Selenoxides allylic, -sigmatropic rearrangements

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

The use of allylic selenides 166 in oxidation reaction leads to intermediate selenoxides 167, which can undergo [2,3]sigmatropic rearrangements to the corresponding allylic selenenates 168. These componds will lead to allylic alcohols 169 after hydrolysis (Scheme 48). This is also a versatile procedure for the synthesis of optically active allylic alcohols, provided that either an asymmetric oxidation or an optically active selenide is used for the rearrangement. Detailed kinetic and thermodynamic studies of [2,3]sigmatropic rearrangements of allylic selenoxides have also been reported.290-294... 

The [2,3]sigmatropic rearrangement of allylic selenides has proven to be a useful method for the preparation of allenic alcohols. Selenide 170 was obtained by a free-radical selenosulfonation of the corresponding enyne. Oxidation with mCPBA afforded the allenic alcohol 171 in 89% yield via an intermediate selenoxide (Scheme 49).295... 

Asymmetric [2,3]sigmatropic rearrangements can proceed via optically active selenoxides. It has been shown that the Davis oxidant 158 can be used for the oxidation of selenides such as 172. The reaction product, after oxidation and rearrangement, is the allylic alcohol 173 formed with 35% ee (Scheme 50).279,282 Also Sharpless conditions (Ti(/ -PrO)4, (+)-DIPT, /-BuOOH) have been applied to this reaction and the product has been obtained in 69% ee. When, however, the phenyl selenide moiety in 172 is replaced with an or/ < -nitrophenyl selenide, the selectivity is increased to 92% ee in the allylic alcohol 173 using Sharpless conditions.296 Other selenides such as 2 -pyridyl or ferrocenyl selenides gave much lower selectivities. 

Stereoselective syntheses of di- and trisubstituted olefins, (5, 400-402 6, 30-31). The stereochemistry of [2,3] sigmatropic rearrangements has been reviewed (103 references). The review covers the rearrangements of allyl sulfoxides and allyl selenoxides, as well as Stevens and Wittig rearrangements. ... 

The following example demonstrates, however, that there can be differences in the behavior of the selenoxide diastereomers. In the oxidation of a steroid selenide, a 1 1 ratio of allylic alcohol and diene was obtainedlf>. One of the diastereomers, probably the (Sef )-diastereomer, underwent sigmatropic rearrangement, the other diastereomer displayed the syn elimination which seems to be favored because the rearrangement is suppressed as a result of steric hindrance. 

In contrast to the few examples17-19 for the stereoselective sigmatropic rearrangements of acyclic allylic selenoxides, many examples9 16 20-29 have been reported for cycloalkenyl se-lenides. Table 8 shows examples demonstrating the pronounced selectivity for the anticipated suprafacial course of the rearrangement of cycloalkenyl selenoxides. 

Asymmetric selenoxide elimination of the optically active vinyl selenoxides affords optically active allenes and cyclohexylidenes. On the other hand, asymmetric [2,3]sigmatropic rearrangement of allylic selenoxides, selenimides, and selenium ylides leads to the formation of the corresponding optically active allylic alcohols, amines, and homoallylic selenides, respectively. 

It has been demonstrated that cycloadducts (129), which are enolphosphates obtained by regio- and stereospecific [4 + 2] cycloaddition reactions of dienes (130) to a variety of dienophiles, are functionalized versatile synthons having fixed stereochemistry. Their [2,3] sigmatropic rearrangement via allylic sulfoxides and selenoxides (131) provides a direct sterospecific entry to new functionalized bi- and tricyclic allylic alcohol systems (133). The latter has been transformed into the corresponding a-hydroxy ketones (132), key structural subunits of natural products and valuable synthetic intermediates (examples are given in Scheme 32). ... 

Amine oxides, sulfoxides, and selenoxides all undergo the [2,3] sigmatropic rearrangement. The equilibrium between allyl sulfoxides and allyl sulfenates lies on the side of the sulfoxide, but the equilibrium can be pushed toward the sulfe-nate by reduction of its O-S bond. 

Enantioselective oxidation of Z-aryl cinnamyl allylic selenide (83) with oxaziridine (—)-(69) gave 1-phenyl allyl alcohol (85) via an allyl selenoxide-selenate [2,3] sigmatropic rearrangement (Scheme... 

A number of useful enantioselective syntheses can be performed by attaching a chiral auxiliary group to the selenium atom of an appropriate reagent. " Examples of such chiral auxiliaries include (49-53). Most of the asymmetric selenium reactions reported to date have involved Inter- or intramolecular electrophilic additions to alkenes (i.e. enantioselective variations of processes such as shown in equations (23) and (15), respectively) but others include the desymmetrization of epoxides by ringopening with chiral selenolates, asymmetric selenoxide eliminations to afford chiral allenes or cyclohexenes, and the enantioselective formation of allylic alcohols by [2,3]sigmatropic rearrangement of allylic selenoxides or related species. 

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