Selenoxides, allyl rearrangement

Entries 1 and 2 in Table 8 are examples of an overall antarafacial 1,3-transposition of a hydroxy group by selenium compounds20,21. Treatment of the alcohols with 2-nitrophenyl seleno-cyanate in the presence of tributylphosphine gave the selenide with inversion of configuration. Oxidation with hydrogen peroxide led to the selenoxide, which rearranged suprafacially to the allylic alcohol. 

Selenoxide-Selenate Rearrangement 4.63.3 Introduction of Allylic Nitrogen... 

As)fmmetric oxidation of allylic selenides gives allylic alcohols via a [2,3]-sigmatropic selenoxide-selenate rearrangement (eq 5). In both oxidations, eqs 4 and 5, the configuration at the selenoxide is that predicted based on the sulfoxide model. 

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. 

Rearrangement of Allylic Sulfoxides, Selenoxides, and Amine Oxides... 

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. 

A number of useful enantioselective syntheses can be performed by attaching a chiral auxihary 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 desymmefrization 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]sigmafropic rearrangement of allylic selenoxides or related species. 

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. ... 

In separate experiments, it was shown that alkenes lacking the OH-trap, such as the geranylse-leninic acid (3), prepared from the corresponding diselenide, cleanly rearrange to linalool (4) (75 %)8 moreover the allyl phenyl selenoxide 5 gave the -alcohol 6 (71 %) exclusively. 

Both classes of selenium compounds have either been postulated as reaction intermediates or could usually only be observed as short-lived intermediates. Allyl selenoxides (X = R), the oxidation products of allyl selenides, display fast rearrangement to allylic selenates, which are hydrolyzed to allylic alcohols under the standard reaction conditions of the oxidation. 

Allylic selenoxides were observed in special cases as short-lived intermediates at low temperature5. Kinetic measurements showed that the rearrangement occurs with reasonable rates4 at temperatures between — 80 °C and —20 C. The rearranged allyl selenates are easily hydrolyzed to the allyl alcohols, i.e., special reagents for cleavage, as in the case of the sulfenates, are not necessary. 

Like allyl sulfoxides, allylic selenoxides rearrange via a highly ordered five-membered transition state. The arguments, already presented for the allyl sulfoxide rearrangement (Section 4.11.2.1.2.), apply for the rationalization of the high E selectivity of double-bond formation. Table 7 shows some examples7,8,12-15 for the strong preference for E double bonds (see also reference 2, Table V-2, p 148). Trisubstituted (A)-allyl alcohols are also obtained from allyl selenides with a substituent at C-2 of the allylic moiety (entries 8-10)7,8. 

The influence of a stereogenic center at C-l seems to override that of the stereogenic heteroatom. In selenoxides monosubstituted at C-1, the R1 substituent will occupy the equatorial position. One of the diastereoniers will rearrange via the exo, the other via the endo, transition state to give the same allylic alcohol, e.g., for the (C)-alkene as shown overleaf. 

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. 

Utilization of a selenium-initiated electrophilic cyclization (lactonization, etherification) in conjunction with a [2,3] sigmatropic selenoxide rearrangement provided a convenient, stereoselective access to highly substituted bicyclic lactones or ethers (entries 6-8)25-29. The electrophilic addition of phenylselenenyl halogenides to dienes can occur in a 1,2-(anti)-or 1,4-fashion furnishing allylic selenides in both cases. 

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. 

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