A reactivity/affinity switch for parallel kinetic resolution: alpha-amino acid quasienantiomers and the resolution of cyclopropene carboxylic acids

A new type of parallel kinetic resolution (PKR) is reported in which quasienantiomers with very similar reactivities give products whose chromatographic properties diverge upon the addition of fluoride. This concept of a reactivity/affinity switch is applied to the PKR of cyclopropene carboxylic acids with all-carbon quaternary centers. This is the first application of alpha-amino acid quasienantiomers in PKR, and it is a complementary approach for acyltransfer systems where the asymmetry is induced by the nucleophile rather than the leaving group. Excellent diastereoselectivities (ranging from 90:10 to 99.5:5) and good yields were obtained for both quasienantiomeric products, and the reactions can be run on significant scale because the separation is trivial. High-level DFT calculations (B3LYP functional with the 6-31+G(d,p) basis set) provided transition-state structures with relative energies that are in accord with the experimental observations.     

Enantioselective [6pi]-photocyclization reaction of an acrylanilide mediated by a chiral host. Interplay between enantioselective ring closure and enantioselective protonation

The [6pi]-photocyclization of the anilides 1a and 5 was studied in the absence and in the presence of the enantiomerically pure chiral lactam 4. The relative configuration of the products was unambiguously established by single-crystal X-ray crystallography and by NMR spectroscopy. A significant enantiomeric excess was observed upon reaction of compound 1a to its photocyclization products at -55 degrees C employing lactam 4 as a chiral complexing agent in toluene as the solvent (66% yield). The trans product ent-3a was obtained in 57% ee, and the minor diastereoisomer (trans/cis = 73/27), cis product ent-2a, was obtained in 30% ee. DFT calculations were conducted modeling the complexation of intermediates 8 and ent-8 to host 4. In agreement with steric arguments concerning the conrotatory ring closure of 1a, the formation of ent-8 is favored leading to the more stable complex 4.ent-8 as compared to 4.8. Whereas the enantioselectivity in the photocyclization to trans compound ent-3a increased upon reduction in the reaction temperature, the enantiomeric excess in the formation of cis compound ent-2a went through a maximum at -15 degrees C (45% ee) and decreased at lower temperatures. Deuteration experiments conducted with the pentadeuterated analogue of 1a, d(5)-1a, revealed that the protonation of the intermediates 8 and ent-8 is influenced by chiral amide 4. In the formation of ent-3a/3a, both the enantioselective ring closure and the enantioselective protonation by amide 4 favor the observed (6aS,10aS)-configuration of the major enantiomer ent-3a. In the formation of ent-2a/2a, the enantioselective ring closure (and the subsequent diastereoselective protonation) favors the (6aR,10aS)-configuration that is found in compound 2a. Contrary to that, the enantioselective protonation by amide 4 shows a preference for ent-2a with the (6aS,10aR)-configuration.     

Apparate

Ohne Zusammenfassung     

Influence of cobalt ions on the electrochemical properties of lamellar manganese oxides

Evaluation of Co-doping on the electrochemical properties of the sol-gel birnessite and the new lithiated manganese oxide Li_0.45MnO_2+δ is reported. For both compounds the synthesis of Co-doped materials via a solution technique is described. We demonstrate the interest of Co-doped structures with the selected content of 0.15 Co per mole of oxide as the optimum composition. In the case of Li_0.45Mn_1−yCo_yO_2+δ. prepared at 300 °C, a mixture of a lamellar phase and a cubic one is identified while the Co-doped birnessite appears as a single phase. A probable substitution of Mn by Co ions explains the better specific capacity of 185 mAh/g found and the excellent stability observed over 40 cycles in the voltage range 4.2−2.0 V. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Ein neues Reagens zum Nachweis geringer Mengen Wasserstoffsuperoxyds, speciell in Pflanzen

Ohne Zusammenfassung     

Monitoring changes in the band structure of graphite following chemical reaction: An X-ray-induced photo-electron study of intercalation

Dramatic changes in the valence band of graphite, resulting from the insertion of bromine between the layers, are directly measurable using the ESCA technique.     

Zur Frage der Rothf?rbung der Carbols?ure

Ohne Zusammenfassung