3-Bromo-2-fluoropropene (4) is prepared in a new three-step synthesis from ammonium α -fluoroacrylate (1) in 31% overall yield. Glycine and alanine ester imines are efficiently alkylated by 4 to give, after deprotection, 2-amino-4-fluoropent-4-enoic acid (9) in 63% overall yield, and the α -methylated derivative 13 in 26% overall yield, respectively. Preliminary results indicate that 4 is potentially a new α -carbonyl cation equivalent. Key words: alkylation, amino acid ester imines, β -fluoroallyl bromide, 2-amino-4-fluoropent-4-enoic acid, α -carbonyl cation equivalentThe interest in partially fluorinated organic compounds has grown continuously over the last few years. 1 Nevertheless, the synthesis of highly functionalized molecules containing a limited number of fluorine atoms still remains a significant challenge to synthetic organic chemists.Recently, we reported the syntheses of racemic 2 and optically active 3 γ -and δ -fluoro-α -amino acids using easily accessible fluorinated building blocks. Glycine ester imines have been alkylated with 1-bromo-2-fluoroalkanes 4 in good yields, in spite of the deactivating influence of the fluorine substituent in β -position to the reaction center. 5 We then became interested in the application of 3-bromo-2 -fluoropropene (4) , which should be a more reactive alkylating reagent compared to the saturated β -fluorinated alkyl bromides.Until now β -fluoroallylic compounds have been infrequently used as building blocks. Only a few C-C bond formation reactions of these compounds have been described in the literature. 6 For example, one such compound has been used for the alkylation of Schöllkopf's bislactim ether. 6, 7 However, they have been already shown to participate well in substitution reactions with non-carbon nucleophiles 6 and esters of β -fluoroallylic alcohols have also been used for hetero-Cope rearrangements. 8 3-Bromo-2-fluoropropene (4) , previously prepared in a three-step synthesis from methyl vinyl ether in low overall yield, has been used for O -alkylation by Schlosser et al. 9 We report here a more efficient preparation of 4 and its use for C -alkylation of amino acid ester imines.Attempts to prepare 3-bromo-2-fluoropropene (4) by bromofluorination of allylic bromide followed by dehydrobromination have been thwarted by lack of regioselectivity in the addition step. 10, 11 However, we found that 3-bromo-2-fluoropropene (4) could be efficiently obtained in three steps via 2-fluoroallylic alcohol 3 starting with ammonium α -fluoroacrylate (1) 12 (Scheme 1 ) . The direct reduction of 1 with LiAlH 4 could not be accomplished. However, compound 3 could be synthesized by treatment of 1 with SOCl 2 , followed by reduction of the α -fluoroacrylic acid chloride with LiAlH 4 in diethyl ether at -20 ˚C. Although the reduction step was nearly quantitative, the overall yield of 3 was only 37% in this sequence. The most convenient synthesis of 3 involved the reduction of the benzyl ester 2 with LiAlH 4 in diethyl ether. The ester 2 was prepared under mild conditions by ...