Still—’Wittig rearrangement

In order to establish the correct absolute stereochemistry in cyclopentanoid 123 (Scheme 10.11), a chirality transfer strategy was employed with aldehyde 117, obtained from (S)-(-)-limonene (Scheme 10.11). A modified procedure for the conversion of (S)-(-)-limonene to cyclopentene 117 (58 % from limonene) was used [58], and aldehyde 117 was reduced with diisobutylaluminium hydride (DIBAL) (quant.) and alkylated to provide tributylstannane ether 118. This compound underwent a Still-Wittig rearrangement upon treatment with n-butyl lithium (n-BuLi) to yield 119 (75 %, two steps) [59]. The extent to which the chirality transfer was successful was deemed quantitative on the basis of conversion of alcohol 119 to its (+)-(9-methyI mande I ic acid ester and subsequent analysis of optical purity. The ozonolysis (70 %) of 119, protection of the free alcohol as the silyl ether (85 %), and reduction of the ketone with DIBAL (quant.) gave alcohol 120. Elimination of the alcohol in 120 with phosphorus oxychloride-pyridine... 

Transformation of allyl ethers into homoallylic alcohols by treatment with base. Also known as Still-Wittig rearrangement. Cf. Sommelet-Hauser rearrangement... 

A. G. Wee and L. Zhang, The Still—Wittig rearrangement in the preparation of a synthetically useful 4C-hydroxymethyl-hex-2-enopyranoside, Synth. Commun. 23 325 (1993). 

Sn2 substitution, 225 StilL-Wittig rearrangement, 227 Tebbe reaction, 231 types, 208... 

The Still-Wittig rearrangement of stannylated cis-ethers (11) gives trans homoallylic alcohols (12) selectively (equation 5). If the underlying alcohols (R = Me) are rearranged under Biichi s conditions (DMF acetal, xylene, reflux), trans selectivity is achieved, too, but yields are lower (41-49%). ... 

Alkenic stereoselection in the Still-Wittig rearrangement results presumably from conformational control in a very early transition state (Scheme 2). There, the substituent R prefers equatorial (15) over axial (16) orientation, if the 1,2-allylic strain is smaller than the 1,3-allylic strain, and vice versa. This implies trans products in the former vide supra) and (Z)-products in the latter case vide infra). 

Scheme 2 Cisitrans selectivity of the Still-Wittig rearrangement...

Unequalled in elegance is the double Still-Wittig rearrangement of (48), which gave the bis-(ZZ)-homoallylic alcohol (49) in 95% isomeric purity (equation 16). (Z)-Selectivity has also been noted for the Wittig rearrangement of an acid dianion (equation 17). ... 

The conversion of the cyclopentenyl alcohol (92) into the homoallylic alcohol (93) occurs by a dia-stereoselective Still-Wittig rearrangement on the less hindered face of the molecule (equation 23). °... 

Allylic alcohols with a trans double bond give product mixtures in the Still-Wittig rearrangement cf. equation 7 ). Advantageously, such substrates e.g. 98) can be converted to homoallylic alcohols with 100% chirality transfer by the Biichi rearrangement (equation 25). ... 

Pu]ii, K., Hara, O., Sakagami, Y. Highly stereoselective synthesis of (E)-substituted allylsilanes via the Still-Wittig rearrangement. Biosci. Biotechnol. Biochem. 1997, 61, 1394-1396. 

Hart, S. A., Trindle, C. O., Etzkorn, F. A. Solvent-dependent stereoselectivity in a Still-Wittig rearrangement an experimental and ab initio study. Org. Lett. 2001, 3, 1789-1791. 

Still-Wittig rearrangement of the ( )- and (Z)-17-ethylidene-16a ethers (93) and (94) afforded the 20a- and 20p-methyl steroids (95) and (96) with complete stereochemical control (equation 23). As in other examples, sp stereochemistry depends upon double bond geometry in the starting allylic ether. 

Still-Wittig rearrangements have been used to introduce an allylic hydroxymethyl substituent to a cyclohexene ring (equations 24 and 25). ° These reactions proceed with retention of stereochemistry. Al-... 

Still-Wittig rearrangement of the nonracemic (/ )-propargylic ethers (100a) and (100b) has been found to give the (/ )-allenes (100c) and (lOOd) with complete asymmetric transfer (equation 25a). The stereochemistry is consistent with a five-membered cyclic transition state. 

Chirality transfer in the rearrangement of allyloxymethyl stannanes is complete, even in cases where the rearrangement itself is not selective for one product, as shown by the examples in Scheme 6.17 [85]. Recall from Scheme 6.7b and c that in the Still-Wittig rearrangement, one product double bond configuration is formed selectively only when the educt has the Z configuration. This is due to severe strain in one of the two transition structures (e.g., between the isopropyl and the methyl in Scheme 6.17a). In 1985, Midland reported that rearrangement of the Z-... 

Scheme 6.17. I2,3]-Still-Wittig rearrangements of allyl ethers [85],...

The Still-Wittig rearrangement is hig functionality. A sila-version of the rear toxic tin compounds. 

The Still-Wittig rearrangement is highly (Zj-selective if assisted by a proximal oxy functionality. A sila-version of the rearrangement may have advantages in avoiding the toxic tin compounds. 

HalometiiyUributylstannanes. These valuable reagents (e.g., for Still-Wittig rearrangement) are prepared by a two-step procedure consisting of the hydroxymethylation of BUjSnH with paraformaldehyde and treatment with PhjP-CBr or PhjP-NIS. 

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