Selenylation allylic oxidation

An analogous reaction occurs when allylic selenoxides are generated in situ by oxidation of allylic selenyl ethers.277... 

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>. 

The phenylselenyl radical adds irreversibly to the central carbon atom of 2-methylbuta-l, 2-diene (Id) with a rate constant of 3 x 106 M-1 s-1 (23 1 °C) (Scheme 11.7) [45], On a synthetic scale, PhSe addition to cumulated Jt-bonds has been investigated by oxidizing phenylselenol with air in the presence of mono-, 1,1-di- or 1,3-di-substituted allenes to provide products of selective fi-addition. Trapping of 2-phenyl -selenyl-substituted allyl radicals with 02 did not interfere with the hydrogen atom delivery from PhSeH (Scheme 11.7) [31]. 

In connection with the synthesis of cytochalasans, electrophilic selenylation of cycloalkenyl silanes provided exclusively the selenides with the endocyclic double bond. Treatment of the selenides with oxidizing agents (periodate, 3-chloroperbenzoic acid) resulted in the formation of allylic alcohols with the CO bonding at the less hindered /l-alkene face33 - 35. The methylthio group in the molecule is not oxidized under the conditions used for the oxidation of the selenide. 

To use a selenate instead of a sulfoxide rearrangement for the preparation of allylic alcohols can be advantageous if silicon is present in the starting material. Vedejs found that the allylsilane (213) could be selenylated to give (214 equation 79) this reaction includes a rapid 1,3-shift of the initially formed selenide. Oxidation and 2,3-rearrangement to the alcohol (215) followed uneventfully. 

Thermolysis of 24 at 180-190°C for 6 days gave a 35% yield of the desired endo product 35a contaminated with the exo isomer 35b. The ratio of these adducts was quite acceptable (4 1 endo exo. With the basic cytochalasin ring system in hand, what remained was the introduction of the required unsaturation adjacent to the lactone and oxidation of the cyclohexene ring. The former was achieved in 70-76% yield by selenylation followed by oxidative elimination to afford solely the ( )-olefin 36. Hydroxylation was initiated by nitrogen acylation followed by stereo- and regioselective epoxidation of the A olefin. Deacylation and desilylation then afforded cytochalasin F (37) in 51% yield. Cytochalasin B (20) was obtained in a similar way by isomerization of the intermediate epoxide to the allylic alcohol with aluminum isopropoxide, followed by deacylation and desilylation. 

Isomerization of epoxides to allylic alcohols can be carried out under mild conditions using diethylaluminium dialkylamides (usually prepared in situ). A useful conversion of olefins into allylic acetates or ethers involves treatment of the olefin with phenyl selenyl bromide or chloride, addition of an alcohol or acid to give a P-substituted selenide (139), and selective oxidative elimination e.g. truns-cyclodode-cene gave 3-acetoxy-trans-cyclododecene in 85% yield. 

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