Stereoselective aminoxylation of biradical titanium enolates with TEMPO

Abstract

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 a-cyclopropyl, as well as a-trifluoromethyl groups, are well tolerated. This transformation can therefore produce the a-aminoxylated adducts in excellent yields with high diastereomeric ratios (d.r.). In turn, parallel additions to the a,b-unsaturated N-acyl counterparts give the corresponding g-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.