Stereoselective Aminoxylation of Biradical Titanium Enolates with TEMPO

A highly efficient and straightforward aminoxylation of titanium(IV) enolates from (S)-N-acyl-4-benzyl-5,5-dimethyl-1,3-oxazolidin-2-ones with TEMPO has been developed. A wide array of functional groups on the acyl moiety, including alkyl and aryl substituents, olefins, esters, or alpha-cyclopropyl, as well as alpha-trifluoromethyl groups, are well tolerated. This transformation can therefore produce the alpha-aminoxylated adducts in excellent yields with high diastereomeric ratios (d.r.). In turn, parallel additions to the alpha,beta-unsaturated N-acyl counterparts give the corresponding gamma-adducts with complete regioselectivity in moderate to good yields. Removal of the piperidinyl moiety or the chiral auxiliary converts the resultant adducts into enantiomerically pure a-hydroxy carboxyl derivatives, alcohols, or esters in high yields under mild conditions. Finally, a new mechanistic model based on the biradical character of the titanium(IV) enolates has been proposed.