Oxidation of alcohols by transition metal complexes—iv : The rhodium catalysed synthesis of esters from aldehydes and alcohols

Both RhH(CO)PPh₃)₃ and a catalyst made in _situ from RhCl₃·3H₂O, PPh₃ and Na₂CO₃ catalyse the reaction of a range of aldehydes with simple primary alcohols to give esters together with alcohols formed by reduction of the aldehydes. The proportion of ester can be increased by adding an efficient hydrogen acceptor. The reaction can also be used to produce 5- and 7-membered lactones from aromatic dialdehydes. Propan-2-ol and the _{in situ} catalyst reduce some aromatic aldehydes to the corresponding alcohols without concomitant ester formation.     

Alstonia scholaris: Struktur des Indolalkaloides Narelin

Alstonia scholaris: The structure of the indole alkaloid nareline Besides the known akuammidine, picralinal, picrinine and pseudoakuammigine a new indole alkaloid called nareline (M=352) was isolated from Alstonia scholaris R. BR. , which belongs to the plant family of Apocynaceae. Its structure 2 was deduced by single crystal X-ray diffraction. 2 represents the absolute configuration. The spectroscopic data of 2 and its derivatives (Scheme 1) as well as their chemical behavior support this structure. In biogenetic sense nareline is related to the bases akuammiline ( 4 ) and picraline ( 5 ) (Scheme 2). In contrast to those the C-atom 5 is exocyclic and represents an aldehyde group which forms together with the oxygen atom of the N (4)-hydroxylamine group a cyclic half acetale. - By oxidation (CrO₃/CH₃COOH) of 2 the oxindol derivative 19 (oxonareline) is formed which contains a cyclic acetal as a partial structure element (Scheme 4).     

Temperature weighted histogram analysis method, replica exchange, and transition paths

We analyzed the data from a replica exchange molecular dynamics simulation using the weighted histogram analysis method to combine data from all of the temperature replicas (T-WHAM) to obtain the room-temperature potential of mean force of the G-peptide (the C-terminal beta-hairpin of the B1 domain of protein G) in regions of conformational space not sampled at room temperature. We were able to determine the potential of mean force in the transition region between a minor alpha-helical population and the major beta-hairpin population and identify a possible transition path between them along which the peptide retains a significant amount of secondary structure. This observation provides new insights into a possible mechanism of formation of beta-sheet secondary structures in proteins. We developed a novel Bayesian statistical uncertainty estimation method for any quantity derived from WHAM and used it to validate the calculated potential of mean force. The feasibility of estimating regions of the potential of mean force with unfavorable free energy at room temperature by T-WHAM analysis of replica exchange simulations was further tested on a system that can be solved analytically and presented some of the same challenges found in more complex chemical systems.     

The synthesis and characterization of α- and β-1, 1-diiodo-3,4-benzo-1-telluracyclopentane,C8H8Tel2

Elemental Tellurium reacts with α,α′-dichloro-o-xylene and NaI in 2-methoxyethanol to form 1,1-diiodo-3,4-benzo-1-telluracyclopentane in 83% yield. C₈H₈TeI₂ is molecular in acetone, methylene chloride and methyl ethyl ketone, but ionic in DMF. Two crystalline modifications of the compound have been isolated from 2-methoxyethanol. The yellow-orange or α form is monoclini?, space group P2₁/c; the orange-red or β form is also monoclinic, space group I2/c. Infrared, optical and mass spectral data, along with solution UV, NMR, molecular weight and conductivity data, suggest that the two crystalline modifications are plymorphs, possibly differing in the degree and type of heavy atom interaction. In 2-methoxyethanol the β form undergoes a solution phase transformation to the stable α form. Both forms are thermochromic.     

PHOTOREDUCTION OF SOME NITROBIPHENYL ETHER HERBICIDES TO NITRO RADICAL ANIONS BY β-CAROTENE AND RELATED COMPOUNDS

Abstract —The nitrobiphenyl ether herbicides; 4-nitrobiphenyl ether, bifenox, nitrofen, acifluorfen, acifluorfen-methyl, acifluorfen-ethyl, and oxyfluorfen were reduced to their corresponding nitro radical anions by visible light in the presence of β-carotene, lycopene, retinol, retinal, retinoic acid and retinyl acetate in anaerobic solutions at high pH. It was more difficult to obtain spectra of bifenox, nitrofen and oxyfluorfen than acifluorfen derivatives, probably due to their poor solubilities. In neutral solutions the steady-state concentration of the nitro radical anions was low due to their faster rate of dispropor-tionation and the poor solubility of β-carotene. In the presence of retinal, the nitro radical anion of acifluorfen was produced at pH 7.4. Compounds containing conjugated double bonds such as crotonaldehyde and 2.4-hexadienal also reduced acifluorfen and its derivatives to their respective nitro radical anions in the presence of light. Ubiquinone-50 which does not contain conjugated double bonds in the side chain did not reduce acifluorfen under similar conditions.     

Substituenteneffekte auf azido-tetrazolo-gleichgewichte elektronische und sterische effekte bei s-triazolo[4,3-c]tetrazolo[1,5-a]pyrimidinen1

The azido tetrazolo valence isomerism of twenty 9-methyl-s-triazolo[4,3-c]tetrazolo[1,5-a]pyrimidines with different substituents at position 5 has been studied by means of ¹H NMR spectroscopy. All the compounds have been found to be tetrazoles in the solid state and in (CD₃)₂SO solution; in CF₃COOH azido and tetrazolo isomers are in equilibrium. From equilibrium constants K and thermodynamic data determined it is concluded that in this series K depends on both electronic effects and steric requirements of the 5-substituents. A linear relation between K and σ was found for 5-arylderivatives because ΔS° keeps approximately constant and the substituent mainly operates on ΔH°.     

The isomerization of optically-active propargyl alcohols to terminal acetylenes.

The isomerization of optically-active secondary propargyl alcohols, RCHOHCC(CH₂)_nCH₃, to terminal acetylenic alcohols, RCHOH(CH₂)_n+1 C=CH, by potassium 3-aminopropylamide (KAPA) proceeds without loss of configuration at the hydroxy center.     

Der massenspektrometrische Verlust von Vinylalkohol aus 1-Amino-3-aryloxy-2-propanolen. 24. Mitteilung über das massenspektrometrische Verhalten von Stickstoffverbindungen

Loss of vinyl alcohol from 1-amino-3-aryloxy-2-propanols under electron impact Under electron impact compounds of type 1 (see Scheme 1) split off 44 mass units from the molecular ion. This unusual reaction was studied using derivatives and deuterium labelled compounds. It could be demonstrated that for this fragmentation reaction 16 is the important structural feature from which H₂(C3)?C(2)HOH (44 mass units) is lost. The preferred reaction mechanism involves a transition state in which four members of the side chain are involved (Scheme 2, mechanism 2).     

Ein Dreikomponenten-Zweiphasensystem für die schnelle Flüssigkeits-Chromatographie

Zusammenfassung Für die Hochdruck-Flüssigkeits-Chromatographie wird ein Dreikomponenten-Zweiphasensystem beschrieben. Die Mischungslücke besteht zwischen dem wenig polaren Methylenchlorid und dem stark polaren Wasser und kann durch Zugabe von Äthanol als Lösungsvermittler geschlossen werden. Durch den Anteil der stark polaren Komponente in den jeweils koexistenten Phasen ist die Polarität von stationärer und mobiler Phase als stationäre gebunden und aus der mobilen immer wieder regeneriert. Durch Verteilungs-Chromatographie in der Mischungslücke ist eine Vielzahl von Stoffklassen der Trennung zugänglich, so z.B. Corticosteroide, Östrogene, Phenole, Phenolcarbonsäuren usw.

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A three-component two-phase system for high speed liquid chromatography

Summary High pressure liquid chromatography with a three-component two-phase system is described. For the less polar methylenechloride and the highly polar water a wide range of incomplete miscibility exists which can be decreased with ethanol. The water content of the coexistent phases determines the polarity of the stationary and the mobile phases. The more polar phase is adsorbed on silicagel as the column support material and serves as the stationary phase. Numerous compounds can be separated by partition chromatography between the coexistent phases, for instance; corticosteroids, estrogens, phenols, phenol-carboxylic acids and so on