Acyclic -Wittig rearrangements

A high level of enantioselectivity in an acyclic system has been reported in the rearrangement of tricarbonylchromium(O) complexes of allyl benzyl ethers using chiral lithium amide base 73 (equation 38) . Upon treatment with 1.1 equivalents of lithium amide 73 and 1 equivalent of LiCl at —78 to —50°C, ether 74 afforded the rearrangement product R)-75 in 80% yield with 96% ee. The effect of substituents on the chemical yields and enantioselectivity of the [2,3]-Wittig rearrangement was also studied (see Table 3). 

Aza-Wittig rearrangement in the acyclic series are harder to control.141 The most reliable turn out to be those of allyl amides such as 192 in which the allyl group bears a (3-silyl substituent, whose function is to stabilise the anionic transition state of the reaction.142 143... 

The enantioselective [2,3]-Wittig rearrangements of acyclic substrates are grouped by substrate structure in Table 3. 

ACYCLIC r2.31-WITTIG REARRANGEMENT OF PROPARGYL-ALLYL ETHERS... 

However, stereochemically well-defined acyclic ethers are in demand too, for instance, for the stereospecific Wittig rearrangement. 

Finally, an aza-Wittig rearrangement of acyclic enantio-enriched N,N-diallyllic a-amino alkylithium prepared via a tin-to-lithium exchange was reported, the process proceeding predominantly with inversion of configuration at the lithiumbearing carbon terminus [361]. 

Mikami, K., Nakai, T. Acyclic stereocontrol via [2,3]-Wittig sigmatropic rearrangement. Synthesis 1991,594-604. 

Mikami K, Azuma K-I, Nakai T. [2,3]-Wittig sigmatropic rearrangement of crotyl propargyl ether system. An emerging tool for control of acyclic stereochemistry. Tetrahedron 1984 40(12) 2303-2308. 

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