The rearrangement of (E)-didehydroamino acid derivatives to the corresponding Z-derivatives under acid or basic catalysis as well as under the influence of radicals has been investigated. The condensation of N-benzyloxycarbonyl or N-tert-butoxycarbonyl protected alkyl 2-amino-2-(dimethoxyphosphoryl)acetates with aldehydes or ketones in dichloromethane in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene furnishes (Z)-didehydroamino acid ester derivatives diastereoselectively in excellent yields and with high purity.