(E)-Dehydro analogues of alpha-(fluoromethyl)putrescine and -ornithine derivatives were synthesized and evaluated in vitro as irreversible inhibitors of a preparation of ornithine decarboxylase (ODC, EC 4.1.1.17) obtained from rat liver. The key step in the synthesis of (E)-alpha-(fluoromethyl)dehydroornithine (17) and -putrescine (14) was the addition of propenylmagnesium bromide to fluoroacetonitrile. The resulting unstable conjugated imine salt was reduced regioselectively in situ with NaBH4 or was quenched with a solution of NaCN to give the corresponding unsaturated alpha-(fluoromethyl) amine and alpha-amino nitrile, respectively. These were transformed into 17 and 14 via a four-step sequence involving (a) phthaloyation of the amine function; (b) allylic bromination of the methyl group; (c) Gabriel reaction; and (d) hydrolytic cleavage of the protective groups. (E)-alpha-(Difluoromethyl)dehydroornithine (10) and -putrescine (7) were prepared from ethyl tert-butyl 2-(difluoromethyl)-2-(2-propenyl)malonate and di-tert-butyl 2-(difluoromethyl)-2-(2-propenyl)malonate, respectively, via a sequence similar to that reported previously for the synthesis of the saturated analogues. Compounds 17, 14, 10, and 7 proved to be much more potent enzyme-activated irreversible inhibitors of ODC than the corresponding saturated analogues. The increase in potency is particularly marked in the alpha-fluoromethyl series. The apparent dissociation constants (KI) and the times of half-inactivation of enzyme (tau 50) at infinite concentration of inhibitors are 2.7 microM and 2.6 min for 17 and 42 microM and 0.2 min for 14. The KI and tau 50 of the corresponding saturated analogues are 75 microM and 1.6 min for the ornithine derivative and 56 microM and 4.4 min for the putrescine derivative.