Allylic selenoxide 2,31-rearrangements

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 most widely used heteroatom variant of the Mislow-Evans rearrangement involves the use of allylic selenoxides in place of allyl sulfoxides. Early studies on allylic selenoxide rearrangements were carried out by Sharpless and Lauer and Reich,and several recent natural product syntheses have featured impressive applications of the method. As Reich pointed out, capture of the selenate esters resulting from the rearrangement can be easier than in the sulfur analogues because the selenates are easily hydrolyzed. Nevertheless, the addition of a selenophilic trap is often beneficial as highlighted in a number of the exanples in this section. 

En route to a total synthesis of the anticancer compound FR901464, Koide and coworkers carried out a diastereoselective allylic selenoxide rearrangement upon oxidation of either allyl selenide 251 [Scheme 18.641. Optimization studies using the preformed allyl selenide 251 identified the o-nitrophenyl selenide as an effective aryl substituent and N,N-dimethylaminopyridine as the best selenophilic base additive in the formation of rearrangement product 252 (see top of Scheme 18.641. Reactions were slower and diastereoselectivity, a crucial parameter here, was lower using other bases or with less than 3 equiv of DMAP. Using the optimized conditions, a one-pot method for overall 1,3-allylic alcohol transposition was... 

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... 

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. ... 

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. 

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. 

If the chiral allylic selenoxides were obtained by enantioselective or diaste-reoselective oxidation of the allylic selenides, the formation of the corresponding chiral allylic alcohols is expected after the hydrolysis of the intermediate chiral allylic selenenates obtained by chirality transfer in the rearrangement... 

In fact, allylic selenoxides (207b) are so unstable with respect to their rearrangement products that they are never (209 is an exception) isolated. The oxidation of an allylic selenide leads, after hydrolysis, directly to the rearranged alcohol. Equation (77) demonstrates how such a rearrangement can be incorporated in the diastereoselective 1,3-transposition of an allylic alcohol. 

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... 

By a Reaction Involving the Rearrangement of an Allylic selenoxide The transposition of allylic selenoxides to allyl alcohols is a well-established reaction24-27,189) which parallels the one described with sulfur analogs190 . It was found, however,45 that phenylselenoxy derivatives rearrange much more easily than their phenylthio or their methylseleno analogs. 

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