The title amino acid derivatives have been prepared in two steps from 3-alkoxy-1-isocyanopropenes. The key step is the alkoxycarbonylation of 3-alkoxy-1-isocyano-1-lithiopropenes with alkyl chloroformates to afford the corresponding 4-alkoxy-2-isocyano-3-butenoates, which are readily converted to the title compounds by acidic hydrolysis.