Asymmetric -Wittig rearrangement

Denmark, S.E. and Miller, PC., Asymmetric [2,3]-Wittig rearrangements with chiral, phosphorus anion-stabilizing groups, Tetrahedron Lett., 37, 6631, 1995. 

Asymmetric Wittig rearrangements.7 High 1,2-asymmetric induction obtains in the [2,3]Wittig rearrangement of the chiral dioxolanes 1 and 3. In each case the... 

Scheme 4.26 [2,3] Wittig rearrangement for asymmetric synthesis of allenic alcohol 102.

The bis(oxazoline) S, 5)-(115) has been used as an external chiral ligand to induce asymmetric diastereoselective lithiation by r-BuLi during [2,3]-Wittig rearrangement of achiral substrates, (fj-crotyl propargylic ethers.It is believed that the enantios-electivity is determined predominantly at the lithiation step. 

Chiral bis(oxazoline) 27 is an effective chiral coordinating agent for enantiocontrol in the [2,3]-Wittig rearrangement. The rearrangement of (Z)-crotyl benzyl ether 84 with f-BuLi/(5, 5)-27 (1.5 equivalents each) in hexane provided [2,3]-shift product (l/ ,25 )-85 in 40% ee (equation 46The feasibility of the asymmetric catalytic version was also examined. In this case, the rearrangement with 20 mol% of 27 in ether was found to provide the same level of enantioselectivity (34% ee). 

Recently, the asymmetric thia-[2,3]-Wittig rearrangement using di-(Z)-menthyl phos-phonate 156 as the chiral auxiliary to 157 has been developed by Masson and colleagues (equation... 

Asymmetric Wittig rearrangement (12, 96). The Wittig rearrangement has been used to control the configuration of two of the four chiral centers in a synthesis... 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Properly designed strategies based on the [2,3]-Wittig Rearrangement are powerful tools for asymmetric synthesis as exemplified by the many examples presented. 

Kiyooka, S.-I. Tsutsui, T. Kira, T. Complete asymmetric induction in the [1,2]-Wittig rearrangement of a system involving a binaphthol moiety. Tetrahedron Lett. 1996, 37, 8903-8704. 

Enantioselective deprotonation to yield a configurationally defined organolithium offers great potential for future asymmetric [2,3]-Wittig rearrangements. The first steps in this area were taken by Nakai and Tomooka, who treated 182 with r-BuLi in the presence of the bis-oxazoline ligand 114. The product 183 is formed in 89% ee.133... 

Chiral lithium bases have been used for enantioselective deprotonation to yield configurationally stable a-oxy carbanions. This holds potential for asymmetric [2,3]-Wittig rearrangement in stereoselective synthesis. Thus, treatment of propargylic ether 72 with (S,S)-3 in THF at — 70 °C to —15 °C afforded propargylic alcohol 73 in 82% yield and in 69% ee of the shown enantiomer96,97. This product was successfully employed as a precursor of (-l-)-Aristolactone (Scheme 55). 

Asymmetric Wittig rearrangement. Three laboratories " have reported enantio- and iy/i-selective [2,3] Wittig rearrangements. Thus rearrangement of the optically active allylic ether 1 provides the allylic alcohol (3R,4R i-2 as the major product, with complete... 

Nakai, T. Tomooka, K. Asymmetric [2,3]-Wittig Rearrangement as a General Tool for Asymmetric Synthesis, Pure Appl. Chem. 1997,69, 595-600. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

Asymmetric induction of the same kind controls the configuration at the allylic methine carbon in compound (134) (134) results from a chain-elongating Wittig rearrangement (mechanism Scheme 6) of sulfone (133 equation 35). Surprisingly, the sense of the asymmetric induction in the rearrangement of silyl ether (135 equation 36) is opposite to that of more than a dozen other ( )-ethers. ... 

Table 10 Asymmetric Induction in Wittig Rearrangements of Allylic Ethers ...

The first asymmetric total synthesis of (+)-astrophylline was accomplished in the laboratory of S. Blechert. The Still variant of the [2,3]-Wittig rearrangement was used to generate the 1,2-trans relationship between the substituents of the key cyclopentene intermediate. The tributylstannylmethyl ether substrate was transmetalated with n-BuLi, which initiated the desired [2,3]-sigmatropic shift to afford the expected homoallylic alcohol as a single enantiomer. 

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