Cyclopropenone ketals allylzincation

The first example of enantioselective allylzincation of an alkene was also reported for the cyclopropenone ketal 78 as substrate. The chiral allylzinc complex 135 was prepared from the corresponding bis-oxazoline derived from (,V)-valine by deprotonation with n-BuLi and transmetallation with allylzinc bromide. This reagent reacted with 78 and afforded the allylated product 136 with high optical purity ( = 99%) (equation 66)101. 

Allylzincation of the monosubstituted cyclopropenone ketal 137 with the chiral reagent 138 proceeded regioselectively so as to generate the less substituted secondary cyclo-propylzinc species 139. After hydrolysis, the resulting cyclopropanone ketal was obtained with high enantiomeric excess ( = 99%). The reaction was very slow at 20 °C but was considerably accelerated under high pressure (1 GPa) (equation 67)102. 

Scheme 7-118 Enantio- and diastereoselective allylzincation of a cyclopropenone ketal. by coordination with chiral amines.

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