A liquid membrane electrode based on a lipophilic derivative of vitamin B12 Is described which exhibits a selectivity sequence N02" > SCN" >> CI04~> Cl" ~NOg", with a preference for N02" over NOg" and Cl" by >1042 and 1048, respectively. Response times are on the order of seconds and the emf drift of the cell assembly is smaller than 5 ¿tV/h. The potentiometrlcally observed reversible response of the membrane system can be correlated with the coordination of the substrate anion to the lipophilic Co(III) complex using spectrophotometrlc techniques. The cationic Co(III) complex acts as a very selective charged carrier for anions.

SummaryIncorporation of the lipophilic Co(I1I)-cobyrinate octadecyl-cobester 1 and of its ionic aqua-cyano perchlorate derivative 2 into poly(viny1 chloride)/bis(l-butylpentyl) adipate liquid membranes induces a selectivity, measured potentiometrically, of about lo3 for SCN-and NO; with respect to Cl-, but only of about 4 for C10; us. C1-. This is in contrast to classical anion-exchanger membranes, which exhibit a selectivity sequence C10; > SCN->> NO; > C1-in accordance with the Hofmeister series. The Co(1II)-corrins 1 and 2, when components in solvent polymeric membranes, undergo exchange of axial ligands and behave as highly selective carriers for SCN-and NO;.An anion-selectivity sequence with a preference for lipophilic and a rejection of hydrophilic anions [ 11 is characteristic of liquid membranes based on various classical anion exchangers [2] such as quarternary ammonium salts or metal-phenanthrolinates (Hofmeister series [3]). For dissociated anion exchangers, where the complexation between the cationic sites and the counterions is negligible, the anion selectivity is controlled by the distribution coefficients of the anions between the aqueous sample phase and the membrane phase [1] [4]. A different behaviour is expected for associated anion exchangers. In this case, the parameters determining the selectivity strongly depend on the stability constants of the complexes of the exchanger sites with the counterions, provided that the mobilities of these sites are large compared to those of the counterions [1][4]. During our search for components with a selective interaction between cationic sites and counterions, vitamin B,, and its derivatives appeared as promising candidates. This was supported by the available information on their complexation behaviour towards anionic ligands in aqueous solutions [5]. For exploratory potentiometric studies on anion-selective properties of vitamin B,, derivatives in membranes, the lipophilic Co(II1)-complex 1 was prepared. This was followed by the synthesis of 2 and 3, which were used for an evaluation of structural effects and for comparison with Rktey's cholestano-cobaloxime 4 (chlorobis(cholestane-2,3-dione dioxime)pyridinecobalt) [6]

The preparation and X-ray crystal structure of a monomeric cobalt(ii)corrinate derived from vitamin BI2 is reported.

A concise, stereocontrolled strategy for the total synthesis of allopumiliotoxin A alkaloids is described. A much improved second generation total synthesis of enantiopure (+)-allopumiliotoxin 267A (3) was accomplished in 10 steps and 11% overall yield from the commercially available oxazolidinone precursor of alcohol 32 and 17 steps and 4% overall yield from N-[(benzyloxy)carbonyl]-l-proline. The first synthesis of (+)-allopumiliotoxin 323B‘ (4) rigorously confirms the complete stereostructure of 4 and establishes that the major C(15) epimer isolated from dendrobatid frogs has the 15S configuration. The total synthesis of 4 was realized in 5 steps and 17% overall yield from alkyne 39 and aldehyde 20; the synthesis proceeded in 13 steps and 6% overall yield from (S)-2-methyl-1-penten-3-ol and 17 steps and 3.5% overall yield from N-[(benzyloxy)carbonyl]-l-proline, the precursors, respectively, of alkyne 39 and pyrrolidine aldehyde 20. The first total synthesis of allopumiliotoxin 339A (5) also confirmed the full stereostructure of this alkaloid. The synthesis of enantiopure 5 was achieved in 5 steps and 32% overall yield from alkyne 45 and pyrrolidine aldehyde 20; the synthesis proceeded in 17 steps and ∼7% overall yield from N-[(benzyloxy)carbonyl]-l-proline and 16 steps and ∼6% overall yield from the commercially available oxazolidinone precursor of 45. These syntheses provide the best illustrations to date of the substantial utility of iodide-promoted iminium ion−alkyne cyclizations for constructing highly functionalized nitrogen heterocycles.

Treatment of heptamethyl cob(1)yrinate (2) in toluene/tetrahydrofurane (ca. 4 : 1) with methyl p-toluenesulfonate under exclusion of 0, and with protection from light leads to the selective formation of the heptamethyl Cop-methylcob(I1I)yrinate (perchlorate lb) in 75% yield. In contrast, methylation of 2 with methyl iodide under the same conditions results in the isomeric heptamethyl Coa -methylcob(III)yrinate (perchlorate la) in 73 % yield, besides 7 YO of 1 b. This complementary diastereofaceselectivity of the methylation at the Co-center results from alkylation under kinetic control and apparently involves two mechanistically distinct alkylation processes. A radical mechanism is considered to account for the stereochemically unusual outcome of the reaction with methyl iodide.Soweit bekannt, ist die bei der Reaktion der sogenannt ccsupernukleophilen)) [I] Cob(1)yrinsiiurederivate mit Alkylhalogeniden oder (primaren) Alkyltosylaten resultierende, der Struktur des Coenzyms B,Z [2] analoge 8-Konfiguration der Co-gebundenen Alkylgruppe in <(komplettenn Alkylcob(II1)yrinaten sowohl auf kinetische als auch auf thennodynamische Faktoren zuruckzufuhren, einschliesslich der dann moglichen Co-Koordination der Nukleotidbase von der a -Seite her [3]'). Ahnlich durfte eine thermodynamisch bedingte Praferenz fur die matiirlichei) (d. h. B-stiindige) Alkylgruppenkonfiguration ebenfalls fur die ctinkompletten)) Alkylcob(II1)yrinsiiurederivate vorliegen, denen der dirigierende Einfluss der a -standigen Nukleotidfunktion fehlt [3]. Jedenfalls uberwiegt nach einer von Friedrich et al. durchgefiihrten stereochemischen Korrelation*) bei mehreren, in wasseriger Losung thermo-und photolytisch aquilibrierten Methylcob(II1)yrinsaurederivaten das Koordinationsisomere mit Cop-Methyl-Konfiguration deutlich [6]. Die Moglichkeit, dass die Konfiguration am Metal1 (Cop us. Coa) durch mechanistische Faktoren der Alkylierung beeinflusst wird, scheint hingegen noch unbeachtet geblieben zu sein. ') Z. B. entsteht Cop-Athylcob(II1)alamin aus Cob(1)alamin und p-Toluolsulfonsaure-athylester in einer (auch) am Athyl-C-Atom (weitgehend) stereospezifischen Reaktion [4]. In H20 aquilibriertes Methylcob-(1II)aIamin liegt zu > 97% in der Cu~-Methyl-Konfiguration vor [5]. UVjVlS-und CD-spektroskopische Korrelation rnit Cop-und Coa-Methylcob(lIl)alamin 15-71. ')