Enantiomerically pure natural and unnatural alpha-amino acids have been synthesized from a chiral methyleneoxazolidinone by means of a highly diastereoselective 1,4-conjugate addition of alkyl iodides in aqueous media. The zinc-copper conjugate addition reaction exhibits high chemoselectivity, with the possibility of using functionalized iodides, to afford a single diastereomer in short reaction times and with good yields.