Epoxidation from selenoxides

Allylic alcohols can also be obtained from epoxides by ring opening with a selenide anion followed by elimination via the selenoxide (see Section 6.8.3 for discussion of selenoxide elimination). The elimination occurs regiospecifically away from the hydroxy group.116 117 118... 

Certain chiral epoxides can be prepared from fl-hydroxyselenides (e.g., 43), typically intermediates for allylic alcohol synthesis. The novel reactivity of these substrates seems to be restricted to those cyclic compounds in which the hydroxy and the selenoxide groups can achieve an antiperiplanar disposition [95TL5079],... 

Another synthetic application of selenoxides is the conversion of epoxides into allylic alcohols (equation 594) [168. Dehydrogenation occurs almost exclusively away from the hydroxyl group. 

Alkyl phenyl selenoxides bearing a P-hydrogen undergo facile syn elimination to form olefins under much milder conditions than the corresponding sulfoxides. The selenium anion formed from (PhSe)2 is an excellent nucleophile and easily opens the epoxide to give the corresponding hydroxyl selenide. This intermediate is not isolated but... 

Additions of various nucleophiles to vinyl selenoxides - and vinyl selenones have been described. Thus potassium hydroxide and amines add to vinyl selenones and directly produce epoxides and aziridines, respectively, whereas 3-methoxyoxetanes have been obtained from 7-hydroxy-a-alkenyl selenones and st ium methoxide (Scheme 111). ... 

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

Both the selenoxide and the selenonium ylide routes have been applied to cyclobutyl derivatives, themselves readily available 57) from selenoacetals of cyclo-butanones on one hand and primary alkyl halides, epoxides, or carbonyl compounds on the other. 

Oxidation to the selenoxide 39 and thermal elimination (chapter 32) produces a new alkene 40 that forms an epoxide 41 on the outside face again. At this point every carbon atom in the six-membered ring is functionalised, five oxygen-based and one nitrogen, and this all started with symmetrical cyclohexadiene. Each functional group was selectively introduced from an alkene. 

Sulfides and selenides both stabilize an a-carbanion (see Section 1.1.5.2) and alkylation followed by elimination provides a route to substituted alkenes. Some examples of selenoxide eliminations are given in Schemes 2.24—2.26. Like the other syn eliminations described in this section, the regioselectivity of selenoxide elimination can be poor. Elimination normally takes place preferentially towards a conjugating (3-substituent or away from an electronegative p-substituent. This latter facet allows a good method for converting epoxides into allylic alcohols. 

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... 

A one-step synthesis of deoxycarbonyl derivatives from epoxides uses o-nitro-selenoxides as intermediates (Scheme 1). The reaction is, however, considerably... 

Starting materials, like 141, which are accessible from alkenes by azido-selenenylation, afforded only in a few cases vinyl azides exclusively as shown by the example 141 142 (Scheme 5.19). In most cases, after oxidation of the vicinal phenylseleno azides and elimination reaction on the intermediate selenoxide, mixtures of allyl and vinyl azides were obtained. Ring opening of trialkylsilyl-substituted epoxides was utilized several times for stereoselective synthesis of vinyl azides. Thus, treatment of the frani -contigured oxiranes 143 with azidotrimethylsilane and boron trifluoride etherate yielded cis-conflgured products 145. This result was explained by the intermediate 144 which should undergo anti-elimination. On the other hand, when subjected to sodium azide in dimethylformamide, epoxides 146 were transformed into vinyl azides 148 via... 

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