Synthesis of γ-fluoro-α-amino acids by Claisen rearrangement

A series of N-protected glycine and alanine esters 4–7 of different fluorinated allylic alcohols 1 was prepared using the dicyclohexylcarbodiimide/dimethylaminopyridine method. All fluorinated esters of this type failed to react in an esterenolate Claisen rearrangement under the general conditions of the Kazmaier variant of this rearrangement. Change of the solvent from tetrahydrofuran to the less coordinating diethyl ether enabled the rearrangement of N-Boc-protected glycine esters 4a–c of 2-fluoroalken-3-ols 1a–c to form N-Boc-2-amino-4-fluoroalk-4-enecarboxylic acids 8a–c, while the rearrangement failed with N-Boc-alanine esters and all amino acid esters of 2-fluoroallylic alcohol (1e). This might be due to competing deprotonation of the position β to fluorine. Similarly to the esters 4a–c, the TFA-protected glycine esters 5a–c of 2-fluoroalken-3-ols 1a–c were rearranged. Deprotection of the Boc or the TFA group under salt-free conditions yielded the free amino acids 11a–c, which might be seen as mimics for N-alkylasparagines a group of lipoproteins.