Selenols oxidation

The 7 a-bromo steroid (9) can also be treated with sodium phenyl selenolate (41). The resultant 7 P-phenyl selenide (13) can be oxidized and the corresponding phenyl selenoxide elirninated to form the 7-dehydtocholesteryl ester (11). 

Treatment of 2-aminoselenazoline with aqueous alkaline base (63) or with hydrogen sulfide (64) leads to ring rupture and gives 2-ureidoethane-selenol and 2-thioureidoethaneselenol. which are not isolated but give by oxidation l,T-(diselenodiethy ene)-bis(2-thiourea) (H2N-C-NH-CH -... 

The oxidative addition reaction of the type shown in Equation (9) were used to prepare some of the first selenolate and tellurolate complexes still used today. 

Stable hindered selenoketones222 are easily reduced to give selenolate ions with various reducing agents and oxidized by air to form diselenides (Scheme 39). 

The lO-E-4 chalcogen(IV) species diphenylselenium(IV) dibromide (1, Fig. 1) and diphenyltellurium(IV) dibromide (2, Fig. 1) oxidize thiophenol to diphenyl disulfide in nearly quantitative yield as shown in equations (13) and (14). Tellurium(IV) dihalides 6-11 also oxidize thiophenol to diphenyl disulfide and benzene selenol to diphenyl diselenide. Similarly, the 12-Te-5 molecule dioxatellurapentalene 45 (Fig. 19) is a mild oxidant for ethylmercaptan, thiophenol, and benzene selenol giving diethyl disulfide, diphenyl disulfide, and diphenyl diselenide in essentially quantitative yield. As shown in equation (15), 1,1,5,5,9,9-hexachloro-1,5,9-tritelluracyclododecane oxidizes six molecules of thiophenol to diphenyl disulfide and 1,5,9-tritelluracyclododecane in 90% yield. In contrast, 12-Te-5 pertellurane 44 and 12-Se-5 perselenane 46 do not oxidize thiophenol to diphenyl disulfide. Instead, these molecules undergo a nucleophilic addition of thiophenol followed by cleavage of the tellurium-carbon or selenium-carbon bond. ... 

Although in humans only MsrBl is a selenoprotein, the depletion of selenium from the diet of mice led to increases in both R and S stereoisomers. This was not initially explained, yet a subsequent study has shown that small molecule selenols (organic selenocysteine homologues) could act as efficient electron donors in vitro for MsrA enzymes. ° This effect has only been shown in vitro, but the possibility that small molecular selenium reductants, or more likely that some selenoproteins that contain reduced selenols (in redox-active motifs) is quite intriguing. Several small selenoproteins do not have real roles and reside in nearly all subcompartments of the cell (mitochondria, ER) where electron donors for Msr enzymes are probably critical to maintain protein stability. Low selenium nutritional status would then have a significant impact on all methionine oxidation, as Future studies to address selenium nutrition and methionine oxidation could prove to be... 

Selenoprotein A is remarkably heat stable, as seen by the loss of only 20% of activity on boiling at pH 8.0 for lOmin (Thrner and Stadtman 1973). Although selenoprotein A contains one tyrosine and no tryptophan residues, it contains six phenylalanine residues and thus has an unusual absorbance spectrum (Cone et al. 1977). Upon reduction, a unique absorption peak emerges at 238 nm, presumably due to the ionized selenol of selenocysteine, which is not present in the oxidized enzyme. The activity of selenoprotein A was initially measured as its ability to complement fractions B and C for production of acetate from glycine, in the presence of reducing equivalents (e.g., dithiothreitol). Numerous purification schemes were adopted for isolation of selenoprotein A, all of which employed the use of an anion exchange column to exploit the strongly acidic character of the protein. 

Oxidation of an a-silyl selenide with hydrogen peroxide leads to an aldehyde, silanol and selenol as the primary products [253]. Evidently, the fragmentation proceeds only after contrapolarization at Se, the intermediate now having an a-d-a array. 

Acyl radical cyclization to cyclohexanones.n Acyl radical cyclization to five-membered rings is well known, but this reaction is also useful for synthesis of substituted cyclohexanones as shown by a recent synthesis of the a-methylenecyclo-hexanone 3. Thus treatment of the selenol ester 1 with Bu3SnH and AIBN in C6H6 at 80° provides a 1 1 mixture of cyclohexanones 2 in 91 % yield. Oxidation of 2 with... 

The dihalides are susceptible to decomposition by hydrolysis. The dibromides oxidize thiols to disulfides, selenols to diselenides, and diphenyl tellurium to diphenyl tellurium oxide. Hydrazine reduces the dihalides to the parent compounds1. 

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