Fluorine in Peptides: The Synthesis of alpha-Fluoro-beta-Amino Dipeptides by Direct Deoxofluorination/Rearrangement of N-Seryl Dipeptides

This article describes the stereo- and regioselectivity of the deoxofluorination of N-terminal dipeptides bearing a serine residue to generate, after rearrangement, a-fluoro-beta-amine-terminated dipeptides. The ratio of the rearranged a-fluorinated regioisomer is increased, relative to the non-rearranged beta-fluoro isomer, with N-alkylated amides. Otherwise, an intramolecular H-bond between the free amine and the amide NH suppresses formation of the key aziridinium intermediate required for a-fluorination. N-Methyl and N-allyl amides give exclusively a-fluorination products. Subsequent deprotection of the N-allyl amide to give a a-fluoro-beta-amino dipeptide product is demonstrated.