An efficient method for the synthesis of phosphorated 1,2-oxazabuta-1,3-dienes derived from phosphine oxides and phosphonates has been developed by means of base-mediated dehydrohalogenation of readily available α-halooximes. New functionalized α-amino phosphorus derivatives are easily available, in a regioselective way, through conjugate addition of amino nucleophilic reagents such as ammonia, primary or secondary amines, and optically active amino esters to these highly reactive nitroso derivative Michael acceptors. Phosphorated 1,2-oxazabuta-1,3-dienes also react with enamines in a formal [3 + 2] cycloaddition to afford highly functionalized phosphorated N-hydroxypyrrole derivatives.