Wittig rearrangement stereocontrol

The aza-[2,3]-Wittig rearrangement [47] and the related aza-[3,3]-Claisen rearrangement [48] of vinylaziridines are elegant examples of expansion of the aziridine ring in a stereocontrolled fashion (Scheme 38). 

A novel approach to stereocontrol in 2,3-Wittig rearrangements involves the use of chiral chromium tricarbonyl complexes (Table 8a). The reactions, which were carried out with racemic ethers, show excellent diastereoselectivity as a consequence of a preferred transition state in which the benzylic oxygen and the R -substituent and the vinylic grouping and Cr(CO)3 grouping adopt anti orientations (Lc). 

Lactones, stereocontrolled synthesis using organometallics 82H( 18)357. Lactones, [2,3]-sigmatropic Wittig rearrangement in synthesis of ... 

A related reaction is the [2,3] Wittig rearrangement.33,37 This goes via a live-membered transition state - we shall not go into any more detail about that - but it too is a useful reaction both for making homoallylic alcohols and because of the stereocontrol that can be achieved in the process. Allylic ether 150 gives38 only the diastereoisomer shown of alcohol 152. The [2,3]-sigmatropic rearrangement 151 creates an -alkenc at the expense of a Z-alkene and two new chiral centres at the expense of one. The immediate product of the [2,3]-shift is an oxyanion instead of a carbanion. 

A stereocontrolled entry into congugated silylalkynes has been based on a combination of the highly diastereoselective (2,3)-Wittig rearrangement followed by the stereospecific fragmentation of the... 

Thomas and Hoegenauer achieved aliphatic 1,8-stereocontrol in the S5mthesis of racemic patulolide C 157 and epipatulolide C 158 via [2,3]-Wittig rearrangement of bisallylethers such as 160. ° The stereochemistry of the racemic product was confirmed by completion of the synthesis and comparison with natural epipatulolide C. Thomas reported several other... 

The [1,2]-Wittig rearrangement has found several applications in the stereocontrolled conversion of 0-glycosides to C-glycosides. In a representative case, treatment of 73 with -BuLi provided 74 in 77% yield with > 98 2 The products of these reactions are potentially of biological... 

The stereocontrolled synthesis of homoallylic alcohols and unsaturated ketones via a benzotriazole-mediated [2,3]-Wittig rearrangement... 

A thorough investigation of the Wittig rearrangement of unsymmetrical bis-allylic ethers, e.g. (156), has been made. " Importantly, lithiation occurs almost exclusively on the allyl moiety that is less-substituted at the a- and y-positions, resulting in the formation of only one l,5-dien-3-ol, e.g. (157). A degree of stereocontrol can also be achieved. 

With the key intermediate 28 synthesized, we next investigated the critical stereocontrolled [2,3]-Wittig rearrangement of 28 to construct the requisite decalin system 26, which has both a hydroxymethyl group at C9 with correct stereochemistry and an cjco-methylene function at C8 (Scheme 6). After... 

Marshall has reported an innovative application of the Wittig rearrangement in the synthesis of the 14-membered cembrenoid carbocycles (Scheme 16.21) [104], The stereoselectivity of this process is coupled with the conformational preferences of the macrocyclic ether. The rearrangement reaction of the 17-membered macrocycle 187 provided a 90% yield of the 14-membered alcohols, with 188 as the major diastereomer (dr=81 7 6 7). This represents a remarkable example of macrocyclic stereocontrol (see also Chapter 1) to effect a stereoselective ring-contraction by use of a Wittig rearrangement. 

In parallel with the Parker example shown in Scheme 9.22, the stereocontrolled construction of 95 also capitalized on a [2,3]-Wittig rearrangement to create the C4-C5 bond. 

A well-developed concept to achieve high stereocontrol in the formation of a quaternary chiral center has been introduced by Fraser-Reid, using the Claisen rearrangement along two lines. In the first approach a type III branched-chain sugar is prepared by Wittig... 

In a stereocontrolled route to thromboxane B2, Corey and coworkers used the Eschenmoser rearrangement for the preparation of lactone (91 Scheme 14). The product of the 3,3-sigmatropic shift, amide (90), is directly iodolactonized, thus avoiding often troublesome amide hydrolysis conditions. Another application involving a carbohydrate derivative was demonstrated by Fraser-Reid and coworkers (Scheme 15). Reductive elimination of benzylidene (92), followed by in situ alkylation, Wittig reaction, DIBAL-H reduction and rearrangement, led to amide (94), which was transformed into the corresponding pyranoside diquinane by double radical cyclization. 

Anderson, J. C., Roberts, C. A. The tri-n-butyltin group as a novel stereocontrol element and synthetic handle in the aza-[2,3]-Wittig sigmatropic rearrangement. Tetrahedron Lett. 1998, 39,159-162. 

For instance, the one-pot tandem reaction [2,3]-Wittig-anionic oxy-Cope rearrangement affords the unsaturated aldehydes 571 starting from the bis-allylic ethers 570 at a high level of stereocontrol (equation 224). It was shown that the efficiency of chirality transfer in anionic oxy-Cope rearrangements is determined only by the orientational preference of the oxyanionic bond in the precursors having a single carbinol carbon chiral center . 

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