HETEROATOM-DIRECTED LITHIATIONS OF CHIRAL ALKYL CARBAMATES - A POWERFUL TOOL FOR ENANTIOSELECTIVE SYNTHESIS

Chiral alkyl carbamates, bearing hetero-substituents in the beta- or g amma-position, are deprotonated by sec-butyllithium/TMEDA with efficie nt selection between the diastereotopic alpha-methylene protons. Depen ding on the substitution pattern and the reaction conditions, either s teric or chelate effects act as stereodirecting devices. In several ca ses, this substrate-induced stereoselectivity is enhanced or is overri dden by means of sec-butyllithium/(-)-sparteine, which exhibits a stro ng preference for the abstraction of the pro-S proton. Altogether, an efficient and simple protocol for the stereoselective generation of sy nthetic equivalents for hetero-substituted alpha-oxyalkanide synthons is presented.