Wittig rearrangement asymmetric induction

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... 

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. 

Ethyl lactate was used for diastereocontrol and asymmetric transmission in a sequential 2,3-Wittig-oxy-Cope rearrangement, affording product in 91% ee (eq 9). - 4 Excellent asymmetric induction has also been noted in the Lewis acid-mediated ene reaction of (5)-ethyl lactate-derived intermediates (eq 10). ... 

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 ...

Hoffmann, R., Brueckner, R. Asymmetric induction in reductively initiated [2,3]-Wittig and retro-[1,4]-Brook rearrangements of secondary carbanions. Chemische Berichte 1992, 125, 1471-1484. 

The sensitivity of the five-centered transition state of the [2,3] Wittig rearrangement to perturbation by steric or stereoelectronic factors presents an enhanced opportunity for substrate-, auxiliary- and/or reagent-induced asymmetric induction. For selected [2.3] Wittig substrates. 

As is the case for the [2,3] Wittig rearrangement, the stereochemical consequences of the [2,3] ylide rearrangement are sensitive to perturbation by external steric and stereoelectronic factors, presenting a useful opportunity for both substrate- and reagent-based asymmetric induction. Rearrangements of carbene-derived ylides of allylic sulfide 1 provide a simple example of substrate-directed diastereosclcction, in which diastereoface selectivity results from attack on the exocyclic olefin via the less-hindered equatorial approach vector112. 

A remarkable example of a stereoselective [1,2] Wittig reaction is the rearrangement of the ally] ether 8, in which both retention of configuration at the migrating center and a high level of asymmetric induction at the allylic center are observed50. 

A similar attempt to employ a carbohydrate template for asymmetric induction in the [1,2] Wittig rearrangement of methyl 6-0-benzyl-2.3,4-tri-O-methyl-a-D-glucopyranoside was modestly successful, affording a 2 1 mixture of diastereomeric products51. 

Bottom Intramolecular asymmetric induction in [2,3]-Wittig rearrangements. 

Scheme 6.11. Asymmetric induction and chirality transfer in [2,3]-Wittig rearrangements of allylic benzyl [78], ally [78], and trimethylsilylpropargyl [79] ethers.

Hydroxy-amides. - Wittig rearrangement of acetamide (424) [LDA, -85 C] provides almost exclusively the erythro-a-hydroxy-amide (425). The likely transition-state geometry suggests that the presence of vinylic substituents larger than methyl should at least maintain this level of selectivity during rearrangement. Unfortunately similar reactions of chiral amides derived from prolinol result in only moderate asymmetric induction. 

A study of chiral induction in the asymmetric [2,3]-Wittig rearrangement of allyl benzyl ether in the presence of a bis(oxazoline) chiral ligand [(5,5)-Box-r-Bu] has been reported (Scheme 45). °... 

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