Selenium-mediated oxidations

Selenium-mediated allylic oxidations producing allylic alcohols have been discussed above however, in some cases oxidation proceeds further to give the a, -unsaturated carbonyl compounds directly, or mixtures of alcoholic and ketonic products. That the regioselectivity observed in these allylic oxidation reactions closely resembles that found in classical selenium dioxide oxidations is in accord with initial formation of the intermediate allylic alcohol before in situ oxidation to the carbonyl compound. This process was studied by Rapoport and was explained mechanistically as an elimination of the intermediate allylic selenite ester via a cyclic transition state, analogous to Ssi (rather than 5n20 solvolysis (Scheme 21). Of the two possible transition states (78) and (79), the cyclic alternative (78) was preferred tecause oxidation exclusively yields trans aldehydes. 

There are still other selenium-mediated oxidative transformations that do not depend on selenium dioxide. Thus, selenylation, oxidation, and selenoxide elimination constitute a valuable reaction sequence for introducing a,j0-unsaturation into a carbonyl compound, as... 

Drabowicz J, Mikolajczyk M (2000) Selenium at Higher Oxidation States. 208 143 -176 Ehses M, Romerosa A, Peruzzini M (2002) Metal-Mediated Degradation and Reaggregation of White Phosphorus. 220 107-140... 

The conversion of the polystyrene-supported selenyl bromide 289 into the corresponding acid 290 allowed dicyclohexylcarbodiimide (DCC)-mediated coupling with an amidoxime to give the 1,2,4-oxadiazolyl-substituted selenium resin 291 (Scheme 48). Reaction with lithium diisopropylamide (LDA) and allylation gave the a-sub-stituted selenium resin 292, which was then used as an alkene substrate for 1,3-dipolar cycloaddition with nitrile oxides. Cleavage of heterocycles 293 from the resin was executed in an elegant manner via selenoxide syn-elimination from the resin <2005JC0726>. 

Biological action is very important in Se redox transformations. Rates of abiotic selenium redox reactions tend to be slow, and in soils and sediments, Se(VI), Se(IV), Se(0) and organically bormd Se often coexist (Tokrmaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGratii 1998). Bacteria use Se(VI) and Se(IV) as eleclron acceptors (Blum et al. 1998 Dungan and Frankenberger 1998 Oremland et al. 1989), or oxidize elemental Se (Dowdle and Oremland 1998), and it is likely that most of the important redox transformations are microbially mediated. 

The addiction to nicotine that makes smoking withdrawal so difficult for many is believed to be mediated by a subset of nicotinic acetylcholine receptors. A bridged bicylcic aryl-benzepine that acts as a partial agonist at those sites is now approved as an aid for tobacco smoking withdrawal. The synthesis starts with the known ben-zonorbomenone (3-1). Treatment of that compound with selenium dioxide leads to the oxidation of the carbon adjacent to the carbonyl group and thus the formation... 

A selenium dioxide induced oxidative cyclization of the 2 -hydroxychalcone 745 is a key step in the total synthesis of ( )-5 -methoxyhydnocarin-D (Scheme 189) <2005T4149>. DMSO containing catalytic amounts of iodine also effectively promotes the oxidative cyclization of 2 -hydroxychalcones to afford flavones <1996CHEC-II>. The DMSO-iodine mediated cyclization of the bis(2 -hydroxychalcone) 746 is a key step in the synthesis of the natural atropisomer4 4 ",7,7 -tetra-O-methylcupressuflavone (Equation 298) <1997TL1087>. Likewise, silica gel supported indium(in) halides catalyze the oxidative cyclization of 2 -hydroxychalcones to afford flavones in excellent yield (Equation 299) <2005TL253>. 

Sodium selenite, at nanomolar concentrations, improved the depressed cardiac performance of the isolated heart subjected to ischemia/reperfusion injury. As ischemia/reperfusion-mediated cardiac function has been reported to occur mainly due to the development of oxidative stress and intracellular Ca2+ overload (Bolli and Marban 1999), it is likely that selenium could protect the heart... 

Since oxidative stress has been reported to produce subcellular redistribution and activation of NF-kB, we measured total NF-kB content in cytosolic and particulate fractions of the hearts subjected to ischemia/reperfusion. In another set of experiments, the activation of NF-kB protein was studied by measuring the total NF-kB and phospho-NF-KB in the heart homogenate. Elevated ratios of NF-kB in particulate versus cytosolic fraction and of phospho-NF-KB and total NF-kB in the hearts subjected to ischemia/reperfusion were reduced by selenium. As oxidative stress has been reported to produce subcellular redistribution and activation of NF-kB and antioxidants have been shown to prevent this alteration (Cargnoni et al. 2002), it is possible that our observation on selenium-induced NF-kB translocation might be due to its antioxidant action. It should be noted that since selenium treatment could reduce the tumor necrosis factor-cc (TNF-ct)-mediated activation of NF-kB (Kim and Stadtman 1997), it suggests a possible involvement of a reduction in the formation of TNF-tx in the hearts subjected to ischemia/reperfusion by selenium. 

The selenium-dioxide mediated allylic oxidation of alkenes was explored by means of 2H and 13C KIEs to clarify the mechanism of ene step.85 Changes of isotopic composition were determined for unreacted 2-methyl-2-butene 33 in reaction with Se02 at 25°C in ferf-butyl alcohol (Equation (49)). 

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