Enantioselective HRGC Separation of Toxaphene Congeners by Ethylated γ‐Cyclodextrins

The enantioselectivity of three batches of octakis‐(2,3,6‐tri‐O‐ethyl)‐γ‐cyclodextrin (TEG‐CD), which differed significantly in their degree of ethylation, is reported for toxaphene congeners. The cyclodextrin composition was determined by high performance liquid chromatography‐mass spectrometry. Columns prepared with almost fully ethylated cyclodextrin showed no enantiomer separation. Increasing the average number of free OH groups up to an optimum of 3.8 allowed to resolve the following toxaphene congeners into enantiomers: Parlar no. 11, 12, 15, 21, 25, 32, 38, 42.1, 42.2, 50, 51, 56, 58, 59, 62, 69 as well as B8‐1412. More free OH groups did not improve the enantiomer resolution. The structure of the polysiloxane used for dilution of TEG‐CD also had an influence on the enantiomer separations achieved. Compared to OV 1701‐OH, the use of PS 086 significantly improved the enantiomer resolutions of the separated congeners. However, neither increasing the column length nor the cyclodextrin amount in the stationary phase led to better separation results.     

Catalytic enantioselective addition of diethylzinc to (hetero)aromatic aldehydes

The asymmetric alkylation with diethylzinc of five heterocyclic aldehydes and benzaldehyde (for comparison) has been studied in the presence of two optically active amino alcohols: (S)-2-amino-1-butanol (AB) and (1S,2R)-N,N-dibutylnorephedrine (DBNE). A number of chiral (hetero)aromatic secondary alcohols were synthesized in high yields (95–98%) with enantioselectivity up to 92% enantiomeric excess (ee) in the presence of DBNE catalyst. Optically active thienyl and 4-pyridyl derivatives were prepared for the first time by catalytic asymmetric alkylation. The influence of the amount of DBNE on the enantioselectivity was investigated. In contrast to benzaldehyde, 2-furan- and 2-thiophene-carbaldehydes, in the case of 3- and 4-pyridinecarbaldehydes the ee values depend directly on the catalyst concentration. © 1998 John Wiley & Sons, Ltd.     

Ein neuartiges Impulsverfahren mit digitaler Auswertung zur automatischen Titration

Zusammenfassung Die Verwendung von Schrittmotoren zum Antrieb von Kolbenbüretten bringt eine Reihe von Vorteilen mit sich, die einerseits in dem praktisch trägheitsfreien Verhalten, andererseits in der Anwendung der Impulstechnik begründet liegen. Es wird eine Bürette beschrieben, die bei einem Zylindervolumen von 10 ml noch einzelne Mikroliter mit einer Genauigkeit von etwa 5% zu dosieren gestattet. Das Volumen wird mittels eines Impulszählers angezeigt. Der Schrittmotor wird in seiner Drehzahl von einem Impulsgenerator entweder manuell oder automatisch in Abhängigkeit vom Zeitdifferential an der Meßkette frequenzgesteuert. Für die Aufzeichnung von Titrations-Diagrammen kommt ein Schreiber zur Verwendung, der für den Papiervorschub ebenfalls einen Schrittmotor aufweist. Damit ergibt sich eine einfache Möglichkeit zur Synchronisation von Kolbenweg und Papiertransport mittels einer Bürette und Schreiber verbindenden Impulsleitung.

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Summary The application of stepping motors for driving piston burettes offers several advantages, based mainly on the low inertia of the driving system and the pulse technique used. A burette is described with a cylinder of 10 ml allowing the delivery of single microliters with an accuracy of about 5%. The volume is indicated by a pulse counter. The stepping motor is regulated by a pulse generator the frequency of which is controlled manually or automatically, depending on the first derivative of the signal of the measuring assembly. Titration diagrams are plotted by a recorder the paper transport of which is achieved by a stepping motor as well. Thus, the synchronisation of the movements of the piston and the paper can be performed free of mechanic coupling systems with a simple electric lead.

     

Mass spectrometry and the analysis of ultratrace quantities of chlorinated compounds in arctic air

Negative chemical ionization mass spectrometry helps to quantify picogram amounts of chlorinated hydrocarbons in arctic air masses. This technique has become a reliable routine method which enables the selective determination of compounds with high electron affinity.