Rearrangement of a-Hydroxy Substituted Allylic Esters

Chelation effects in general override the usual preferences in the formation of lithium ester enolates, and the (Z)-configured enolates are obtained nearly exclusively. Therefore the stereochemical outcome of the rearrangement should only be controlled by the olefin geometry in the allyl moiety and by the transition state (chair vs. boat). If substituted allyUc esters of glycolic add or related a-hydroxy-acids are subjected to rearrangement, synthetically valuable unsaturated a-hydroxyadds are obtained, albeit the yield and stereoselectivity strongly depends on the substrate and the reaction conditions used. 

Stereomeric mixture. Similar observations were made by Fujisawa et al. in the rearrangement of crotyl glycolate le (entry 5) [9]. In this case the yield could be increased by using LHMDS (entry 6), which is the base of choice for this system, especially if the Ireland version is used. 

The major side reaction in these processes appeared to be ester cleavage under liberation of the corresponding alcohol [8]. This mode of decomposition presumably involves formation of a ketene intermediate, which then undergoes further side reactions [10]. The increased prevalence of this side reaction with the enedio-late is not surprising in view of the enolate-destabilizing effect the ionized a-hy-droxyl is expected to have. 

The rearrangement of crotyl lactates If was investigated by Bartlett et al. No significant effect was observed if H MPA was used as cosolvent, but if 1 equiv MgClj was added to the ester, the stereoselectivity could be increased significantly, although unfortunately not the yield [8]. 

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