Stereochemistry 1,2 -Wittig rearrangement

Review articles have addressed advances in photochemical generation and reactions of carbanions,165 the [1,2]-Wittig rearrangement stereochemistry and synthetic application,167 and the aza-Wittig rearrangement.168... 

The stereochemistry of the Wittig rearrangement can be predicted in terms of a cyclic five-membered TS in which the a-substituent prefers an equatorial orientation.293... 

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... 

Tomooka, K. Inoue, T. Nakai, T. Stereochemistry and mechanism of vinyl-migrating [1,2]-Wittig rearrangement of a-lithioalkyl vinyl ethers. Chem. Lett. 2000, 418-419. 

The Wittig rearrangement is primarily used in the transformation of an allylic ether to an a-allyl alcohol (Scheme 26.18).444-445 The transition-state geometry plays an important role to determine the reaction outcome that, in turn, is dependent on the stereochemistry of the double bond (Figure... 

In a cyclic system, the stereochemistry of the product of a [2,3]-Wittig rearrangement may be ensured by the rearrangement s suprafacial nature. [2,3]-Wittig rearrangement of lithiomethyl ethers derived from stannanes provides a valuable method for stereospecific... 

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. ... 

It is remaikable that the ( )/(Z)-stereochemistry of the oxy-Cope product is not dependent on the erythrolthreo ratio of the 2,3-Wittig rearrangement but on the specific rearrangement procedures used thermal rearrangement exhibits higher ( )-selectivity (92-95%) than the anionic oxy-Cope and siloxy-Cope variants. 

In the laboratory of J.C. Anderson, the total synthesis of (+)-kainic acid was accomplished relying on a route that utilized an aza-[2,3]-Wittig rearrangement as the key step to install the correct relative stereochemistry between C2 and C3. The C4 stereocenter was established via an iodolactonization reaction. 

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. 

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