Selenoxides, elimination reaction 1.5 -shifts

There are also stereochemical considerations here and Holmes used the Evans asymmetric aldol reaction (chapter 27) to make the starting material 174 R=Bn. The formation of any allyl vinyl ether reagent involves no change in the stereochemistry of the allyl alcohol - this is acetal exchange at the vinyl ether or acetal centre. The enol ether was added in masked form as a selenium compound 175 (chapter 32) as selenoxides eliminate at room temperature. The stereochemistry is developed directly from that in 177 as it transforms during the [3,3] shift. 

The thermal decomposition (pyrolysis) of alkylaryl selenoxides (selenoxide pyrolysis) to an alkene and an aryl selenic acid Ar—Se—OH often takes place even at room temperature (Figure 4.10). This reaction is one of the mildest methods for introducing a C=C double bond by means of a /3-elimination. The mechanism is described by the simultaneous shift of three electron pairs in a five-membered cyclic transition state. One of these electron pairs becomes a nonbonding electron pair on the selenium atom in the selenic acid product. The Se atom is consequently reduced in the course of the pyrolysis. 

Another method for the conversion of an alkene into an allylic alcohol, but with a shift in the position of the double bond, proceeds from the corresponding p-hydroxyselenide. The p-hydroxyselenide can be obtained from the epoxide by reaction with phenylselenide anion or directly from the alkene by addition of phenylselenenic acid, phenylselenenyl chloride in aqueous MeCN, or by acid-catalysed reaction with A-phenylseleno-phthalimide. The hydroxyselenide does not need to be isolated, but can be oxidized directly with tert-BuOOH to the unstable selenoxide, which spontaneously eliminates phenylselenenic acid to form the E-allylic alcohol. For example, 4-octene gave 5-octen -ol (6.15). Elimination takes place away from the hydroxy group to give the allylic alcohol no more than traces... 

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