Selenoxide-mediated elimination

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>. 

Elimination of selenoxides takes place through an intramolecular, syn elimination pathway. The carbon—hydrogen and carbon—selenium bonds are coplanar in the transition state. The reaction is highly traws-selective when acyclic a-phenylseleno carbonyl compounds are employed. The formation of conjugated double bonds is favored. Endocyclic double bonds tend to predominate over exocyclic ones, unless there is no syn hydrogen available in the ring. Some examples of selenoxide-mediated syn eUmination reaction are given in Scheme 6.23. 

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... 

Allylic anda-Allenic Alcohols. The electrochemical generation of a selenenylat-ing reagent, presumably PhSeOR, from a catalytic amount of diphenyl diselenide in the presence of water or an alcohol (ROH) is the basis of a new method for direct conversion of alkenes to allylic alcohols or ethers (Scheme 13). Regio-specific Markovnikov oxyselenenylation followed by electrochemical oxidation and selenoxide elimination, regenerating a selenenylating species, accounts for a sequence that is obviously closely related to the bromide-mediated method reported last year (5,158). 

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