Syntheses of [Co(bpy)3](2+) yield racemic solutions because the Δ- and Λ-enantiomers are stereochemically labile. However, crystallization and attrition-enhanced deracemization can give homochiral crystal batches of either handedness in quantitative yield. Subsequently, solvent-free oxidation with bromine vapour fixes the chirality because [Co(bipy)3](3+) does not enantiomerize in solution at ambient temperature. This combination of Viedma ripening and the labile/inert Co(II)/Co(III) couple constitutes a convenient method of absolute asymmetric synthesis.
Ethnopharmacological relevanceTurraea robusta and Turraea nilotica are African medicinal plants used for the treatment of a wide variety of diseases, including malaria. The genus Turraea is rich in limonoids and other triterpenoids known to possess various biological activities.Materials and methodsFrom the stem bark of T. robusta six compounds, and from various parts of T. nilotica eleven compounds were isolated by the use of a combination of chromatographic techniques. The structures of the isolated compounds were elucidated using NMR and MS, whilst the relative configuration of one of the isolated compounds, toonapubesin F, was established by X-ray crystallography. The antiplasmodial activities of the crude extracts and the isolated constituents against the D6 and W2 strains of Plasmodium falciparum were determined using the semiautomated micro dilution technique that measures the ability of the extracts to inhibit the incorporation of (G-3H, where G is guanine) hypoxanthine into the malaria parasite. The cytotoxicity of the crude extracts and their isolated constituents was evaluated against the mammalian cell lines African monkey kidney (vero), mouse breast cancer (4T1) and human larynx carcinoma (HEp2).ResultsThe extracts showed good to moderate antiplasmodial activities, where the extract of the stem bark of T. robusta was also cytotoxic against the 4T1 and the HEp2 cells (IC50<10 μg/ml). The compounds isolated from these extracts were characterized as limonoids, protolimonoids and phytosterol glucosides. These compounds showed good to moderate activities with the most active one being azadironolide, IC50 2.4±0.03 μM and 1.1±0.01 μM against the D6 and W2 strains of Plasmodium falciparum, respectively; all other compounds possessed IC50 14.4–40.5 μM. None of the compounds showed significant cytotoxicity against vero cells, yet four of them were toxic against the 4T1 and HEp2 cancer cell lines with piscidinol A having IC50 8.0±0.03 and 8.4 ±0.01 μM against the 4T1 and HEp2 cells, respectively. Diacetylation of piscidinol A resulted in reduced cytotoxicity.ConclusionFrom the medicinal plants T. robusta and T. nilotica, twelve compounds were isolated and characterized; two of the isolated compounds, namely 11-epi-toonacilin and azadironolide showed good antiplasmodial activity with the highest selectivity indices.
The synthesis and crystal structures of bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (dicarbothioamide I) and 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (L 1 ) as well as the syntheses of the palladium(II) chloride and acetate pincer complexes are reported. The stability constant for the palladium complex formation at 258C was found to be (2.04 AE 0.26) Â 10 4 dm 3 mol À1 and (2.30 AE 0.19) Â 10 4 dm 3 mol À1 with D f H ¼ 8 AE 1 kJ mol À1 , D f S y ¼ 108 AE 10 J K À1 mol À1 , and D f H ¼ 17 AE 4 kJ mol À1 and D f S y ¼ 140 AE 20 J K À1 mol À1 for the PdClL 1 and Pd(OAc)L 1 , respectively. The kinetics of formation of the palladium(II) complexes were investigated and the mechanism is proposed to be associative in nature (DH 1 z ¼ 34 AE 2 kJ mol À1 and DS 1 z ¼ À113 AE 8 J K À1 mol À1 , and DH 1 z ¼ 37 AE 3 kJ mol À1 and DS 1 z ¼ À100 AE 8 J K À1 mol À1 for the PdClL 1 and Pd(OAc)L 1 species, respectively). The electrochemical measurements of the acetonitrile solutions revealed irreversible electron transfers consistent with the electrochemical decomposition of the ligand and its coordination complexes.
Even though the isolation of tetrahedral stereoisomers usually presents a synthetic challenge, a highly enantioenriched tetrahedral silver complex could be easily accessed by either crystallization or Viedma ripening. The overall preparation may be regarded as an example of absolute asymmetric synthesis. Experimental results indicate that both crystallization and Viedma ripening follow a similar cluster-controlled mechanism.
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