Valine f-butyl ester

The asymmetric variant of a-oxybenzoylation makes use of lithioenamines. Lithioena-mines formed with / -ketoesters and ( S)-valine f-butyl ester were in fact easily oxidized with benzoyl peroxide53 with high enantioselectivity (78-92% e.e.) to give, after hydrolysis, the corresponding protected tertiary a-ketols having (i ) configuration (Scheme 36). 

For the three types of reactions presented above, f-leucine f-butyl ester has been shown to be the most efficient amino acid derivative. It is often mentioned that valine f-butyl ester affords lower enantioselectivities. Work-up procedures allow recovery of reusable optically pure auxiliary. 

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