Fluoro-α-amino acids. 1—use of 19F NMR spectroscopy for the configurational determination of β-fluoro-α-amino acids through complexation by 18-crown-6 ether |
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Authors: | S Hamman M C Salon C Bguin |
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Institution: | S. Hamman,M. C. Salon,C. Béguin |
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Abstract: | Erythro and threo configurational assignments have been made for 11 β-fluoro-α-amino acids or esters using the effect of complexation of the ammonium group by 18-crown-6 ether on the 19F NMR parameters. For the erythro configurations, 3J(HF) increases and a high-field 19F chemical shift is generally observed; these phenomena are accompanied by a decrease in 3J(HH) and 3J(CF). The opposite effects are observed for the threo configurations. These observations can be explained by a change in the relative population of the conformers around the Cα-Cβ bond on complexation of the ammonium group. This complexation impairs the interactions between the ammonium and fluorine groups and, concomitantly, the steric hindrance between the ammonium and R (methyl, phenyl or carboxylate) groups is increased. |
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