Selenoxides sensitizers

Resin-bound selenium has been used as a linker for alkenes in two ways (a) as an oxidant-sensitive linker (selenoxides readily undergo [5-elimination at room temperature Entries 6-8, Table 3.43 [767-773]), or (b) as a linker cleavable by tin radicals (Figure 3.37 Entries 9 and 10, Table 3.43). The main advantages of selenides as linkers are their stability under a broad variety of (non-oxidizing) reaction conditions, including high temperatures and treatment with acids or bases, and the mild conditions required for their cleavage. 

The excessive replacement of Met by SeMet lowers the protein stability in vitro, but not necessarily in vivo. At the low levels of SeMet normally present in culture media, the substitution of Met by the more oxygen-sensitive SeMet is unlikely to affect the properties of enzymes in an adverse manner. Moreover, since methionine selenoxide is readily converted back to SeMet on reaction with biogenic thiols such as GSH, a mechanism of repair of this type of oxidative damage is available. The reversibility of SeMet oxidation in the... 

Other convenient reagents for the imidation of sulfides and selenides are imidoiodanes such as A-(/>-tolylsulfonyl)-imino(phenyl)iodane (PhI=NTs).304 Unfortunately, these reagents are sometimes difficult to prepare due to their thermal sensitivity and some have even been claimed to be explosive.305 Selenimides are tricoordinate tetravalent compounds and can be isolated in optically active forms. They can be prepared from optically active selenoxides, a reaction which was shown to occur with an overall retention of stereochemistry.306 They can also be obtained by optical resolution of a diastereomeric selenimide and stereochemical issues including kinetics of epimerization by pyramidal inversion were studied in detail.307 Also the enantioselective imidation of prochiral selenides of type 179 is possible by using a combination of A-(/>-tolylsulfonyl)imino(phenyl)iodane (PhI=NTs) and a catalytic amount of... 

Use of dimethyl selenoxide or di(4-anisyl) selenoxide in place of DMSO is claimed to offer some advantages in terms of mildness for polysubstituted or base sensitive benzyl halides. Problems of cost, accessibility and toxicity are not addressed. 

Selenenyl halides are relatively stable, though moisture sensitive, compounds that are generally prepared by the reactions shown in Scheme 7 and behave as electrophihc selenium species. " They react with ketones and aldehydes via their enols or enolates to afford a-seleno derivatives (e.g. (17) in equation 11). Similar a-selenenylations of /3-dicarbonyl compounds, esters, and lactones can be performed, although the latter two types of compounds require prior formation of their enolates. Moreover, the a-selenenylation of anions stabilized by nitrile, nifro, sulfone, or various types of phosphorus substituents has also been reported (equation 12). In many such cases, the selenenylation step is followed by oxidation to the selenoxide and spontaneous syn elimination to provide a convenient method for the preparation of the corresponding a ,/3-unsaturated compound (e.g. 18 in equation 11). Enones react with benzeneselenenyl chloride (PhSeCl) and pyridine to afford a-phenylselenoenones (equation 13). 

Oxidation of selenides. lenides are converted into selenoxides in high yield by oxidation with singlet oxygen (Rose Bengal sensitizer, CH3OH). In the case of -hydroxy selenides, elimination of RSeOH occurs to some extent to form allylic alcohols. ... 

Selenomethionine is the predominant selenium moiety in plants whereas selenocysteine is probably formed from glycine and selenite in mammalian tissues. Analytical approaches are typically based on degradation of the original matrix to these and other amino acids followed by their determination. For this purpose, GC with element-selective or mass spectro-metric detection provides a valuable alternative to HPLC due to improved sensitivity. Selenoamino acids are derivatized with isopropylchloroformate and bis(p-methoxyphenyl) selenoxide, and with ethyl chloroformate. Figure 3 shows comparative selenium-specific GC-AED chromatograms for ethylated extracted enzymatic yeast hydrolyzates of archived selenized yeast employed in the Clark trial , a reference fresh selenized yeast and reference selenomethionine. 

Singlet oxygen (air, visible light and sensitizer) and benzyl phenyl selenide [which may be oxidized by O2 to the selenoxide, which may in turn oxidize Os(VI) back to... 

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