Asymmetric synthesis of α-hydroxy carboxylic acids: direct oxidation of chiral amide enolates using 2-sulfonyloxaziridines

The direct oxidation of chiral amide enolates to optically active mandelic acid using 2-sulfonyloxaziridine $\text{1}$ is described. The diastereoselectivity is counterion dependent.     

Unsymmetrical selenides from selenocyanates and methyl grignard

The addition of methyl Grignard to diethyl acetamido(cyanoselenobenzyl)malonates 3 and 4 at ?78° followed by hydrolysis yields the 3-(4-and 3-methylselenophenyl)alanines 1 and 2.     

The chromatographic properties of a mixed-bed stationary phase combining reversed-phase and strong anion exchange properties

Summary The feasibility of using columns prepared by mixing together ion exchange and reversed-phase packing materials, for the simultaneous separation of ionised and unionised molecules, has been examined using a range of test solutes. Columns prepared in this way showed chromatographic properties which were intermediate between those of the individual phases. The dual nature of the retention mechanism allowed the retention of ionisable molecules to be adjusted using pH whilst that of uncharged compounds was unaffected. The simultaneous chromatography of model compounds and their glucuronide, sulphate and glycine conjugates, under a variety of conditions, was demonstrated.     

The carbonyl frequency of cyclopentanone as a function of its distortion,a vibrational model for 2- and 7-norbornanone

Using normal coordinate analysis, the infrared spectrum of cyclopentanone has been computed as the geometry is modified from the planar form to those in which the geometry about the 5-membered ring looks like 2-norbornanone and also like 7-norbornanone. The computed v _co of cyclopentanone is 1739.9 cm^–1 compared to the experimental value of 1740 cm^–1.As the cyclopentanone ring is deformed to resemble either the 2-norbornanone or the 7-norbornanone ring system, the computed v _co is too low using the force fields that give good agreement with experimental value for cyclopentanone. The carbonyl force constant had to be increased from 9.7 to 10.0 mdynes/Å for the 2-norbornanone-like model and to 10.4 mdynes/Å for the 7-model system to give v _co values in agreement with the experimental values.

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Zusammenfassung Mit Hilfe der Normalkoordinaten-Analyse wird das Infrarotspektrum des Zyklopentanons berechnet, wobei die Geometrie von der planaren Form bis zu der Form, bei der die Geometrie am Fünfring derjenigen des 2-Norbornanons oder auch des 7-Norbornanons ähnlich wird, variiert wird. Der berechnete Wert von v _co des Zyklopentanons ist 1739,9 cm^–1, während der experimentelle Wert 1740 cm^–1 beträgt. Bei den angegebenen Deformationen des Zyklopentanonrings wird der berechnete Wert von v _co zu niedrig, wenn man die Kraftfelder zugrunde legt, die für das Zyklopentanon gute Übereinstimmung mit dem Experiment liefern. Die Carbonylkraftkonstanten mußten von 9,7 auf 10,0 m dyn/Å für das 2-Norbornanon-Modell sowie auf 10,4 m dyn/Å für das 7-Norbornanon erhöht werden, um den Wert von v _co in Übereinstimmung mit dem Experiment zu bringen.

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Résumé Le spectre infra-rouge de la cyclopentanone a été calculé en employant l'analyse en coordonnées normales lorsque l'on modifie la géométrie depuis la forme plane jusqu'aux formes o u les configurations sur le noyau pentagonal ressemblent à celles de la 2-norbornanone et de la 7-norbornanone. La fréquence _co calculée pour la cyclopentanone est 1739.8 cm^–1 comparée à la valeur expérimentale de 1740 cm^–1. Lorsque l'on déforme le cycle de la cyclopentanone comme indiqué ci-dessus, la valeur calculée de _co est trop faible si l'on emploie les champs de force qui donnent un bon accord avec la valeur expérimentale pour la cyclopentanone. La constante de force du carbonyle doit etre augmentée de 9,7 à 10,0 mdynes/Å pour le modèle analogue à la 2-norbornanone et de 9,7 à 10,4 mdynes/Å pour le modèle analogue à la 7-norbornanone.

     

The effect of platinum oxide films on reaction kinetics at platinum electrodes

It has been known for some time that the pre-treatment of platinum electrodes often effects subsequent electrochemical reactions. Part of the effect of pre-treatment is due to the fact that anodized or chemically oxidized platinum electrodes become coated to some degree with a film of platinum oxide. This work was concerned with quantitative measurement of kinetic parameters as a function of the extent of oxide film formation. Whenever possible, variations in reaction mechanisms are proposed.

Most of the experimental evidence has been taken from current-potential curves but the techniques of chronopotentiometry and microscopy were also used.

The reduction of vanadium^V chromium^VI, arsenic^V, iodate and oxygen were investigated as well as the oxidation of vanadium^IV, arsenic^III, oxalic acid, and formic acid. The presence of the platinum oxide film effected the reactions studied in a variety of ways but in every case some variation in the kinetic parameters of the reactions studied was recorded. For a number of cases, a modified oxygen bridge theory was found useful.     

Forces in molecular recognition: Comparison of experimental data and molecular mechanics calculations

Summary NMR studies of the rotation barrier of the disaccharide of the glycopeptide antibiotic vancomycin have been used to test the performance of computer simulation techniques using molecular mechanics. In the absence of any solvated water, no correlation could be found between experiment and calculation. By introducing solvent water molecules into the binding region of the antibiotic, the NMR results could be simulated both qualitatively and quantitatively within experimental error without using massive computational resources.     

Simple correction for perturbations of peak amplitudes in statistical models of overlap

Summary The application of statistical models of overlap (SMOs) to saturated separations is made possible by theory that addresses variable peak amplitudes. These amplitudes cause peak widths to vary, and this variation can be modeled by a random variable whose effect on the probability of overlap is expressed by a convolution integral. Modified probabilities of overlap are derived for both homogeneous and nonhomogeneous one-dimensional separations, and the new probabilities are compared to results determined from published computer simulations. The new theory can describe overlap at saturations that are 3 to 4 times larger than before. Previously reported experimental chromatograms are reinterpreted to show the capabilities of theory. The theoretical extension is an important step towards making SMOs into practical tools for screening analytical separations.     

Chemically modified "polar patch" mutants of subtilisin in peptide synthesis with remarkably broad substrate acceptance: designing combinatorial biocatalysts

A significant enhancement of the applicability of the serine protease subtilisin Bacillus lentus (SBL) in peptide synthesis was achieved by using the strategy of combined site-directed mutagenesis and chemical modification to create chemically modified mutant (CMM) enzymes. The introduction of polar and/or homochiral auxiliary substituents, such as X=oxazolidinones, alkylammonium groups, and carbohydrates at position 166 at the base of the primary specificity S(1) pocket created SBL CMMs S166C-S-X with strikingly broad structural substrate specificities. These CMMs are capable of catalyzing the coupling reactions of not only L-amino acid esters but also D-amino acid esters as acyl donors with glycinamide to give the corresponding dipeptides in good yields. These powerful enzymes are also applicable to the coupling of L-amino acid acyl donors with alpha-branched acyl acceptor, L-alaninamide. Typical increases in isolated yields of dipeptides of 60-80 % over SBL-WT (e.g. 0 % yield of Z-D-Glu-GlyNH(2) using SBL-WT-->74 % using S166C-S-(CH(2))(2) NMe(3) (+)) demonstrate the remarkable synthetic utility of this "polar patch" strategy. Such wide-ranging systems displaying broadened and therefore similarly high, balanced yields of products (e.g. 91 % Z-L-Ala-GlyNH(2) and 86 % yield of Z-D-Ala-GlyNH(2) using S166C-S-(3R,4S)-indenooxazolidinone) may now allow the use of biocatalysts in parallel library synthesis.     

Mass spectra of transition metal π-complexes. VI—The metastable ion mass spectrum of (η1-Toluene)-tricarbonylchromium

First field free region metastable fragmentations of (η⁶-PhCH₃)Cr(CO)₃ have been examined by means of the linked scan technique. The molecular ion is shown to fragment exclusively by single and multiple CO loss. The ion [C₇H₈Cr(CO)₂]⁺? also fragments directly to [C₇H₈Cr]⁺?.     

Dehydrocyclization of 1-heptene-1-14C over chromia on non-acidic alumina

The dehydrocyclization of 1-heptene-1-¹⁴C has been investigated over a chromia on nonacidic alumina catalyst with the aim of determining the¹⁴C distribution in the ring of the toluene product. As high as 80% of the¹⁴C was consistently found in the methyl position as predicted for direct six-carbon ring formation.

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1--1-C¹⁴ , , C¹⁴ . 80%- C¹⁴ , .