Selenoxides syn-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>. 

Selenoxide syn-eliminations are another reaction type favoured by less polar solvents (Reich, 1979). The planar 5-membered, pericyclic transition state for syn-elimination of [39] was mimicked by the racemic proline-based cis-hapten [39] to give 28 monoclonal antibodies (Appendix entry 8.5) (Zhou et al., 1997). Abzyme SZ-cts-42F7 converted substrate [40] exclusively into... 

Utilizing selenoxide syn elimination as the key step, a two-step procedure has been developed for the conversion of ketones to enones.22 Cyclobutyl phenyl ketone (52) and 3-phenylcyclobu-tanone (55) were both transformed via the phenyl selenides to the corresponding 1-cyclobutenyl phenyl ketone and 3-phenyIcyclobut-2-en-l-one in 74 and 26% overall yield, respectively.22... 

The reaction of a a-phenylselanyl enolate derived from 15-pentadecanolide and fSj-propylene oxide furnished a 1 1 mixture of lactones [50] (Scheme 41). The butyrolactone, resulting from a partial trans lactonization, led to (+)-Squa-mostanal-A in 55 % yield after selenoxide syn elimination and PDC oxidation of the hydroxy group. 

As exemplified by Scheme 15.35, the resulting selenenylated lactones can be easily converted into the corresponding alkenes and alkanes, by selenoxide syn-elimination and reductive deselenation, respectively [89 b, c]. 

Simultaneous introduchon of both sulfur and selenium functions into carbon-carbon unsaturated compounds via a radical mechanism is also demonstrated by selenosulfonahori [149] and selenothiocarboxylatiorl [150] (Scheme 15.70). In these addihon reactions, attack of sulfur-centered radicals at the terminal position of alkenes and the subsequent Sh2 reaction on the selenium lead to the formahon of anti-Markovnikov adducts regioselechvely. The selenosulfonahon can be apphed to a variety of unsaturated compounds, for example alkynes, allenes, and vinylcyclopropanes, and combination with the selenoxide syn-elimination procedure... 

Selenoxide syn-elimination is widely accepted as one of the most expedient methods for olefin formation, because selenoxide elimination occurs at lower temperatures than the corresponding sulfoxide elimination [Ic, li, 182]. The elimination is beheved to occur via a cyclic transition state in which all five participating centers are located on the same plane the selenoxide ehmination is therefore stereospecific (Scheme 15.94). 

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