Post-Rearrangement Enolization

With the exception of the above example, post-rearrangement epimerization is generally an undesired side reaction. McIntosh et al. found that Ireland-Claisen rearrangement of some his-allyUc esters using excess KHMDS and TlPSOTf led to 

The present authors have noted in compiling this review that there are several examples of unusually low diastereoselectivity in systems that would be expected to give good to high levels of stereoselectivity. It seems likely that, at least in some cases, post-rearrangement epimerization is taking place. 

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The difference in yield and stereoselectivity between the Fujisawa and Bardett examples is noteworthy (cf Scheme 4.68 and entry 1, Scheme 4.69), considering that the only differences are the base (LICA vs. LffMDS) and the reaction temperature (THF reflux vs. ambient temperature). The low diastereoselectivity in the Bartlett case might be due to post-rearrangement enolization. 

Also in 1993, Hauske and JuUn reported a similar Ireland-Claisen rearrangement of an acyclic C6 carbamate (Scheme 4.35) [39]. The authors examined three different silyl ketene acetals in the rearrangement, although no experimental details were provided. AU three examples apparently proceeded with complete facial selectivity with respect to the allyUc alkene to afford the syn stereochemistry between the aUyl group and the NHBoc group in the conformation shown. The same rationale for facial selectivity can be applied as for Mulzer s results in the previous scheme. The reason for the low C2,C3 synjanti diastereoselectivity in the propionate example was not addressed. A lack of control of enolate geometry or post-rearrangement epimerization are both possible. 

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