(E)- and (Z)-Di-tert-butyl 2-amino-3-fluoro-2-butene-1,4-dioate [(E)- and (Z)-2] were synthesized in two ways: (a) by elimination of hydrogen fluoride from di-tert-butyl beta,beta-difluoroaspartate under the influence of 1,5-diazabicyclo[4.3.0]non-5-ene and (b) by amination with the ammonium acetate of di-tert-butyl monofluorooxaloacetate (3), obtained via condensation of tert-butyl monofluoroacetate with di-tert-butyl oxalate. Reduction of 2 with sodium cyanoborohydride yielded a mixture of di-tert-butyl monofluoroaspartates in which the erythro isomer constituted the major product. The structure of this isomer (4a) was established by X-ray crystallographic analysis of the corresponding acid 5a. Esterification of 5a to the beta-methyl ester 6, followed by aminolysis, yielded erythro-beta-fluoroasparagine (7). Tests with 5a and 7 in the L-5178Y test system showed that the compounds exhibited toxicity at levels at which no antitumor activity was observed.