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Lipophilic neutral carriers were synthesized which show Li+/Na+ selectivities of up to ca. 80 in highly lipophilic liquid membranes. The sensor membranes exhibit improved response times and increased lifetimes as compared to systems described earlier. They allow reliable measurements of Li+ in blood serum within the clinical concentration range. A 1 :I Li+/ionophore complex of one representative (N,N,N,N'-tetracyclohexyl-5,5-dimethyl-3,7-dioxaazelaamide) has been prepared, and its structure was elucidated by X-ray analysis.Introduction. -Among the different neutral carrier-based membranes recommended for Li+-selective electrodes [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], only few show acceptable Li+/Na+ selectivities for the potentiometric assay of Li' in blood serum during therapy of manic depressive psychosis

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Methionine as Precursor for the Enantioselective Synthesis of a-Branched Vinylglycines and of Other Amino AcidsMethionine is converted by previously published methods into the diastereoisomerically pure 3-thiahutyl-substituted oxazolidinone (7) and imidazolidinones 5 and 6. An X-ray crystal structure determination of cis-3-henzoyl-Z-(rert -hutyl)-4-(3-thiabutyl)oxazolidin-5-one (7) confirms the configurational assignments made by NOE-NMR measurements. Oxidation to sulfoxides and pyrolytic elimination produce vinyl-substituted heterocycles (see 19,21). Diastereoselective alkylations of the enolate 14 from the imidazolidinone 5 and of the dienolate 23 from the vinyl derivative 19 give geminally alkyl-and/or vinyl-substituted heterocycles. Some of these products were hydrolyzed to free amino acids, such as (R)-Zmethyl-(25a) and (R)-2-ethyl-2-vinylglycine (25b) (R)-2-methylhomoserine (27). Runey-Ni desulfurization of 5 and oxidative degradation of 19 lead to enantiomerically pure derivatives of a-aminobutyric acid (see 28) and of glycine (see 31). respectively. A) Einleitung. ~ Die Methode der a-Alkylierung von Aminosauren nach dem Prinzip der Selbstreproduktion des stereogenen Zentrums3) uber die entsprechenden chiralen Enolate aus Oxazolidinonen der Typen

The title compound (I) is converted to the heterocycles (II) by means of previously described procedures [crystal structure of (IIc): space group P2/n with Z=4].