Photochemical Reactions. 149th Communication. Photochemistry of 7,8-dihydro-4-hydroxy-β-ionone and derivatives

The photolysis of 7,8-dihydro-4-hydroxy-β-ionone ( 6 ) was investigated together with its acetate and isopropyl ether 7 and 8 , respectively. Irradiation (λ > 245 nm) of 6 in MeCN or i-PrOH at temperatures between 25° and ?65° leads to the tricyclic ethers 9 , 10 and 13A + B , and to the spirocyclic ethers 11 and 12 , which are all known types of photoproducts, previously obtained on photolysis of 7,8-dihydro-β-ionone ( 1 ). The same types of products are obtained on irradiation of the acetate 7 and the isopropyl ether 8 . On the other hand, irradiation of the hydroxy compound 6 in MeCN or i-PrOH at temperatures between ?35° and ?65° leads to the new tricyclic tertiary alcohols 14 and 15 as the major products. Their formation involves an intramolecular trapping of a carbocation by the neighbouring OH group, thus, supporting the previously proposed mechanism of the transformation 1 → 5. For structure proof, the tricyclic alcohol 14 and the pheny1 carbamate 42 , derived from 9 , were subjected to X-ray analysis.     

Influence of tetrahydrofuran on reactivity, aggregation, and aggregate structure of dimethylcuprates in diethyl ether

The comprehension of factors influencing the reactivity of organocuprates is still far from enabling a rational control of their reactions. Especially the degree of aggregation and structures of organocuprates are the focus of discussion about the factors affecting their reactivity. Therefore, this study combines kinetic measurements and NMR investigations to elucidate the influence of disaggregation via addition of tetrahydrofuran (THF) on the reactivity and aggregate structure of Gilman cuprates. As model systems, Me(2)CuLi.LiI (1.LiI) and Me(2)CuLi.LiCN (1.LiCN) in diethyl ether (DEE) were chosen; as model reaction, the 1,4-addition to 4,4-dimethylcyclohex-2-enone. The kinetic data show for 1.LiI a pronounced acceleration effect upon addition of distinct amounts of THF, whereas the reactivity of 1.LiCN continuously decreases with the addition of THF. Series of NMR diffusion measurements as well as (1)H-(7)Li heteronuclear Overhauser effect (HOE), and (1)H-(1)H nuclear Overhauser effect (NOE) spectra show different structural influences of THF on 1.LiI and 1.LiCN. For 1.LiI, small salt units are separated from the cuprate aggregate by THF. In contrast to this, THF disaggregates the oligomeric structures of 1.LiCN, while the core structures remain intact with salt attached. Thus, the reactivity of 1.LiI seems to be fine-tuned through distinct amounts of salt or THF, whereas the decreasing reactivity of 1.LiCN correlates with the disaggregation of oligomers via THF. Thus, for synthetic chemists with reactivity problems in specific reactions of iododialkylcuprates, the addition of small amounts of THF might be useful to enhance the reactivity. In addition to these structure-reactivity studies, the CN(-) group is shown to be directly attached to the cuprate moiety via a combination of (1)H-(13)C HOE- and (1)H-(1)H NOEs. This represents the first direct experimental evidence in solution for the position of the CN(-) group relative to the cuprate moiety in cyano-Gilman cuprates.     

(57)Fe ENDOR spectroscopy on the iron-sulfur cluster involved in substrate reduction of heterodisulfide reductase

Heterodisulfide reductase (Hdr) from methanogenic archea is an iron-sulfur protein that catalyzes the reversible two-electron reduction of the mixed disulfide CoM-S-S-CoB to the thiol coenzymes, coenzyme M (CoM-SH) and coenzyme B (CoB-SH). It is unusual that this enzyme uses an iron-sulfur cluster to mediate disulfide reduction in two one-electron steps via site-specific cluster chemistry. Upon half-reaction of the oxidized enzyme with CoM-SH in the absence of CoB-SH, an iron-based paramagnetic intermediate is formed, designated CoM-Hdr. In this Communication we report 57Fe pulsed ENDOR at two very different frequencies, 9 and 94 GHz, that identify the iron sites of CoM-Hdr. We find direct evidence for a [4Fe-4S]3+ cluster, and we determine the sign of the 57Fe hyperfine couplings. The 57Fe isotropic hfc values suggest a complex interaction between the cluster and the CoM-SH substrate.     

New pentacyclic triterpene saponins with strong anti-leishmanial activity from the leaves of Maesa balansae

Six new triterpene saponins bearing an oxygen bridge between C-13 and C-28 and with pronounced anti-leishmanial activity were isolated from the methanolic extract of leaves of the Vietnamese medicinal plant Maesa balansae. The structure was established on the basis of detailed NMR (COSY, NOESY, HMQC, HMBC, TOCSY and DEPT) and FAB-MS studies along with chemical degradation. All saponins identified contained the same pentaglycosidic side chain, but a different esterification pattern on the triterpenoid part. Biological evaluation of the individual compounds against visceral leishmaniasis (Leishmania infantum amastigotes) revealed a much better activity in vitro compared to the reference compound Pentostam^®, which is currently used as first-line treatment for leishmaniasis.     

Functional single-wall carbon nanotube nanohybrids--associating SWNTs with water-soluble enzyme model systems

We succeeded in integrating single-wall carbon nanotubes (SWNTs), several water-soluble pyrene derivatives (pyrene(-)), which bear negatively charged ionic headgroups, and a series of water-soluble metalloporphyrins (MP(8+)) into functional nanohybrids through a combination of associative van der Waals and electrostatic interactions. The resulting SWNT/pyrene(-) and SWNT/pyrene(-)/MP(8+) were characterized by spectroscopic and microscopic means and were found to form stable nanohybrid structures in aqueous media. A crucial feature of our SWNT/pyrene(-) and SWNT/pyrene(-)/MP(8)(+) is that an efficient exfoliation of the initial bundles brings about isolated nanohybrid structures. When the nanohybrid systems are photoexcited with visible light, a rapid intrahybrid charge separation causes the reduction of the electron-accepting SWNT and, simultaneously, the oxidation of the electron-donating MP(8)(+). Transient absorption measurements confirm that the radical ion pairs are long-lived, with lifetimes in the microsecond range. Particularly beneficial are charge recombination dynamics that are located deep in the Marcus-inverted region. We include, for the first time, work devoted to exploring and testing FeP(8)(+) and CoP(8)(+) in donor-acceptor nanohybrids.     

Untersuchungen zu Struktur und physikalischen Eigenschaften von Oxiden der LaPd2O4-Familie

Investigations on Structure and Physical Properties of some New Members of the LaPd₂O₄ Family The new mixed valent oxopalladates MPd₂O₄ (M = Pr, Nd, Gd, Y) have been synthesized from the binary oxides in presence of a solid oxygen carrier (KClO₃) by applying hydrostatic pressure (ca. 2 GPa). According to X-ray investigations these oxides crystallize in space group I 4₁/a (No. 88) and are isotypic to LaPd₂O₄ (PrPd₂O₄: a = 5.8382(2), c = 10.1687(5) Å; NdPd₂O₄: a = 5.8419(3), c = 10.1949(6) Å; GdPd₂O₄: a = 5.7799(3), c = 10.0677(8) Å; YPd₂O₄: a = 5.7359(3), c = 9.9829(8) Å). The crystal structure consists of square planar PdO₄ units which are stacked to form columnar arrangements along [1 00] and [0 10], respectively. The PdO₄ squares are connected via common corners to a three-dimensional framework. By substitution of Pr³⁺, Nd³⁺, Gd³⁺ and Y³⁺ for lanthanum the Pd? Pd distances decrease and the electrical conductivities increase. The oxopalladates investigated in this work are black, paramagnetic and semiconducting. At ambient pressure they decompose at temperatures above 880 K into elemental palladium and the binary rare earth oxides.     

Hit-optimization using target-directed dynamic combinatorial chemistry: development of inhibitors of the anti-infective target 1-deoxy-d-xylulose-5-phosphate synthase

Target-directed dynamic combinatorial chemistry (tdDCC) enables identification, as well as optimization of ligands for un(der)explored targets such as the anti-infective target 1-deoxy-d-xylulose-5-phosphate synthase (DXPS). We report the use of tdDCC to first identify and subsequently optimize binders/inhibitors of the anti-infective target DXPS. The initial hits were also optimized for their antibacterial activity against E. coli and M. tuberculosis during subsequent tdDCC runs. Using tdDCC, we were able to generate acylhydrazone-based inhibitors of DXPS. The tailored tdDCC runs also provided insights into the structure–activity relationship of this novel class of DXPS inhibitors. The competition tdDCC runs provided important information about the mode of inhibition of acylhydrazone-based inhibitors. This approach holds the potential to expedite the drug-discovery process and should be applicable to a range of biological targets.

Target-directed dynamic combinatorial chemistry was used for hit-identification and subsequent hit-optimization for the anti-infective target 1-deoxy-d-xylulose-5-phosphate synthase resulting in novel inhibitors with low micromolar affinities.     

Vollständige13C-NMR-Zuordnung von gluco- und idokonfigurierten 1,2-O-Alkyliden-furanurono-6,3-lactonen durch 2D-1H-13C-korrelierte NMR-Spektroskopie

1,2-O-Isopropylidene- and 1,2-O-benzylidene--D-glucofuranurono-6,3-lactones and the readily accessible 1,2-O-isopropylidene- and 1,2-O-benzylidene--L-idofuranurono-6,3-lactones were investigated by NMR spectroscopy. By means of a 2D-¹H-¹³C-correlated NMR experiment all resonances of the sugar carbons could be unambiguously assigned.

     

The Isoinversion Principle—a General Model of Chemical Selectivity

A large number of successful methods for chirality transfer, using either stoichiometric or catalytic chiral auxiliaries, are in use today. However, there is a lack of practical and dynamic selectivity models, i.e. models which take into account the entire reaction sequence, and which allow simple and reliable assessment, optimization and prediction of selectivity in asymmetric syntheses. The models that are available are either too strongly biased to the steric requirement of the particular molecules reacting, but do not go beyond classical considerations of static facial differentiation, or they take a demanding, theoretical approach, which because of its inherent limitations and the great theoretical effort required has not yet found its way into the practical world of the synthetic chemist. The “Isoinversion Principle”, developed on the basis of Eyring's theory, closes this gap. With its aid, the synthetic chemist can determine the characteristic isoinversion temperature T_i for the reaction type of interest from a few temperature-dependent measurements of selectivity parameters. T_i then affords information on interesting questions such as optimization etc. The advantage of this method is that it is useful not only for stereoselectivity, but for any kind of process where selectivity in general (regio-,chemo-, etc) is generated at two or more stages of a reaction sequence, regardless of whether these reactions involve the ground state or a diabatic photoprocess. The present review will demonstrate that this generation of selectivity at two or more stages of a reaction sequence occurs more commonly than is generally thought.     

Asymptotic analysis of quasi-steady n-heptane droplet combustion supported by cool-flame chemistry

A skeletal chemical-kinetic mechanism for n-heptane cool flames is simplified to the maximum extent possible by introduction of steady-state approximations for intermediaries, following procedures employed previously in addressing two-stage ignition. A pair of ordinary differential equations in mixture-fraction space is thereby obtained, describing the quasi-steady structures of the temperature and heptylketohydroperoxide fields. Application of activation-energy asymptotics for the partial-burning regime to this pair of equations is shown to provide convenient expressions for flame structures and the extinction condition associated with maximally reduced chemistry. With the mixture-fraction co-ordinate related to radius, these results are used to address droplet-combustion experiments that have been performed in the International Space Station. Droplet diameters at extinction are predicted as functions of the oxygen concentration in the atmosphere and are compared with experiment. While the results are encouraging concerning qualitative predictions of dependences of extinction diameters on atmospheric conditions, there are noticeable quantitative differences that point to deficiencies in the analysis, likely resulting from a number of oversimplifications. Further investigation is therefore recommended.