Synthesis of (S)-ibuprofen

Only the (S) enantiomer of ibuprofen is active as an anti-inflammatory (in the body, part of the (R) enantiomer is converted into (S)). It can be prepared selectively with the Evans auxiliary: the 4-isobutylphenylacetyl group is attached to the oxazolidinone, its Z enolate is methylated from the face opposite to the benzyl group and hydrolysis with LiOH/H₂O₂ releases (S)-ibuprofen and allows the auxiliary to be recovered.

Note that here the substituent already on the α carbon is the aryl group and the incoming one is the methyl, the reverse of the example on the page; that is why, with the same auxiliary, the descriptor of the product is S.