Still rearrangement

Introduction of the carboxylic acid precursor group [2,3]-Wittig—Still rearrangement... 

The regioselectivity can be better controlled if a-alkoxystannanes are used as substrates. This modification is named the Wittig-Still Rearrangement . Here, the intermediate organolithium compound is produced through transmetallation ... 

The Wittig-Still Rearrangement is also a suitable starting point for performing mechanistic studies about the stereospecificity of this process, and Maleczka and Feng have reported on the stereochemical outcome of the [1,2]-Wittig Rearrangement. 

Kruse, B. Bruckner, R. Does the Wittig-Still rearrangement proceed via a metal free carb-anion Tetrahedron Lett. 1990, 31, 4425—4428. 

Alkylation of the free OH provides the substrate 232 for the Wittig-Still rearrangement 233. The new tetrasubstituted alkene 234 is almost entirely E (>95 5 in most cases) and the migrating CH20 group is transferred across the top face of the allylic system as drawn. The [2,31-sigma-tropic rearrangement is suprafacial. 

In this form the reaction is limited as an anion-directing substituent such as phenyl is needed and this group is incorporated into the structure. A more general approach is to provide an R3Sn group 93 that can be replaced by lithium even if there is no other substituent at all on the carbon atom. This variation is often known as the Wittig-Still rearrangement.15... 

In the first asymmetric total synthesis of (+)-astrophylline, a [2,3]-Wittig-Still rearrangement is used to establish a 1,2-trans substituent relationship of the key cyclopentene intermediate (eq 38).Careful optimization of the reaction conditions leads to the development of a multigram-scale protocol. 

Scheme 17.11 r2.31-Wittig-Still rearrangement en route tn the CP framework. 

Figure 17.7 Anisatin and p-face hond-forming strategy via Wittig-Still rearrangement...

Scheme 17.14 r2.31-Wittig-Still rearrangement for synthesis of sespendole. 

Scheme 17.15 Synthesis of pyricuol via r2.31-Wittig-Still rearrangement.

The [2,3]-Wittig-Still rearrangement of the phenyl sulfide 51 (n = 0) afforded a mixture of 52 and 53 from a and p approach in 8 92 ratio fScheme 17.9T From the corresponding sulfone 51 (n = 2), use of 2 equiv. n-BuLi selectively yielded 54 in 65% yield with the a stereochemistry, resulting from lithium-ion chelation with the sulfonyl methyl and oxymethyl lithium (51a). The synthesis was continued to attach the side chain, and alkylative macrocyclization gave diol 57 (Scheme 17.9). ... 

Scheme 17.12 Anisatin synthesis via the key [2,3]-Wittig-Still rearrangement from 80 to 81.

Meanwhile, Nishikawa and coworkers employed the [2,3]- ttig-Still rearrangement for a partial synthesis of sespendole fScheme 17.14kTreatment of the tributyltinmethyl ether 91 with n-BuLi led to migration on the p-face (91a) to give the equatorial hydroxymethyl derivative 92. It is interesting that this is the opposite stereochemical course as compared to 87a tScheme 17.13). 

Smith employed a [2,3]-Wittig-Still rearrangement to install the trisubstituted (Z)-20-double bond in his group s total S5mthesis of (-)-hennoxazole A (100), an antiviral marine natural product. The transition state conformation 101a is similar to the ground state due to strain fScheme 17.161. 

Scheme 17.1 Synthesis of the hennoxazole A C16-C25 segment via [2,3]- ttig-Still rearrangement.

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