Selenoxide fragmentation

Michael reaction of selenophenols. Selenophenols (and other selenides) undergo enantioselective 1,4-addition to cyclohexenone in the presence of catalytic amounts of cinchona alkaloids. Chemical yields are high optical yields are 10-43%. Usually the optical yield can be enhanced by crystallization. In one case the addition product was converted into an optically active allylic alcohol by hydride reduction followed by selenoxide fragmentation. ... 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

Allylic alcohols. Two laboratories have reported a novel synthesis of allyUc alcohols from two carbonyl compounds. One carbonyl compound (1) is treated with benzeneselenol (2 eq.) under acid catalysis to give a selenoacetal (2). This product can be cleaved to the carbanion (a) by n-butyllithium in THE at -78. The carbanion reacts with a second aldehyde or ketone to form a /3-hydroxyselenide (3). The final step involves oxidation and selenoxide fragmentation to the aUyUc alcohol (4). j3-Hydroxyselenides have been obtained by... 

Furanes. Grieco et al. have developed a general synthesis of 2,4- and 2,3,4-substituted furanes from substituted y-lactones, as illustrated for the conversion of (1), y-decalactone, into 2-hexyl-4-benzylfurane (5). The lactone enolate of (1) is alkylated to give (2). Selenenylation of the enolate of (2) with phenylselenenyl chloride leads to (3). Selenoxide fragmentation of (3) results almost... 

Fragmentation of chiral selenoxides. Selenoxide fragmentation of 7 -phenyl-selenocholesteryl benzoate (1) gives about equal amounts of 2 and 3 fragmentation of the epimeric phenylselenide 4 gives only one product (5). The differing behavior is explained as a result of chirality at selenium of the intermediate selenoxides. ... 

Applications Based on C—Se Bond Cleavage in Selenoketals and Seleno-orthoesters. Primary allylic alcohols are available by homologation of aldehydes (Scheme 4). In this method the selenoxide fragmentation (19) - (20) is best done with t-butyl hydroperoxide in the presence of alumina. 

Reactions.— AUylic amides have been prepared in good yield by the selenoxide fragmentation of -amidoalkylphenylselenides and phase-transfer conditions for the N-alkylation of carboxamides and sulphonamides have been reported. ... 

Cleavage of carbonyl-containing selenoxides and sulfones Fragmentation of epoxy hydrazones Rearrangement of vinylic hydroxycyclopropanes Rearrangement of 3-hydroxy-1,5-dienes (oxy-Cope)... 

The oxidative elimination of primary selenides, readily available from the corresponding alcohol by treatment with an arylselenocyanate and tributylphosphine, is an attractive approach to the synthesis of terminal alkenes." However these reactions are relatively slow when compared with other selenoxide eliminations allowing side reactions, in particular the addition of the areneselenenic acid to the newly formed double bond (Scheme 23), to compete. It has been found that arylselenides with electron-withdrawing substituents fragment more readily, giving improved yields of products, in particular the use of o-nitrophenyl and 2-pyridyl selenides has been recommended (Scheme 37). Often for the elimina-... 

A study has been made of the conversion of selenides and tellurides into their mono-oxides, using A -chlorosuccinimide or Bu OCl followed by alkaline hydrolysis some fragmentation of aliphatic selenoxides occurs at this stage. 

A new site-selective dehydration can convert allylic alcohols (64) and (66) into the dienes (65) and (67) respectively, using 2,4-dinitrobenzenesulphenyl chloride in triethylamine. The intermediate selenenate ester presumably undergoes [2,3] sigmatropic rearrangement to the allyl selenoxide, which then fragments to the diene. Palladium diacetate-triphenylphosphine appears to be an effective catalyst for the elimination of acetic acid or phenol from allylic acetates or phenyl ethers respectively, leading to 1,3-dienes in high yield. ... 

Similarities are noted between the mass spectra of aliphatic and aromatic selenides with analogous sulphides and phosphines. " Selenoxides show the expected fragments derived from the selenate rearrangement product " " Extensions of these studies cover selenium dihalides (no moleculm ion is seen " ), seleninic acids and esters, " and selenones. Mass spectra of diaryl tellurides are very much like those of S and Se analogues. ... 

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