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(Z)-2-Chloroalk-2-en-1-ols are obtained in excellent yields from a wide variety of aldehydes by addition of (E)-chromium vinylidene carbenoids, stereospecifically generated from trichloroalkanes using CrCl(2) in THF at room temperature. [reaction: see text]
[reaction: see text] A new catalytic cycle has been developed to effect all three subgroups of Cr-mediated couplings, i.e., (1) Ni/Cr-mediated alkenylation, alkynylation, and arylation, (2) Co/Cr-mediated 2-haloallylation, alkylation, and propargylation, and (3) Cr-mediated allylation. In the presence of chiral sulfonamide ligands, good asymmetric inductions can be achieved for some of the Ni/Cr-mediated alkenylation, Co/Cr-mediated 2-haloallylation and propargylation, and Cr-mediated allylation.
[reaction: see text] Vicinal stereocontrol during nucleophilic addition of tert-butyl lithiopropionate to eta(6)-anisole chromium tricarbonyl complexes with differing para substituents has been studied. Excellent vicinal double stereoinduction (>99:1) was observed when the para substituent was Si(CH(3))(3), and this has been applied to a stereoselective formal synthesis of (+/-)-erythro Juvabione. Asymmetric synthesis by chiral auxiliary directed nucleophilic addition is also discussed.
[reaction: see text] The combination of CrCl(2) and silyl chlorides converts aryl and conjugated aldehydes into olefinic adducts in good to excellent yields. When constrained by structural features, the intermediate vic-diol can be isolated. Available data are consistent with a novel chromium Brook rearrangement.
A total synthesis of phomactin A (1) based on a Cr(II)/Ni(II) macrocyclisation from the aldehyde vinyl iodide 11, leading to 12, followed by elaboration of the epoxyketone 16, which then undergoes spontaneous pyran-hemiacetal formation on deprotection, is described.
Some eucaryotic organisms, including many plants, yeast and mice, have a higher iron uptake during iron deficiency because the capacity to reduce Fe3+ from the environment to Fe2+ is greatly enhanced. To determine whether this occurs in rats, a common experimental model for iron absorption in humans, we compared the in vivo capacity to reduce intraluminal Fe3+ in iron-deficient and normal rats. We also measured potential Fe(3+)-reducing components within the intestinal lumen and on the mucosal surface. Iron-reducing capacity was higher in iron-deficient rats, by a significant (P = 0.026) but modest 20%, in parallel with higher mucosal weight (R2 = 0.501, P = 0.003). In vitro iron reduction by lumen contents was correlated with mucosal weight, even though mucosal tissue was not present in the assays. This capacity was not related to ascorbic acid, glutathione or other nonprotein sulfhydryls. Mucosal ferric reductase activity was higher in iron-deficient rats in parallel with higher tissue weight, but the specific activity did not differ and the higher total activity was not associated with the brush border fraction. The role of endogenous Fe3+ reduction in regulating iron absorption should be investigated in humans and in other experimental models.
Chromium-contaminated soils threaten surface and groundwater quality at many industrial sites. In vadose zones, indigenous bacteria can reduce Cr(VI) to Cr(III), but the subsequent fate of Cr(III) and the roles of bacterial biofilms are relatively unknown. To investigate, we cultured Pseudomonas putida, a model organism for vadose zone bioremediation, as unsaturated biofilms on membranes overlaying iron-deficient solid media either containing molecular dichromate from potassium dichromate (Cr-only treatment) or with deposits of solid, dichromate-coated hematite (Fe+Cr treatment) to simulate vadose zone conditions. Controls included iron-deficient solid medium and an Fe-only treatment using solid hematite deposits. Under iron-deficient conditions, chromium exposure resulted in lower cell yield and lower amounts of cellular protein and carbohydrate, but providing iron in the form of hematite overcame these toxic effects of Cr. For the Cr and Fe+Cr treatments, Cr(VI) was completely reduced to Cr(III) that accumulated on biofilm cells and extracellular polymeric substances (EPSs). Chromium exposure resulted in elevated extracellular carbohydrates, protein, DNA, and EPS sugars that were relatively enriched in N-acetyl-glucosamine, rhamnose, glucose, and mannose. The proportions of EPS protein and carbohydrate relative to intracellular pools suggested Cr toxicity-mediated cell lysis as the origin. However, DNA accumulated extracellularly in amounts far greater than expected from cell lysis, and Cr was liberated when extracted EPS was treated with DNase. These results demonstrate that Cr accumulation in unsaturated biofilms occurs with enzymatic reduction of Cr(VI), cellular lysis, cellular association, and extracellular DNA binding of Cr(III), which altogether can facilitate localized biotic stabilization of Cr in contaminated vadose zones.
Chromic complexes of N,N',N''-triacetylfusarinine C have been prepared and examined for biological activity in Mycelia sterilia EP-76. The iron transport system of this organism recognizes only the lambda coordination isomer of Cr(III)-triacetylfusarinine C even though the delta configuration predominates in solution. Chromium is excreted into the medium following triacetylfusarinine C-mediated uptake of the metal. Hydrogenation of the double bonds conjugated to the hydroxamic acid functions of triacetylfusarinine C yields four chromatographically distinct ferric complexes. Two of the complexes have the lambda configuration, while the other two have the opposite (delta) configuration. The complexes differ in effectiveness as siderophores in M. sterilia EP-76, the lambda isomers being most active.
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A plasticized Cr3+ ion sensor by incorporating 2,3,8,9-tetraphenyl-1,4,7,10-tetraazacyclododeca-1,3,7,9-tetraene (TTCT) ionophore exhibits a good potentiometric response for Cr3+ over a wide concentration range (1.0 x 10(-6)-1.0 x 10(-1) M) with a slope of 19.5 mV per decade. The sensor response is stable for at least three months. Good selectivity for Cr3+ in comparison with alkali, alkaline earth, transition and heavy metal ions, and minimal interference are caused by Li+, Na+, K+, Co2+, Hg2+, Ca2+, Pb2+ and Zn2+ ions, which are known to interfere with other chromium membrane sensors. The TTCT-based electrode shows a fast response time (15 s), and can be used in aqueous solutions of pH 3-5.5. The proposed sensor was used for the potentiometric titration of Cr3+ with EDTA and for a direct potentiometric determination of Cr3+ content in environmental samples.
A fish meal and a soybean meal sample were bioassayed for available amino acid content according to five different published techniques. The techniques consisted of: 1) a two week chick growth assay (GA) for available lysine; 2) a digestibility assay performed by total fecal collection (TFC); 3) a digestibility assay based on the analysis of ileal contents IA); 4) a procedure based on partial fecal collection and analysis of a dietary marker using gnotobiotic chicks (GF); and 5) procedure 4) using conventional chicks (CONV). The GA gave high lysine availability estimates for both proteins, but values for fish meal were significantly reduced when the level of dietary incorporation was increased. The TFC assay proved difficult in collecting adequate sample and, as a consequence, abnormally high availability values were obtained. Modification of this collection procedure resulted in values comparable to the IA which gave the lowest average values of all the assays (90.5 per cent for fish meal and 91.1 per cent for soybean meal). Mean availabilities calculated from GF and CONV assay were significantly higher than the modified TFC and IA techniques. Significant differences were obtained in several individual amino acids when the GF and CONV assay were compared. A growth assay, using diets formulated on the results of these assays, showed that the available amino acid profile obtained from the CONV assay resulted in significantly improved weight gains and feed conversions as compared to a similar diet based on the chemical analysis of amino acids alone. All diets utilizing available amino acid data resulted in improved feed conversions.
The aim of this paper was to study the conditions favourable to Cr (III) oxidation such as: type of soil, soil water content, presence of organic matter and three oxidisers (MnO2, NaNO3 and Fe2O3). Also concentrations of different species of chromium in water and soils were examined in the area contaminated with tannery wastes containing 5-10% of Cr2O3. The action of oxidisers was studied in batch experiment for 2 h at room temperature at pH=5.35+/-0.2. The highest level of oxidised trivalent chromium was registered for soil with the lowest content of organic matter (0.72% C(org)), when Cr (III) was oxidised by MnO2, Fe2O3 and NaNO3 did not show properties towards Cr (III) oxidation under the conditions described.
After exposure of growing chicks to different concentrations of dietary chromium (Cr) as potassium chromate (K2CrO4) for a period of 3 w, the Cr uptake by different internal organs was determined. More Cr was accumulated in the kidney, liver, pancreas and spleen than in the blood, muscle, heart and lung. A very small amount was found in the brains of the birds given different concentrations of Cr.
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Six metals and/or their compounds have been recognized as carcinogens: arsenic, beryllium, cadmium, chromium, cobalt and nickel. With the exception of arsenic, the main rote of exposure is inhalation and the main target organ is the lung. Arsenic is exceptional because it also produces tumors of skin and lung after oral uptake. With the exception of hexavalent chromium, carcinogenic metals are weak mutagens, if at all, and their mechanisms of carcinogenicity are still far from clear. A general feature of arsenic, cadmium, cobalt and nickel is their property to enhance the mutagenicity and carcinogenicity of directly acting genotoxic agents. These properties can be interpreted in terms of the ability of these metals to inhibit the repair of damaged DNA. However, because carcinogenic metals cause tumor development in experimental animals even under exclusion of further carcinogens, other mechanisms have to be envisaged, too. Evidence will be discussed that carcinogenic metal compounds alter patterns of gene expression leading to stimulated cell proliferation, either by activation of early genes (proto-oncogenes) or by interference with genes downregulating cell growth. Special reference will be devoted to the effects of cadmium and arsenic on gene expression, which have been studied extensively. Possible implications for occupational safety and health will be discussed.
Twenty-six workers were studied (9 chrome-platers exposed to chromium dioxide and 17 workers exposed to metallic chromium dust) in order to investigate the macroscopic and cytological changes of the nasal mucosa due to exposure to water-soluble hexavalent chromium or to metallic chromium dust in the electroplating industry and the role of different valencies in the onset of nasal disease. Experimental and epidemiological data have shown that hexavalent chromium, which has strong oxidative power, induces more noticeable toxic effects on tissues and mucous membranes than other compounds. The correlation between the degree of local toxic effects and the chemical state of chromium was demonstrated in both the macro- and the microscopic investigations and in particular in the cytological examinations: cases of atypia were found only in workers exposed to hexavalent chromium. Evidence of atypia raises the question of whether hexavalent chromium may act as a carcinogenic agent on the rhinosinusal mucosa. For this reason, the introduction of cytological nasal examination in health surveillance programmes for this category of workers acquires considerable importance. Sample collection from the nasal mucosa by brushing is the method of choice since it is simple, non-invasive and gives good diagnostic results.