A lithium amide conjugate addition approach to the synthesis of β-amino acid derivatives is described. Lithium amides derived from α-methylbenzylamine, such as lithium (α-methylbenzyl) benzylamide undergo highly diastereoselective 1, 4-conjugate addition to a variety of α, β-unsaturated carbonyl compounds. The benzyl substituents on the amino group can be readily removed by hydrogenolysis to afford a wide range of β-amino acid derivatives. The enolate intermediate can be trapped by electrophiles such as alkylhalides and (camphorsulphonyl) oxaziridine to give α-alkyl and α-hydroxy-β-amino acids in a highly stereocontroled fashion. The synthetic utility of the methodology is demonstrated by the syntheses of numbers of natural products and other important synthetic intermediates such as taxol C-13 side chain, cispentacin, and (+) -negamycin. The origin of the stereoselectivity is briefly discussed.