High diastereoselectivity in the conjugateaddition of lithiatedSchöllkopf'sbislactimethers to E- and Z- prop-2-enylphosphonates was utilized to achieve a direct asymmetric synthesis of all four diastereoisomers of 2-amino-3-methyl-4-phosphonobutanoic acid, i.e. (+)-(2S, 3R)-4a, (+)-(2S, 3S)-4b and their corresponding enantiomers. Their relative configuration was definitively assigned from an NMR