Stereoselectivity 2,3 -Wittig rearrangement

Recently this [2,3]-Wittig rearrangement has received much attention and has been developed into a useful method for the stereoselective synthesis of homoal-lylic alcohols. 

The benzyl anion of l-benzyloxy-2,2,4,4-tetramethylpentan-3-ones undergoes intramolecular nucleophilic addition to the carbonyl group without competing Wittig rearrangement or decomposition the stereoselectivity observed is consistent with avoidance of interaction between aryl and r-butyl groups. 

Somfai s group has demonstrated that the aza-[2,3]-Wittig rearrangement of the viny-laziridines 134 to 135 is useful for the stereoselective construction of the indolizidine alkaloids 136 . ... 

With allylic alcohols, there is the possibility of a [2,3] variant of the Wittig rearrangement that can compete with the [1,2] rearrangement described above (Eq. 22, the indicated electron flow is for the [2,3] rearrangement). The reaction is expected to be a one-step, pericyclic process without a distinct carbanion intermediate. This rearrangement has proven to be useful synthetically because its concerted nature can lead to high stereoselectivity. ... 

Table 8. Stereoselective Ester Enolate [2,3] Wittig Rearrangement of Substituted Methyl (Z)-2-[(4,4,4-Trifluorobut-2-enyl)oxy]acetates (Z)-25 to y.ti-U nsaturated anti-( )-a-Hydroxy-/3-(trifluoromethyl) Esters (E)-2617...

In a general sense, [1,2]-Wittig rearrangements have only a limited application in synthesis because yields and selectivities are frequently moderate at best. For example, the valuable stereoselective conversion of 117 to 118 (which also works with higher homologues) proceeds in only 14% yield.85 86... 

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). 

Q -(Trialkylstannyl) ether, obtained from the stannylation of a-chloroallyl ether by Bu3SnLi, allowed the stereoselective synthesis of lithioether and then, after Wittig rearrangement, the corresponding alcohol (Scheme 49) . ... 

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. ... 

Other Enantioselective Reactions. Enantioselective epoxide elimination by chiral bases has been demonstrated. More recently, the enantioselective [2,3]-Wittig rearrangement of a 13-membered propargylic ally lie ether has been performed using the lithium amide of (f ,f )-(l) as the base for deprotonation (eq 15). For this particular substrate, THF is a better solvent than ether, although pentane produces better results in a related transformation (eq 16). In fact, a change in solvent in this type of reaction has been shown to lead to a reversal of the stereoselectivity of the transformation. ... 

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. ... 

Tomooka, K, Kikuchi, M, Igawa, K, Suzuki, M, Keong, P H, Nakai, T, Stereoselective total synthesis of zaragozic acid A based on an acetal [1,2] Wittig rearrangement, Angew. Chem. Int. Ed., 39, 4502-4505, 2000. 

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