Chirality transfer rearrangement

Stereospecific Wittig rearrangement.1 Wittig rearrangement of the optically active (Z)-allylic ether 1 (9, 475 for a related reaction) gives (R,E)-2 with complete chirality transfer. Rearrangement of (E)-l results in two products, again with complete chirality transfer.1... 

BROOK Silaketone rearrangement Rearrangement of sila ketones to silyi ethers (with chirality transfer). 

It may be of interest to note that the stereospecific transformation shown in equation 15 has been cited as the first reported observation of an 1 - 3 chirality transfer. It is evident that on rearrangement of optically active 6d to 7d, the chiral center at C-a is eliminated and a new one created at C-y. The term self-immolative asymmetric synthesis has also been used to describe syntheses of this kind. As pointed out by Hoffmann , quantitative 1 - 3 chirality transfer will follow from the suprafacial - course of rearrangement, provided the reactant has a uniform configuration at the j8, y-double bond. This stereochemical prediction has also been confirmed by the results obtained in several other [2,3]sigmatropic rearrangements, subsequently reported " . 

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... 

Under more basic conditions, a-elimination predominates and insertion of the carbene 40 to the solvent gives racemic 22. Non-basic and poorly nucleophilic conditions allow neighboring group participation to form the rearranged substitution product 23 with complete chirality transfer. The participation can be considered as an intramolecular nucleophilic substitution, and does occur only when it is preferable to the external reactions. Under slightly basic conditions with bases in HFIP, participation is allowed, and the weak base can react with the more electrophilic vinylic cation 21 (but not with the iodonium ion 19). A suitably controlled basicity can result in the formation of cycloalkyne 39, which is symmetrical and leads to racemization. These reactivities are illustrated in Scheme 6. 

After the prototropic isomerizations, these rearrangements are the second most important synthetic methodology. In the concerted reactions a highly selective central to axial chirality transfer is possible, but this has already been exploited before the timeframe covered by this review and has been summarized [378]. 

There exist early examples of this transformation [507, 508], but due to the symmetric structure of the alkene part, only isotope labeling, etc., allowed the exclusion of a prototropic rearrangement. Furthermore, due to the high reaction temperatures of 340 °C and above, several different products are formed. A low-temperature version (77 K) of this reaction via the radical cation has been reported [509]. The chirality transfer has been studied and a detailed mechanistic investigation has been conducted [510] typical experiments in that context were the reactions of substrates such as 155 and 157 (Scheme 1.70). 

Keywords Aza-Claisen rearrangement 3-Aza-Cope rearrangement Chirality transfer Asymmetric induction Charge acceleration... 

This type of stereoselective reaction, in which a chiral unit is created and another destroyed, has been termed self-immolative by Mislow39 and an asymmetric transfer process by Pracejus40. Both terms are in use. but the latter is gaining ground in the mutated form of "chirality transfer 41, which is most often applied to sigmatropic rearrangement reactions such as the following37 ... 

The starting material is an O-protected a-hydroxyaldehyde which is submitted to an (E)- or (Z)-selective olefination. After O-deprotection, the allylic alcohol is rearranged to the y.d-un-saturaled ethyl ester with >99% chirality transfer. 

The self-immolative 1,3-chirality transfer from C-5 to C-3 the simple diastereoselection observed in the connection of C-2 with C-3 which results from the enol ester olefin geometry and the chair-like transition state of the [3.3]-sigmatropic rearrangement. 

Chirality transfer also belongs to this class of methods. Thus, the configuration of (S)-(E)-Ar,jV-dimethyl-3-trimethylsilyl-4-hexenamide [(S)-(Zi)-2 on p 422] was solely assigned81 on the basis of the established stereochemistry of the Eschenmoser Claisen rearrangement (see P 475). 

Review on chirality transfer via sigmatropic rearrangements R. K. Hill in Asymmetric Synthesis, J. D. Mosher, Ed., Vol. 3, p 503, Academic, Orlando 1984. 

The FWB rearrangements of zinc carbenoids occur with complete chirality transfer (equation 54)51. 

---