Selenation synthesis

Selenation of the 1,4-dicarbonyl precursor 89 using the reagent (Me2Al)2Se gave the benzo[c]selenophene derivative 90 in good yield <06TL2887>. The synthesis and structural studies of 4,7-dimethoxybenzo[c]tellurophene have also been reported <06AG(E)5666>. 

Common routes for the synthesis of selenates and tellurates(IV and VI) are the reactions of metal oxides or carbonates with the repsective acids (see Section 4). The disadvantage of this procedure is, that one usually obtains hydrates or, at higher acid concentrations, acidic compounds. Because the oxides E03 and E02 (E=Se, Te) are solids under ambient conditions, solid-state reactions with the respective metal oxides are an alternative route to prepare the anhydrous compounds. 

More recently, the direct synthesis of methanol from methane, using metallic gold as catalyst, was reported, involving a purported CH3-Au intermediate. Selenic acid was used as a stoichiometric oxidant as it is known to oxidize gold metal. Moderate turnovers (30) were achieved (Equation (6)).14... 

Hassett, D. J., McCarthy, G. J., Kumarathasan, P. Pflughoeft-Hassett, D. 1990. Synthesis and characterization of selenate and sulphate-selenate ettringite structure phases. Materials Research Bulletin, 25, 1347-1354. 

The synthetic applicability of the electrochemical oxy-transposition may be demonstrated by the one-step synthesis of d -dihydroactinidiolide 60 from the carboxylic acid 59 in 73 % yield. The lactone 60 may be produced by electrochemical intramolecular oxyselenylation followed by elimination of selenic acid (Scheme 3-21)68>. 

Alternative methods for the preparation of selenals and selones which could be applied to the synthesis of related selenium and tellurium heterocyclic systems have been recently reviewed <2005COFGT-II(3)397>. 

The requirements for epithelial cells are somewhat different (Reiss and Dibble, 1988). Mouse keratinocytes (MK-1 cells) enter a GO-phase within 24 h when confluent cultures are fed a serum-free, low Ca2+ (< 0.1 mM) medium supplemented with insulin, transferrin and sodium selenate (see 5.8). Addition of EGF (10 ng/ml) causes cells to enter S-phase after 10-12 h although the percentage of cells responding is not known. Insulin is not essential for this effect but apparently leads to a threefold increase on the rate of DNA synthesis measured 22-24 h after addition of EGF. TGF/ (100 pM) completely abolishes the effect of EGF. 

The literature on inorganic open-framework materials abounds in the synthesis and characterization of metal silicates, phosphates and carboxylates. Most of these materials have an organic amine as the template. In the last few years, it has been shown that anions such as sulfate, selenite and selenate can also be employed to obtain organically templated open-framework materials. This tutorial review provides an up-to-date survey of organically templated metal sulfates, selenites and selenates, prepared under hydrothermal conditions. The discussion includes one-, two-, and three-dimensional structures of these materials, many of which possess open architectures, The article should be useful to practitioners of inorganic and materials chemistry, besides students and teachers. The article serves to demonstrate how most oxy-anions can be used to build complex structures with metal-oxygen polyhedra. 

It is only recently that it has been possible to prepare open-framework metal selenates. The difficulty in the synthesis of these compounds lies in the instability of Se(Vl) in the basic medium. It, therefore, becomes necessary to prepare them in an acidic medium. 

Homopropargylic alcohols are readily available substrates that can be used for the synthesis of 7-lactones. Cul-catalyzed selenation with PhSeBr at the alkyne terminus affords alkynyl aryl selenides. These react with an excess of /i-toluenesulfonic acid monohydrate, in dichloromethane at 60°C, to form a selenium-stabilized vinyl cation intermediate. The cation is then intramolecularly trapped by the tethered hydroxyl group to afford a cyclic selenoketene acetal, which readily adds a molecule of water to give the 7-lactone products (Scheme 55) <2006SL587>. 

In a sequel to the synthesis of 5-carbaphosphatrane (108), reported in SPR 33, Kawashima et al described the oxidation, sulfurization and selenation of... 

How have selenium-accumulators been able to absorb so much Se without any damage to themselves These plants are able to separate inorganic S (as sulphate) from inorganic Se (as selenate or selenite), when they enter in the plants, and to channel the Se into the synthesis of nonprotein amino acid analogues, which are not therefore incorporate into protein synthesis. The adapted plants then sequester them in the vacuoles of the leaves, where they are perfectly harmless to the plants but intensely harmful to any unsuspecting grazing animals. In the non-adapted plants selenium toxicity may be attributed to the replacement of cysteine by selenocysteine and the production of disfunctional proteins in which S-S bond between polypeptide chains are replaced by the more labile Se-Se bonds. Fig (14). 

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