.     

Cottoquinazolines E and F from Neosartorya fischeri NRRL 181

Two new norfumiquinazolines, cottoquinazolines E and F ( 1 and 2 , resp.), together with pyripyropene A ( 3 ), were isolated from the fungus Neosartorya fischeri NRRL 181. The structures of the new isolates were established by spectroscopic methods, including UV, IR, HR‐ESI‐MS, and extensive 1D‐ and 2D‐NMR techniques. To the best of our knowledge, this is the first report on norfumiquinazolines and pyripyropenes produced by the genus Neosartorya.