Selenium ylides epoxidation

Epoxide formation. Aldehydes are attacked by the selenium ylides derived from the cation. The -selenonium oxide intermediates decompose hy selenide elimination, resulting in epoxide products. 

A number of benzo- or dibenzo-fused seven membered phosphorus heterocyclic systems have also been studied. These include the benzo-fused oxa-bridged phosphaalkene 76 prepared by thermolysis of 2,3-diphenylindenone 23-epoxide (as a source of the carbonyl ylide 1,3-dipole intermediate) in the presence of /-butylphosphaalkyne. This bridged phosphaalkene is unusually stable even without inert gas blanketing . Reaction of 76 with sulfur or grey selenium stereoselectively affords the thia- or selenaphosphiranes 77 (X = S, Se respectively). <00T6259>... 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

Selenium and tellurium ylides take part in chemistry which is analogous to the reactions discussed thus far, and the subject has been well reviewed. Both alkenes and epoxides are formed in their reactions. 

The reaction of dichlorocarbene with p-hydroxyalkyl selenides is believed to take place selectively on the selenium atom, thus producing the corresponding p-hydroxyalkyldichloromethylenesele-nonium ylide (40 Scheme 197). This, after an intramolecular proton transfer leading to (41 Scheme 197), which probably occurs in a six-membered transition state, collapses selectively to die epoxide or to the carbonyl compound depending upon the substitution pattern around the substituted carbon. The regiochemistry of Ae latter reaction depends upon the stereochemistry of the starting material, and die regioisomeric ratio without doubt reflects the ratio of the two conformers (43 and 44 Scheme 197)... 

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